UBC Theses and Dissertations

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Measurement of low frequency electric fields using electrodeless breakdown of gases 1983

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MEASUREMENT OF LOW FREQUENCY ELECTRIC FIELDS USING ELECTRODELESS BREAKDOWN OF GASES by DANIEL E. FRIEDMANN B.A.Sc, The University of B r i t i s h Columbia, 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Engineering Physics) We accept t h i s Thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA JANUARY 1983 © Daniel E. Friedmann, 1983 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department o f ir/U<s I AJ e. e R IAJ 6 p H V S ( C S : The U n i v e r s i t y of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date T A ^ I ft ( 9 8 3 DE-6 (3/81) i i ABSTRACT There i s a need for an e l e c t r i c f i e l d meter to measure environmental f i e l d s under high voltage transmission l i n e s and t h e i r associated switchyards. This thesis describes a new e l e c t r i c f i e l d meter based on the electrodeless breakdown of gases i n i n s u l a t i n g vessels. When a glass bulb f i l l e d with a gas (e.g. neon) i s placed i n an a l t e r n a t i n g e l e c t r i c f i e l d i t emits l i g h t i n the form of pulses. The number of pulses per cycle of the e l e c t r i c f i e l d i s proportional to the f i e l d magnitude. An e l e c t r i c f i e l d meter i s constructed by conveying the l i g h t from the gas f i l l e d glass bulb (the sensor) to an e l e c t r o n i c counter (the detector) with an o p t i c a l f i b e r . The r e s u l t i n g meter has a low weight, non m e t a l l i c sensor that can be separated from the detector e l e c t r o n i c s by any desired distance. The sensor shape d i c t a t e s i t s d i r e c t i o n a l s e n s i t i v i t y . A s p h e r i c a l bulb has an i s o t r o p i c response to the f i e l d while a c y l i n d r i c a l bulb gives a maximum response when i t s axis i s aligned with the f i e l d d i r e c t i o n . The size of the bulb i s inversely proportional to the threshold below which the f i e l d can not be measured. A 25mm bulb has a 10kv m-1 threshold. The transmission of the l i g h t s i g n a l from the sensor to the detector i s immune to e l e c t r i c a l noise. The detector e l e c t r o n i c s i s simple because the f i e l d magnitude information i s contained i n the number of pulses not i n the magnitude of the pulses. This thesis presents the t h e o r e t i c a l , experimental and f i e l d test r e s u l t s which explain the operation of the meter and substantiates i t s advantages. The basic p h y s i c a l model for the sensor i s established by describing the r e l a t i o n between o p t i c a l pulses and f i e l d magnitude, the e f f e c t of sensor shape on t h i s r e l a t i o n , the operation of the sensor i n e l l i p t i c a l l y p o l a r i z e d (including harmonic) f i e l d s and the e f f e c t of bulb size and gas composition on the phenomena. i i i The b a s i c p r a c t i c a l c o n s i d e r a t i o n s a r e i n v e s t i g a t e d b y s t u d y i n g t h e e n v i r o n m e n t a l e f f e c t s o n t h e p e r f o r m a n c e o f t h e m e t e r , t h e l i f e t i m e a n d s t a b i l i t y o f t h e m e t e r , t h e e f f e c t o f t h e s e n s o r on t h e f i e l d b e i n g m e a s u r e d a n d t h e g e n e r a l e n g i n e e r i n g o f m e t e r . T h i s w o r k h a s r e s u l t e d i n a f u l l y t e s t e d p r o t o t y p e m e t e r whose b a s i c o p e r a t i o n i s w e l l u n d e r s t o o d . i v T a b l e o f Contents Page 1 . 0 I n t r o d u c t i o n 1 1 . 1 E n v i r o n m e n t a l E l e c t r i c F i e l d s 5 1 . 2 P r i n c i p l e of O p e r a t i o n o f E x i s t i n g E l e c t r i c F i e l d Meters 8 1 . 3 P o s s i b i l i t i e s f o r a New E l e c t r i c F i e l d Meter 10 2 . 0 E x i s t i n g Work R e l a t e d t o E l e c t r o d e l e s s Breakdown 12 3 . 0 P h y s i c a l Model 14 3 . 1 I n t r o d u c t i o n 14 3 . 2 The Breakdown Mechanism 15 3 . 3 Breakdown i n I n s u l a t i n g V e s s e l s 2 0 3 . 3 . 1 S t r o n g Breakdown 2 2 3 . 3 . 2 Weak Breakdown 2 5 3 . 3 . 3 S t a r t up C o n d i t i o n s 2 5 3 . 3 . 4 The Pennin g E f f e c t 2 5 3 . 3 . 5 F i n i t e C o n d u c t i v i t y E f f e c t s 27 3 . 4 Breakdown Guided by V e s s e l W a l l s 2 9 3 . 4 . 1 I n t r o d u c t i o n 2 9 3 . 4 . 2 P o s s i b l e E f f e c t s 2 9 3 . 4 . 2 . 1 F i n i t e C o n d u c t i v i t y S c r e e n i n g 31 3 . 4 . 2 . 2 W a l l C o l l i s i o n Guidance 3 2 3 . 4 . 2 . 3 E x t e n s i o n t o C y l i n d e r s o f F i n i t e w i d t h 3 3 3 . 5 Breakdown i n P l a n a r R o t a t i n g F i e l d s 3 4 3 . 5 . 1 I n t r o d u c t i o n 3 4 3 . 5 . 2 Breakdown i n C i r c u l a r l y P o l a r i z e d F i e l d s 3 4 V Page 3.5.2.1 C y l i n d r i c a l Tubes 35 3.5.2.2 S p h e r i c a l Bulbs 36 3.5.3 E x t e n s i o n t o E l l i p t i c a l l y P o l a r i z e d F i e l d s 40 3.6 Summary 43 4.0 E l e c t r i c F i e l d Meter D e s i g n 45 4.1 I n t r o d u c t i o n 45 4.2 F u n c t i o n a l D e s c r i p t i o n o f GEM 46 4.2.1 The Sensor 46 4.2.1.1 The B u l b 48 4.2.1.1.1 B u l b Manufacture 48 4.2.1.2 The Holder 50 4.2.1.2.1 H o l d e r Manufacture 51 4.2.1 .3 E n g i n e e r i n g C o n s i d e r a t i o n s 52 4.2.1.3.1 Humidity 52 4.2.1.3.2 Temperature 52 4.2.1.3.3 Harmonics 53 4.2.2 The F i b r e 54 4.2.3 The D e t e c t o r 56 4.2.3.1 E l e c t r i c a l D e s i g n 58 4.2.4 O v e r a l l E n g i n e e r i n g c o n s i d e r a t i o n s 56 4.3 Summary 62 5.0 5.1 5.2 E x p e r i m e n t a l R e s u l t s I n t r o d u c t i o n E x p e r i m e n t a l Apparatus 64 64 66 v i Page 5.2.1 I n t r o d u c t i o n 66 5.2.2 Apparatus f o r G e n e r a t i n g U n i f o r m F i e l d s i n a F i x e d D i r e c t i o n and S t u d y i n g P u l s e E m i s s i o n 66 5.2.3 Apparatus f o r G e n e r a t i n g P l a n a r R o t a t i n g F i e l d 69 5.3 Study o f the B a s i c Phenomenon 73 5.3.1 S t a n d a r d Phenomena 73 5.3.1.1 G e n e r a l P u l s e E m i s s i o n 73 5.3.1.2 Rate o f P u l s e E m i s s i o n as a F u n c t i o n o f A p p l i e d F i e l d Magnitude 75 5.3.1.3 D e t e r m i n a t i o n of E D 78 5.3.1.4 P u l s e E m i s s i o n : Dependence on A p p l i e d Waveform 79 5.3.2 P o s s i b l e Mechanisms t o Reduce the T h r e s h o l d 82 5.4 Sensor Shape I n v e s t i g a t i o n s 86 5.4.1 S p h e r i c a l Bulbs 86 5.4.2 C y l i n d r i c a l Tubes 87 5.5 I n v e s t i g a t i o n s o f P l a n a r R o t a t i n g F i e l d s 94 5.5.1 C y l i n d r i c a l Tubes 94 5.5.2 S p h e r i c a l Bulbs 94 5.5.2.1 G e n e r a l P u l s e E m i s s i o n i n C i r c u l a r l y P o l a r i z e d F i e l d s 96 5.5.2.2 Rate o f P u l s e E m i s s i o n on a F u n c t i o n o f F i e l d Magnitude and Shape 96 5.5.2.3 P l a n a r R o t a t i n g F i e l d s and Penning M i x t u r e s 100 5.6 I n v e s t i g a t i o n s of E n g i n e e r i n g Problems 101 5.6.1 F i e l d P e r t u r b a t i o n 101 5.6.2 Meter C a l i b r a t i o n S t a b i l i t y 103 v i i Page 5.6.3 Humidity E f f e c t s 103 5.6 . 4 Temperature E f f e c t s 105 5.7 F i e l d T e s t s 106 5.8 Summary 108 6.0 Summary and C o n c l u s i o n 109 7.0 R e f e r e n c e s 115 Appendix I Appendix I I 117 122 v i i i L i s t o f T a b l e s Page T a b l e 1 Breakdown Parameters 19 T a b l e 2 GEM P r e l i m i n a r y S p e c i f i c a t i o n s 63 T a b l e 3 Meter C o n f i g u r a t i o n s and A p p l i c a t i o n s 114 i x L i s t o f F i g u r e s Page F i g u r e 1 E q u i p o t e n t i a l s around a human body 7 F i g u r e 2 F i e l d s t r e n g t h s a p p l i c a b l e t o the b u l b 21 F i g u r e 3 C a l i b r a t i o n curve ( f r e q u e n c y v e r s u s f i e l d ) 24 F i g u r e 4 O p t i c a l p u l s e s f o r weak breakdown 26 F i g u r e 5 Geometry of f i e l d s f o r c y l i n d r i c a l b u l b 30 F i g u r e 6 Geometry o f R o t a t i n g a p p l i e d f i e l d s 37 F i g u r e 7 Photo of p r o t o t y p e meter 47 F i g u r e 8 Photo o f t y p i c a l b u l b s 49 F i g u r e 9 Phasors f o r a p p l i e d r o t a t i n g f i e l d s w i t h harmonics 55 F i g u r e 10 B l o c k diagram o f d e t e c t o r 57 F i g u r e 1 1 C i r c u i t diagram of d e t e c t o r 60 F i g u r e 1 2 E x p e r i m e n t a l s e t up 67 F i g u r e 1 3 Apparatus f o r g e n e r a t i n g r o t a t i n g f i e l d s 70 F i g u r e 14 E l e c t r i c c i r c u i t f o r g e n e r a t i n g r o t a t i n g f i e l d s 71 F i g u r e 1 5 Photo o f apparatus f o r g e n e r a t i n g r o t a t i n g f i e l d s 72 F i g u r e 16 O p t i c a l p u l s e s f o r s t r o n g breakdown 74 F i g u r e 17 C a l i b r a t i o n c u r v e s f o r d i f f e r e n t b u l b p r e s s u r e 76 F i g u r e 18 C a l i b r a t i o n c u r v e s f o r d i f f e r e n t b u l b dimensions 80 F i g u r e 19 O p t i c a l p u l s e s f o r square wave 83 F i g u r e 20 O p t i c a l p u l s e s f o r Penning b u l b 84 F i g u r e 21 Time i n t e g r a t e d photo o f d i s c h a r g e i n a c y l i n d r i c a l tube 89 F i g u r e 22 C a l i b r a t i o n o f c y l i n d r i c a l tube 91 F i g u r e 23 A n g u l a r response o f c y l i n d r i c a l tube 93 F i g u r e 24 C a l i b r a t i o n c u r v e f o r s t r o n g breakdown 95 F i g u r e 25 C a l i b r a t i o n curve i n c i r c u l a r l y p o l a r i z e d f i e l d 97 F i g u r e 26 C a l i b r a t i o n c u r v e s i n e l l i p t i c a l l y p o l a r i z e d f i e l d s F i g u r e 27 F i e l d p e r t u r b a t i o n by b u l b F i g u r e 29 E l e c t r i c f i e l d under a 500kV l i n e F i g u r e I I T h e o r e t i c a l c a l i b r a t i o n c u r v e s i n e l l i p t i c a l l y p o l a r i z e d f i e l d s Page 99 102 F i g u r e 28 Bulb measurement s t a b i l i t y graph 104 107 F i g u r e I Geometry of c o n d u c t i n g s h e l l i n the a p p l i e d f i e l d 118 1 26 x i ACKNOWLEDGEMENT The i n i t i a l s t i m u l u s f o r t h i s work came from F. Heminsley who was i n t e r e s t e d i n d e v e l o p i n g a meter f o r m o n i t o r i n g e n v i r o n m e n t a l f i e l d s . The i n t i t i a l i n v e n t i o n r e s u l t e d from a c o l l a b o r a t i v e e f f o r t o f J e f f Young and the author which r e s u l t e d i n p a t e n t a p p l i c a t i o n No. 06/142,815 (US). The author i s i n d e b t e d t o R.R. Parsons, R.R. H e e r i n g and R.A. Nodwell f o r the encouragement they p r o v i d e d w h i l e the p r o j e c t was b e i n g d e v e l o p e d i n the A p p l i e d S c i e n c e 459 L a b o r a t o r y . S u b s e q u e n t l y d u r i n g the MASc program the B.C. S c i e n c e C o u n c i l not o n l y p r o v i d e d f i n a n c i a l s u p p o r t f o r the p r o j e c t b u t a l s o encouraged the development t o the commercial e x p l o i t a t i o n o f the d e v i c e . Some f i n a n c i a l s u p p o r t f o r the equipment was a l s o p r o v i d e d by NSERC. The development of the p h y s i c a l models and the p l a n n i n g and e x e c u t i o n of the experiments was a j o i n t e f f o r t w i t h F.L. C u r z o n . Many o t h e r s have a l s o g r e a t l y c o n t r i b u t e d t o the development o f e x p e r i m e n t a l f a c i l i t i e s , the e x e c u t i o n of experiments and the c o n s t r u c t i o n of the meter. Most n o t a b l y M. F e e l e y , G. A u c h i n l e c k , F. E a s t o n , R. A l l a n , A. Cheuck, E. W i l l i a m s , L. De S i l v a , D. P a r f e n i u k , D. Gayton and P. Wong. The author i s v e r y t h a n k f u l l t o J . Rothe and F. Heminsley f o r t h e i r c o n t i n u o u s encouragement and h e l p w i t h f i e l d t e s t s . 1 1.0 INTRODUCTION E l e c t r i c power i s c e n t r a l t o the energy systems o f Canada and the de v e l o p e d c o u n t r i e s . Even w i t h p r e s e n t e f f o r t s t o reduce the energy i n t e n s i v e n e s s of the economy through c o n s e r v a t i o n and h i g h e r p r i c e s , s u b s t a n t i a l growth i n the e l e c t r i c s e c t o r i s expe c t e d to c o n t i n u e . To move l a r g e amounts of power e c o n o m i c a l l y and r e l i a b l y over l o n g d i s t a n c e s , e l e c t r i c u t i l i t i e s need overhead A.C. t r a n s m i s s i o n l i n e s which o p e r a t e a t v o l t a g e s of 500 kV and above. In 1977 the U n i t e d S t a t e s alone had 20,000 km 1 o f t r a n s m i s s i o n l i n e s o p e r a t i n g a t 500 kV. S i n c e then the t o t a l l e n g t h o f t r a n s m i s s i o n l i n e s i n the U.S. has d o u b l e d . Furthermore the v o l t a g e o f the l i n e s has i n c r e a s e d c o n t i n u o u s l y . L i n e s o p e r a t i n g a t 765 kV a r e now common, w h i l e e x p e r i m e n t a l l i n e s are b e i n g o p e r a t e d a t v o l t a g e s above 1000 kV. Because of t h i s growth ( i n mil e a g e and v o l t a g e ) t h e r e i s an evergrowing number of people b e i n g exposed t o i n c r e a s i n g l y l a r g e r e l e c t r i c f i e l d s g e n e r a t e d by these p o w e r l i n e s and t h e i r a s s o c i a t e d s u b s t a t i o n s . Most o f those exposed ( e s p e c i a l l y t o the l a r g e r f i e l d s ) work i n the e l e c t r i c power i n d u s t r y . However o t h e r s l i v i n g and/or o p e r a t i n g equipment near p o w e r l i n e s or s u b s t a t i o n s a l s o r e c e i v e s u b s t a n t i a l exposure t o e l e c t r i c f i e l d s . Accompanying t h i s e x p a n s i o n of the e l e c t r i c power i n d u s t r y t h e r e i s a growing c o n c e r n among the p u b l i c , the E l e c t r i c a l U t i l i t i e s and the r e g u l a t o r y b o d i e s about the p o s s i b l e h e a l t h e f f e c t s of these f i e l d s . The c o n c e r n i s e x e m p l i f i e d by a number o f h e a r i n g s on the proposed c o n s t r u c t i o n of h i g h e r v o l t a g e l i n e s i n the U.S.2 2 There are t h e o r e t i c a l reasons t o b e l i e v e t h a t low f r e q u e n c y (50 t o 60Hz) e l e c t r i c f i e l d s can cause b i o l o g i c a l e f f e c t s : 1. n e u r o l o g i c a l or o t h e r e f f e c t s can be caused by 50 - 60Hz body c u r r e n t s i n d u c e d by the e x t e r n a l e l e c t r i c f i e l d . A l t h o u g h 60Hz i s p a r t i a l l y chosen f o r t r a n s m i s s i o n of AC power because of i t b e i n g the lowest f r e q u e n c y g i v i n g the i l l u s i o n o f c o n t i n u o u s l i g h t i n g w i t h i n c a n d e s c e n t lamps, i t i s a hazardous f r e q u e n c y . E x p e r i m e n t s ^ show t h a t t h r e s h o l d f o r s t i m u l a t i o n of nerve, s k e l e t a l muscle and c a r d i a c muscle are minimal near 60Hz; 2. e x t e r n a l f i e l d s c o u l d d i r e c t l y i n t e r f e r e i n b i o l o g i c a l p r o - c e s s e s t h a t i n v o l v e or a r e a f f e c t e d by the p r e s e n c e of e l e c t r i c f i e l d s , such as hormone and enzyme r e c o g n i t i o n p r o c e s s e s on c e l l membrane s u r f a c e s , bone growth p r o c e s s e s , e t c . ; 3. s m a l l a r c d i s c h a r g e s which r e s u l t when a p e r s o n a t one p o t e n t i a l touches an o b j e c t a t another p o t e n t i a l may a l s o l e a d t o p h y s i o l o g i c a l and p s y c h o l o g i c a l e f f e c t s and 4. unwanted s i g n a l s i s p r o s t h e t i c d e v i c e s ( f i l l i n g s i n t e e t h , pace- makers, h e a r i n g a i d s , e t c . ) can be u n c o m f o r t a b l e and dangerous. These t h e o r e t i c a l concerns have been p a r t i a l l y s u p p o r t e d by e xperiment. S t u d i e s p u b l i s h e d by Presman 4 o f r e s u l t s o b t a i n e d i n the S o v i e t Union and o t h e r s t u d i e s from E a s t e r n c o u n t r i e s c l a i m a number of a d v e r s e h e a l t h e f f e c t s such as headaches, f a t i g u e , i r r i t a b i l i t y and s e x u a l impotency among 500 kv s w i t c h y a r d w orkers. However, s t u d i e s by groups i n Sweden and the U.S.A. r e p o r t e d no s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e o b s erved between those workers who had been exposed t o s t r o n g e l e c t r i c f i e l d s and those who had n o t . R e c e n t l y s t u d i e s i n Canada^ s u p p o r t the f i n d i n g s i n the U.S.A. and Sweden. In a l l these s t u d i e s , the 3 number of p e o p l e i n v e s t i g a t e d was s m a l l so t h a t no r e l i a b l e i n f o r m a t i o n can be i n f e r r e d c o n c e r n i n g h e a l t h e f f e c t s which have a low f r e q u e n c y o f o c c u r r e n c e ( e . g . m o r t a l i t y r a t e s ) . A l s o , adverse e f f e c t s w i t h l o n g dormant p e r i o d s would n o t have been d e t e c t e d by the s t u d i e s done so f a r . F i n a l l y , the a c t u a l exposure, of the people i n v e s t i g a t e d , t o e l e c t r i c f i e l d s i s not known, s i n c e t h e r e has not been an e l e c t r i c f i e l d d e t e c t o r t h a t makes c o n t i n u o u s m o n i t o r i n g p o s s i b l e . F o r t h i s r e a s o n , a d e v i c e was d e s i r e d which c o u l d p r o v i d e c o n t i n u o u s m o n i t o r i n g of e l e c t r i c f i e l d e x p o s u r e s . A survey of e x i s t i n g d e v i c e s ( a l l of which are based on i n d u c e d c u r r e n t between two metal p l a t e s ) shows t h a t they have a number of d i s a d v a n t a g e s 6 . F i r s t l y , most d e v i c e s have d i r e c t i o n a l dependent s e n s i t i v i t i e s and are a d v e r s e l y i n f l u e n c e d by harmonic c o n t e n t of the a p p l i e d f i e l d . S e c ondly, they are made o f metal and thus c r e a t e a h a z a r d to workers u s i n g them because of the r i s k o f f l a s h o v e r . The i n i t i a l m o t i v a t i o n f o r the work d e s c r i b e d i n t h i s t h e s i s was to d e v e l o p a m o n i t o r i n g d e v i c e which would not s u f f e r from the e f f e c t s c i t e d above. I t would then be p o s s i b l e t o c a r r y out b e t t e r s t u d i e s on the c o r r e l a t i o n between exposure t o e l e c t r i c f i e l d s , and p o s s i b l e adverse h e a l t h e f f e c t s . However such a d e v i c e would have o t h e r p r o m i s i n g a p p l i c a t i o n s . F o r example, many crane o p e r a t o r s have been k i l l e d (2 i n B.C. l a s t y e a r ) when the crane touches the p o w e r l i n e . E x i s t i n g e l e c t r i c f i e l d d e t e c t o r s do not r e l i a b l y warn o p e r a t o r s of heavy c o n s t r u c t i o n equipment of the dangers posed by overhead t r a n s m i s s i o n l i n e s ^ . T h i s T h e s i s d e s c r i b e s the t h e o r e t i c a l , e x p e r i m e n t a l and e n g i n e e r i n g work done to d e v e l o p a new e l e c t r i c f i e l d meter. The meter i s based on a p r i n c i p l e which i s t o t a l l y d i f f e r e n t from t h a t o f e x i s t i n g meters. The new meter i s based on the e l e c t r i c a l breakdown of a low p r e s s u r e gas i n an 4 i n s u l a t i n g e n v e l o p e . T h i s phenomenon i s known as e l e c t r o d e l e s s breakdown and d u r i n g the c o u r s e o f d e v e l o p i n g the e l e c t r i c f i e l d meter, many new r e s u l t s were o b t a i n e d c o n c e r n i n g the p h y s i c a l p r o c e s s e s o c c u r r i n g i n e l e c t r o d e l e s s breakdown. The remainder of the i n t r o d u c t i o n d e s c r i b e s the t y p i c a l f i e l d s t o be measured, the e x i s t i n g e l e c t r i c f i e l d meters and i n t r o d u c e s the concept of the new e l e c t r i c f i e l d meter GEM (Gaseous E l l e c t r i c F i e l d M e t e r ) . S e c t i o n 2 p r e s e n t s a review o f the l i t e r a t u r e r e l a t e d t o the e f f e c t on which GEM i s based. S e c t i o n 3 i s devoted t o a d e s c r i p t i o n o f the P h y s i c a l Model f o r GEM, i t c o n t a i n s a l l the b a s i c t h e o r y . The d e s i g n o f GEM, which c o n t a i n s the a c t u a l p h y s i c a l and e n g i n e e r i n g c o n s i d e r a t i o n s , i s g i v e n i n S e c t i o n 4 . S e c t i o n 5 i s a d e t a i l e d a c c o u n t o f the major e x p e r i m e n t a l r e s u l t s o b t a i n e d i n the l a b o r a t o r y and i n p r e l i m i n a r y f i e l d t e s t s which form the e x p e r i m e n t a l b a s i s o f S e c t i o n s 3 and 4 . F i n a l l y , S e c t i o n 6 c o n t a i n s the c o n c l u s i o n s and expe c t e d f u t u r e developments. .1 Environmental E l e c t r i c F i e l d s Environmental e l e c t r i c f i e l d s are due to the p o t e n t i a l difference between energized conductors (powerlines) ( t y p i c a l l y 100V to 1000kV) and the ground (Ov). The f i e l d ' s magnitude and d i r e c t i o n depends on the number, d i s t r i b u t i o n and voltage of the conductors, the ground cover, the environmental conditions and any objects present nearby. T y p i c a l f i e l d s under 3 phase transmission li n e s are approximately e l l i p t i c a l l y p olarized i n a v e r t i c a l plane. The major axis of the e l l i p s e i s nearly v e r t i c a l due to the presence of the h o r i z o n t a l ground plane ( e l e c t r i c f i e l d s are perpendicular to the surface of good conductors). The e l l i p s e i s a r e s u l t of adding the f i e l d s from each phase. At an equal distance from each wire the actual f i e l d s cancel out to zero (a major property of three phase transmission). However, when the distances to each wire are not i d e n t i c a l the resultant f i e l d does not vanish. Thus the magnitude of the e l e c t r i c f i e l d under a transmission l i n e exhibits a two hump camel shape having a minimum near the center conductor (approximate cancellation) and a maximum near each outer conductor (l e a s t c a n c e l l a t i o n ) . A t y p i c a l f i e l d i s sketched below. CONDUCTORS ~\$~ rrrs HUxU '•v-/_~n APPART o o o 6 The f i e l d i n a s w i t c h y a r d i s much more complex due to a l a r g e number of p o w e r l i n e s of d i f f e r e n t v o l t a g e s , h e i g h t s and d i r e c t i o n s . However, the f i e l d a t any g i v e n p o s i t i o n i s s t i l l e l l i p t i c a l l y p o l a r i z e d . The d e s c r i p t i o n so f a r has i g n o r e d the p r e s e n c e of l a r g e c o n d u c t i n g o b j e c t s . These o b j e c t s can s e v e r e l y d i s t o r t the e l e c t r i c f i e l d l i n e s . Fo r example the f i e l d near the head of a person can be 10 times l a r g e r ^ than the ambient f i e l d i n the absence of the p e r s o n . T h i s enhancement i s due to a c o n c e n t r a t i o n of f i e l d l i n e s near the head (see F i g u r e 1 ) . An e l e c t r i c f i e l d meter must not o n l y measure these complex f i e l d s b u t must o p e r a t e i n extremes o f weather c o n d i t i o n s (±40°C, 0 - 100% r e l a t i v e h u m i d i t y ) and extreme e l e c t r i c a l n o i s e ( a t 60 Hz and i t s h a r m o n i c s ) . 7 8 1.2 P r i n c i p l e o f O p e r a t i o n o f E x i s t i n g E l e c t r i c F i e l d Meters I n v a r i a b l y p r e s e n t e l e c t r i c f i e l d meters c o n s i s t o f two meta l h a l v e s s e p a r a t e d by an i n s u l a t o r . When immersed i n an e l e c t r i c f i e l d the f i e l d i n d u c e s an a l t e r n a t i n g charge (or c u r r e n t ) which i s d i r e c t l y p r o p o r t i o n a l t o the magnitude o f the s i n u s o i d a l f i e l d . The meters work by measuring a q u a n t i t y p r o p o r t i o n a l t o the charges in d u c e d on the s e n s i n g e l e c t r o d e s ( m e t a l h a l v e s ) o r by measuring a q u a n t i t y p r o p o r t i o n a l t o the c u r r e n t between the e l e c t r o d e s . i n both cases the meters respond t o the average v a l u e o f the r e c t i f i e d s i g n a l , b u t a r e c a l i b r a t e d t o r e a d i n rms (assuming a u n i f o r m s i n u s o i d a l f i e l d ) . The c a l i b r a t i o n c o n s t a n t depends on the shape and s i z e o f the s e n s i n g e l e c t r o d e s as w e l l as on the f i e l d geometry. T y p i c a l l y the e l e c t r o n i c s r e q u i r e d t o d e t e c t the s i g n a l from the e l e c t r o d e i s housed between or i n s i d e the metal e l e c t r o d e s . The meters y i e l d a maximum r e a d i n g when a l i g n e d w i t h the d i r e c t i o n o f the f i e l d and a minimum ( c l o s e to z e r o ) when p e r p e n d i c u l a r t o the f i e l d . These meters have the f o l l o w i n g f a v o u r a b l e a t t r i b u t e s : 1. many of the measurement s t a n d a r d s have been devel o p e d f o r them; 2. they can be m i n i a t u r i z e d t o the s i z e o f a c i g a r e t t e box. However, i f a re a d o u t i s r e q u i r e d ( r a t h e r than j u s t s t o r a g e of the e l e c t r i c f i e l d r e a d i n g a t d e f i n i t e time i n t e r v a l s ) w i r e s (or an o p t i c a l f i b e r l i n k ) must be used t o connect the d e v i c e t o the r e a d o u t meter. The meters have the f o l l o w i n g d i s a d v a n t a g e s : 1 . they a r e m e t a l l i c and t h e r e f o r e dangerous because o f the r i s k of f l a s h - o v e r from the h i g h v o l t a g e equipment t o the sensor; 9 2. they a r e d i r e c t i o n a l l y s e n s i t i v e ( i e . the s e n s i t i v i t y depends on the o r i e n t a t i o n o f the d e v i c e i n the e l e c t r i c f i e l d ) ; 3. because the o u t p u t i s p r o p o r t i o n a l t o the average r e c t i f i e d s i g n a l the p r e s e n c e of harmonics i n the e l e c t r i c f i e l d i n t r o d u c e s an e r r o r which depends on the amplitude of the harmonic components. The e r r o r a l s o v a r i e s w i t h the phase d i f f e r e n c e between the harmonics and the fundamental; 4. they are made of metal which g r e a t l y p e r t u r b s the f i e l d and thus a meter c a l i b r a t e d i n a unform f i e l d may not be a c c u r a t e i n a non-uniform f i e l d ; 5. a s e l f c o n t a i n e d meter ( i e . one i n which the e l e c t r o n i c s and d i s p l a y are t o g e t h e r w i t h the s e n s i n g e l e c t r o d e s ) i s heavy; 6. a meter i n which the d i s p l a y e l e c t r o n i c s and the s e n s i n g e l e c t r o d e s a r e s e p a r a t e d ( f o r example meters f o r c r a n e s ) i s s u s c e p t i b l e t o e l e c t r i c a l n o i s e because o f the c o n n e c t i n g m e t a l l i c w i r e s . I f the two p a r t s are s e p a r a t e d by an o p t i c a l f i b e r ^ the e l e c t r o n i c s w i t h the s e n s i n g e l e c t r o d e s becomes more c o m p l i c a t e d and r e q u i r e s i t s own b a t t e r y o r power so u r c e ( t o modulate the l i g h t e m i t t i n g d i o d e ) . E n v i r o n m e n t a l e f f e c t s o f temperature and h u m i d i t y on these meters a r e n o t w e l l documented. However one s t u d y 1 6 shows t h a t t h e r e are no adverse e f f e c t s f o r temperatures from 0° t o 40°C ( i n f o r m a t i o n i s not g i v e n f o r temperatures below z e r o ) . Over the temperature range o f 0 t o 40°C v a r y i n g h u m i d i t y was a l s o found t o have no adverse e f f e c t . 10 1.3 P o s s i b i l i t i e s f o r a New E l e c t r i c F i e l d Meter An e l e c t r i c f i e l d meter i s made of two p a r t s : 1. the senso r which responds w i t h a s i g n a l p r o p o r t i o n a l t o the e l e c t r i c f i e l d and 2. the d e t e c t o r which d e t e c t s , a m p l i f i e s and d i s p l a y s (or s t o r e s ) the s i g n a l . As can be seen some of the p r e s e n t meters combine the sensor and d e t e c t o r i n t o one package. However o t h e r s (mostly f o r use as warning d e v i c e s ) s e p a r a t e the senso r and d e t e c t o r . T y p i c a l l y ( w i t h one e x c e p t i o n ^ ) the l i n k i s a w i r e . A new meter which overcomes the main d i s a d v a n t a g e s ( m e t a l l i c s e n s o r , d i r e c t i o n a l s e n s i t i v i t y , n o i s e s e n s i t i v i t y , e t c . ) i s d e s i r e d . One of the main d i s a d v a n t a g e s , d i r e c t i o n a l s e n s i t i v i t y , can t h e o r e t i c a l l y be overcome. Three of the a v a i l a b l e s e n s o r s can be p l a c e d i n t h r e e o r t h o g o n a l d i r e c t i o n s . However these s e n s o r s i n f l u e n c e each o t h e r (because they d i s t o r t the f i e l d l i n e s ) u n l e s s they are s e p a r a t e d by 2 or 3 s e n s o r d i m e n s i o n s . D i r e c t i o n a l independence can a l s o be overcome by u s i n g a non m e t a l l i c i s o t r o p i c a l l y shaped s e n s o r . The non m e t a l l i c s e n s o r a l s o reduces f l a s h o v e r r i s k and a f f o r d s the p r o s p e c t o f g r e a t l y r e d u c i n g the c o u p l i n g of e l e c t r i c a l n o i s e from the senso r t o the d e t e c t o r . A sens o r o f s y m m e t r i c a l shape w i t h o u t m e t a l p a r t s t h e r e f o r e becomes the main d e s i g n o b j e c t i v e . The need t o measure and q u a n t i f y the s i g n a l from the s e n s o r w i t h a d e t e c t o r which n e c e s s a r i l y employs e l e c t r i c a l components d i c t a t e s another o b j e c t i v e . The senso r and d e t e c t o r must be s e p a r a t e d from each o t h e r by a s i g n a l t r a n s m i s s i o n system which i s immune t o e l e c t r i c a l n o i s e , and which does not a d v e r s e l y 11 a f f e c t the s e n s o r ' s performance ( i e . does not d i s t o r t the f i e l d ) . In p r a c t i c e a s e n s o r which measures the component of the e l e c t r i c f i e l d a l o n g a s p e c i f i e d a x i s and g i v e s a maximum response when the p r e f e r r e d a x i s o f the s e n s o r i s a l i g n e d w i t h the f i e l d i s h i g h l y d e s i r a b l e f o r many a p p l i c a t i o n s . F o r example a c c u r a t e mapping o f e l e c t r i c f i e l d s f o r r e s e a r c h purposes or f o r the manufacture of e l e c t r i c a l equipment. Thus the i d e a l s e n s o r s h o u l d be a v a i l a b l e w i t h s p a t i a l l y i s o t r o p i c or n o n i s o t r o p i c s e n s i t i v i t y . Of the many phenomena t h a t o c c u r i n the presence o f e l e c t r i c f i e l d s , the e m i s s i o n of l i g h t by i n s u l a t i n g s o l i d s and gases i s most p r o m i s i n g . A s e n s o r based on t h i s phenomenon i n v o l v e s no m e t a l , can be made of any shape ( s p h e r i c a l f o r i s o t r o p i c response) and emits a s i g n a l ( e l e c t r o m a g n e t i c r a d i a t i o n ) which can be transmported t o the d e t e c t o r by an i n s u l a t i n g o p t i c a l l i n k . I n i t i a l experiments c a r r i e d out by e x p o s i n g e l e c t r o d e l e s s g l a s s b u l b s f i l l e d w i t h neon t o a l t e r n a t i n g e l e c t r i c f i e l d s r e v e a l e d t h a t the b u l b s emit i n t e n s e p u l s e s o f l i g h t . The number of p u l s e s per second was p r o p o r t i o n a l t o the s t r e n g t h of the a p p l i e d f i e l d w h i l e the p u l s e a m p l i t u d e d i d not seem i m p o r t a n t . The f a c t t h a t the i n f o r m a t i o n i s c o n t a i n e d i n the number o f p u l s e s g i v e s the meter as a whole the advantage of b e i n g a d i g i t a l system, t h e r e f o r e making the d e s i g n of the d e t e c t o r s i m p l e and n o i s e immune. In c o n c l u s i o n , to measure the complex e n v i r o n m e n t a l e l e c t r i c f i e l d s w i t h maximum a c c u r a c y and minimum danger ( t o the person u s i n g i t ) a new meter w i t h a non m e t a l l i c s e n s o r i s r e q u i r e d . The meter can be based on the breakdown of gases i n e l e c t r o d e l e s s g l a s s v e s s e l s . The next s e c t i o n d i s c u s s e s work on e l e c t r o d e l e s s breakdown of gases done by o t h e r s p r i o r to the development of the GEM. 12 2.0 E x i s t i n g Work R e l a t e d t o E l e c t r o d e l e s s Breakdown The breakdown o f gases by e l e c t r i c f i e l d s i n f l u o r e s c e n t l i g h t i n g and neon s i g n s i s a f a m i l i a r s i g h t . However i n a l l these cases metal e l e c t r o d e s a r e i n c o n t a c t w i t h the gas. E l e c t r o d e l e s s breakdown o c c u r s when the e l e c t r o d e s t h a t g e n e r a t e the f i e l d (the h i g h v o l t a g e l i n e s i n t h i s case) a r e s e p a r a t e d from the gas by an i n s u l a t i n g v e s s e l . The p h y s i c s of t h i s type of breakdown i s t o t a l l y d i f f e r e n t , m o s t l y because the e l e c t r o d e s and the gas (which becomes a c o n d u c t i n g plasma a f t e r breakdown) cannot form an e l e c t r i c a l c i r c u i t . I n s t e a d the v e s s e l w a l l s p l a y a major r o l e . Depending on the f r e q u e n c y of the a p p l i e d f i e l d e l e c t r o d e l e s s breakdown takes d i f f e r e n t f o r m s 1 0 . A t low f r e q u e n c i e s ( l e s s than 100Hz) d i s c h a r g e o c c u r s as c u r r e n t p u l s e s almost i n d e p e n d e n t l y i n each h a l f c y c l e . The d i s c h a r g e i s governed by p r i m a r y and secondary p r o c e s s e s as i n D.C. breakdown. The v e s s e l w a l l s p l a y a dominant r o l e i n the d i s c h a r g e . As the f r e q u e n c y i s i n c r e a s e d , p r o d u c t s o f the d i s c h a r g e i n one h a l f c y c l e remain t o a s s i s t the subsequent d i s c h a r g e s , and a d e c r e a s e i n t h r e s h o l d f i e l d ( f i e l d below which the gas w i l l n ot breakdown) r e s u l t s . A t even h i g h e r f r e q u e n c i e s the e l e c t r o n s are swept a s h o r t e r d i s t a n c e i n each h a l f c y c l e : when t h e i r o s c i l l a t i o n a m plitude becomes l e s s than the l e n g t h of the v e s s e l , the w a l l s no l o n g e r p l a y a dominant r o l e , the d i s c h a r g e becomes c o n t i n u o u s and the breakdown t h r e s h o l d d e c r e a s e s . E v e n t u a l l y a t v e r y h i g h f r e q u e n c i e s the e l e c t r o n s s t o p c o l l i d i n g w i t h the gas mo l e c u l e s and the d i s c h a r g e t h r e s h o l d i n c r e a s e s r a p i d l y . The h i g h f r e q u e n c y regime has been s t u d i e d because o f i t s a p p l i c a t i o n t o plasma d i s p l a y p a n e l s . However, the low f r e q u e n c y regime 13 r e c e i v e d a t t e n t i o n f o r a b r i e f p e r i o d d u r i n g the 1950's and then q u i c k l y d i s a p p e a r e d even from many w e l l known t e x t books. In f a c t the p r e l i m i n a r y work on GEM was done w i t h o u t knowledge o f the p u b l i s h e d r e s u l t s . H a r r i e s and von E n g e l 1 1 > 1 2 ' 1 3 d i d most of the work on e l e c t r o d e l e s s breakdown a t 50Hz. One of t h e i r main m o t i v a t i o n s was to u n d e r s t a n d the breakdown of i n s u l a t i o n used i n e l e c t r i c a l equipment ( i n s u l a t i o n d e v e l o p s s m a l l a i r pockets i n which e l e c t r o d e l e s s breakdown o c c u r s and e v e n t u a l l y d e s t r o y s the i n s u l a t i o n ) . They s t u d i e d the breakdown mechanism by o b s e r v i n g the c u r r e n t p u l s e s produced w i t h each d i s c h a r g e . T h i s work l e a d t o the b a s i c e x p l a n a t i o n of why c u r r e n t p u l s e s o c c u r . However they d i d not study the l i g h t e m i s s i o n d u r i n g breakdown, nor d i d they c o r r e l a t e the c u r r e n t or l i g h t p u l s e s t o the a p p l i e d e l e c t r i c f i e l d t o d e r i v e a measure of the f i e l d . T h e i r work was a l s o done o n l y w i t h u n i f o r m s i n g l e phase s i n u s o i d a l f i e l d s i n the l a b o r a t o r y . I t a l s o d i d not c o n c e r n i t s e l f w i t h v e s s e l s of d i f f e r e n t shapes nor w i t h the i n t e r a c t i o n o f on-going d i s c h a r g e s w i t h the v e s s e l w a l l s . The t h e o r y b e h i n d the o p e r a t i o n o f GEM i s d e s c r i b e d i n the n e x t s e c t i o n i n terms of a p h y s i c a l model f o r the s e n s o r . The p h y s i c a l model has i t s s t a r t i n g p o i n t on the work on D.C. breakdown and on H a r r i e s ' and von E n g e l s ' work. 14 0 P h y s i c a l Model 1 I n t r o d u c t i o n The p h y s i c a l model f o r GEM i s a t h e o r e t i c a l a p p r o x i m a t i o n t o the s e n s o r used t o e x p l a i n and u n d e r s t a n d how the s e n s o r works. The model i s based on the l i t e r a t u r e (see S e c t i o n 2) and numerous new experiments c a r r i e d out d u r i n g the development of GEM (see S e c t i o n 5 ) . There a r e many p r a c t i c a l ( e n g i n e e r i n g ) reasons to d e v e l o p a p h y s i c a l model: 1. i t a l l o w s a deeper u n d e r s t a n d i n g o f the b a s i c phenomena and t h e r e f o r e the t h e o r e t i c a l advantages and l i m i t a t i o n s o f the d e v i c e ; 2. i t sheds l i g h t on methods of c o n t r o l l i n g major meter parameters (such as t h r e s h o l d and response to e n v i r o n m e n t a l e f f e c t s ) and 3. i t a l l o w s p r e d i c t i o n o f r e s u l t s which are hard to o b t a i n e x p e r i m e n t a l l y (such as the i n f l u e n c e of h a r m o n i c s ) . T h i s s e c t i o n w i l l d e s c r i b e the b a s i c breakdown phenomena f o r homogeneous l i n e a r f i e l d s f i r s t . R e s u l t s w i l l then be extended to p l a n e o r r o t a t i n g f i e l d s ( i e . more t y p i c a l e n v i r o n m e n t a l f i e l d s ) . P r a c t i c a l c o n s i d e r a t i o n s such as e n v i r o n m e n t a l e f f e c t s , harmonics and meter d e s i g n a r e d e f e r r e d t o S e c t i o n 4. 15 3.2 The Breakdown Mechanism As stated previously the breakdown mechanism at low frequencies i s the same as D.C. The reason for this w i l l become apparent i n Subsection 3.3. Breakdown i n D.C. f i e l d s i s governed by the Townsend 1 4 mechanism which has been i n t e n s i v e l y studied because of the important role i t plays i n the formation of sparks between metal electrodes. Townsend formulated the following theory. Consider N electrons propagating through a gas which i s exposed to an e l e c t r i c f i e l d . The electrons are accelerated by the f i e l d and ionize atoms by c o l l i s i o n s . The number of new electrons produced per unit distance __. i s proportional to the number of electrons. Where , the constant of p r o p o r t i o n a l i t y , i s the f i r s t Townsend c o e f f i c i e n t . Thus the number of new electrons (DN) after a d i s t ance d i s given by dx dN c< N 3x DN = No (e ot d - 1) Where No i s the i n i t i a l number of electrons. Photons emitted by the excited atoms bombard the i n t e r i o r of the containing vessel and produce more electrons which can also contribute to the growth of i o n i z a t i o n within the gas. Let the p r o b a b i l i t y of secondary emission from the walls be % (per electron from the primary a v a l a n c h e ) where & i s known as the second Townsend c o e f f i c i e n t . I f an ava l a n c h e s t a r t s w i t h one e l e c t r o n the number of secondary e l e c t r o n s a f t e r the f i r s t a v a l a n c e w i l l be tf(e*d _ 1 ) I f the p r o c e s s i s c o n t i n u o u s the number of e l e c t r o n s a f t e r many a v a l a n c e s w i l l be 1 + tf(e<*d - 1) + tf2(e*d - 1)2 + . . . . 1 ~ X(e^a - 1) C l e a r l y a l l the gas i n the c o n t a i n e r w i l l be i o n i z e d i f the denominator o f the above e x p r e s s i o n v a n i s h e s , i e . i f T h i s e x p r e s s i o n i s known as the Townsend breakdown c r i t e r i o n . The f o l l o w i n g i m p o r t a n t o b s e r v a t i o n s can be made: 1 . depends on the energy a c q u i r e d by the e l e c t r o n p e r mean f r e e p a t h . The e l e c t r o n must a c q u i r e enough energy t o i o n i z e an atom, o t h e r w i s e the number o f e l e c t r o n s w i l l n o t i n c r e a s e . Thus below a c e r t a i n f i e l d o( tends t o 0 and the Townsend c r i t e r i a cannot be s a t i s f i e d . T h i s f i e l d i s the t h r e s h o l d f i e l d E D f o r av a l a n c h e growth; 17 2. the dependence o f the c r i t e r i o n on V i s l o g a r i t h m i c and t h e r e f o r e weak. Y i s governed by the secondary e m i s s i o n m a t e r i a l ( i e . the v e s s e l w a l l s i n t h i s case) and 3 . the dependance o f the c r i t e r i o n on ot i s l i n e a r , oi i s t h e r e f o r e most i m p o r t a n t f o r t h r e s h o l d c o n t r o l . S i n c e X i s s e t by the secondary e m i t t e r s u r f a c e i t i s c o n s t a n t f o r a g i v e n s e n s o r m a t e r i a l and geometry. Thus oid i s c o n s t a n t , c* i s a f u n c t i o n of the energy g a i n e d per mean f r e e p a t h ( E Q A ) <X = f ( E n A ) A where A i s t h e mean f r e e p a t h and f the a r b i t r a r y f u n c t i o n . Ii f o l l o w s t h a t f ( E 0 X) d = c o n s t a n t A or s i n c e A = c o n s t a n t / p (gas p r e s s u r e ) V 0 = E Q d = 0 (pd) (1) Where 0 i s another f u n c t i o n , p i s the gas p r e s s u r e ( r e l a t e d t o A ) and V Q i s the t h r e s h o l d v o l t a g e . T h i s remarkable r e s u l t i s known as t h e Paschen S c a l i n g Law. What e q u a t i o n (1) says i s i f p d i s h e l d c o n s t a n t and d i s v a r i e d the t h r e s h o l d f i e l d E Q can be c o n t r o l l e d ( E Q v a r i e s i n v e r s e l y w i t h d f o r c o n s t a n t p d ) . T h i s i s one o f the two most i m p o r t a n t r e s u l t s i n the c o n s t r u c t i o n o f low t h r e s h o l d 18 s e n s o r s . The o t h e r c o n s i d e r a t i o n i s the v a l u e of pd. .The shape o f 0 can be deduced from the b a s i c p h y s i c s . A t v e r y low p r e s s u r e t h e r e are al m o s t no m o l e c u l e s and t h e r e f o r e a l a r g e v o l t a g e i s r e q u i r e d t o s t a r t breakdown. At v e r y h i g h p r e s s u r e s t h e r e a r e so many c o l l i s i o n s t h a t a v e r y h i g h v o l t a g e i s r e q u i r e d i n o r d e r f o r the e l e c t r o n s t o g a i n s u f f i c i e n t energy. Thus (? must have a minimum somewhere between these two extremes. A graph o f V Q v e r s u s pd resembles an asymmetric p a r a b o l a . I t i s a t t h i s minimum pd t h a t the sensor must be o p e r a t e d . T y p i c a l l y 0 has a broad minimum near pd 10 t o 60 T o r r mm. T a b l e 1 shows t y p i c a l v a l u e s o f the parameters d i s c u s s e d f o r d i f f e r e n t g a s e s . I n e r t gases are h i g h l y d e s i r a b l e because o f t h e i r l o n g l i f e ( v e r y s t a b l e , no c h e m i c a l d e c o m p o s i t i o n ) . Neon and Argon can be used because of t h e i r low V D . Pyrex i s the b e s t c h o i c e f o r sensor m a t e r i a l because o f i t s a v a i l a b i l i t y , ease o f use, toughness and v a l u e of ^ no t s u b s t a n t i a l l y worse than f o r o t h e r g l a s s e s . 19 gas V 0 (v) pd minimum ( T o r r mm) neon 230 4 argon 220 1 .5 a i r 350 6.0 T a b l e 1 a) Breakdown parameters f o r d i f f e r e n t gases m a t e r i a l p y r e x m e t a l e l e c t r o d e p h o t o m u l t i p l i e r c o a t i n g v e r y low ( l e s s than 1 0 - 3 ) 0.01 0.3 T a b l e 1 b) Breakdown parameter f o r d i f f e r e n t m a t e r i a l s T a b l e 1 Breakdown Parameters 20 3.3 Breakdown i n I n s u l a t i n g V e s s e l s In a normal spark gap the breakdown s t a r t s as d e s c r i b e d i n the p r e v i o u s s u b s e c t i o n and c o n t i n u e s as a steady d i s c h a r g e w i t h the i o n i z e d gas (plasma) f o r m i n g p a r t of the e l e c t r i c a l c i r c u i t . However i n an i n s u l a t i n g v e s s e l the breakdown i s i n t e r r u p t e d by the v e s s e l w a l l s . Thus i n a D.C. f i e l d one breakdown o c c u r s , the charges s e p a r a t e and an i n t e r n - a l f i e l d which c a n c e l s the a p p l i e d f i e l d i s s e t up. I n s t e a d o f a c o n t i n - uous d i s c h a r g e t h e r e i s one s m a l l p u l s e d i s c h a r g e . The f o l l o w i n g d i s c u s s i o n d e v e l o p s the model f o r an a l t e r n a t i n g l i n e a r f i e l d and s p h e r i c a l v e s s e l s w i t h no c o n d u c t i v i t y . C l e a r l y the o r i e n t a t i o n of the v e s s e l w i t h r e s p e c t t o the a p p l i e d f i e l d does not m a t t e r . Suppose t h a t a s p a t i a l l y u n i f o r m f i e l d E^, o f f i x e d d i r e c t i o n i s a p p l i e d and v a r i e s i n time a c c o r d i n g to the e x p r e s s i o n = s i n W t where E^ i s the amplitude of the f i e l d , t i s the time and W the f r e - quency. In what f o l l o w s the magnitude of e l e c t r i c f i e l d s i s denoted by the a p p r o p r i a t e symbol w i t h the v e c t o r s u b s c r i p t o m i t t e d . E l e c t r i c a l breakdown w i l l o c c u r a t h i g h enough f i e l d s t r e n g t h s , and w i l l e v o l v e i n accordance w i t h the c o n v e n t i o n a l Townsend T h e o r y 1 4 . A f t e r breakdown the b u l b i s f i l l e d w i t h p a r t i a l l y i o n i z e d gas i n which i o n s and e l e c t r o n s are s e p a r a t e d by E A . The charge s e p a r a t i o n produces a space charge f i e l d , Eg which c o u n t e r a c t s the e f f e c t o f (see F i g u r e 2 b ) . S i n c e Eg i s produced by the e f f e c t s o f the a p p l i e d f i e l d , Eg i s l e s s than or e q u a l t o Ej^. I f the c o n d u c t i v i t y of the p a r t i a l l y i o n i z e d gas i s l a r g e enough the 21 • ( a ) F i g u r e 2 F i e l d S t r e n g t h s A p p l i c a b l e t o the Bulb a) F i e l d s t r e n g t h s v e r s u s time. E A - s i n u s o i d a l a p p l i e d f i e l d ( v e r t i c a l l i n e s a r e superposed o p t i c a l p u l s e s ) E j - sawtooth i n t e r n a l f i e l d . E g- stepwave space charge f i e l d . " E Q - breakdown f i e l d s t r e n g t h . b) Geometry of E A , _Eg and E j 22 i n t e r n a l f i e l d , E j i s r a p i d l y brought to z e r o 1 1 so t h a t - + Eg I f , however, the degree o f i o n i z a t i o n f o l l o w i n g breakdown i s too low, the t o t a l s e p a r a t i o n of i o n s and e l e c t r o n s w i l l produce a space charge f i e l d , Eg, which i s weaker than E A . Hence a r e s i d u a l i n t e r n a l f i e l d E j w i l l r emain i n the b u l b , and w i l l be i n the same d i r e c t i o n as E^. I t i s c o n v e n i e n t t o d i s t i n g u i s h between these two cases by the terms ' s t r o n g breakdown' (_Ej- = 0 j u s t a f t e r breakdown) and 'weak breakdown' (Ej g r e a t e r than 0 a f t e r breakdown). One would e x p e c t l a r g e a m p l i t u d e o p t i c a l p u l s e s t o o c c u r i n s t r o n g breakdown, and lower amplitude p u l s e s to a r i s e from weak breakdown. The s e p a r a t e d charges r e s i d e , between breakdowns, on the i n n e r s u r f a c e o f the g l a s s s h e l l , where th e y a r e h e l d i n p l a c e by p o l a r i z a t i o n f o r c e s . The charges are t h e r e f o r e immobile, and Eg can o n l y be changed by subsequent breakdowns. The e x t r a charges t r a n s p o r t e d to the g l a s s on such o c c a s i o n s e i t h e r n e u t r a l i z e those a l r e a d y t h e r e , or i n c r e a s e t h e i r c o n c e n t r a t i o n 1 1 - 1 ^ . 3.3.1 S t r o n g Breakdown The e v o l u t i o n of the f i e l d s f o r s t r o n g breakdown i s shown i n F i g u r e l a . The a p p l i e d f i e l d has a s i n u s o i d a l waveform, and the v e r t i c a l l i n e s s i g n i f y the o c c u r r e n c e o f o p t i c a l p u l s e s . Eg, which r e p r e s e n t s the space charge f i e l d i n the b u l b i s the s t e p p e d waveform below the time a x i s . I t i s c o n s t a n t between o p t i c a l p u l s e s i n accordance w i t h the assumed i m m o b i l i t y of charges a d h e r i n g t o the g l a s s . E j , which i s the 23 sum of E^ and Eg i s the saw-tooth wave form. Each time that E-j- reaches the breakdown f i e l d (E Q) of the gas i n the bulb, a breakdown occurs, accompanied by a f l a s h of l i g h t , and E-j- i s reset to zero. The process recurs when E-j- again reaches a value of E Q . Changes i n E-j- between breakdown correspond to changes i n E A . Therefore the change i n E A between o p t i c a l pulses i s simply E 0(see Figure 2). Thus for strong breakdown the average number of pulses (n) per cycle of the applied f i e l d i s given by the expression 1^ n = 2[2E A/E Q] where the square brackets denote the integer part of the r a t i o 2E A/E 0. If the frequency of the applied f i e l d i s f A then the frequency (fg) of the o p t i c a l pulses i s given by the expression f B = 2[2'E A/E 0Jf A (2) Thus f g depends i n a stepwise fashion on E A ( s o l i d l i n e i n Figure 3) A step occurs each time tha E A increases by E^/2, where E^ i s the average breakdown f i e l d of the gas. The step i n the pulse frequency has a size of 2 f A (see Figure 3). Since, i n strong breakdown the conditions ins i d e the bulb are i d e n t i c a l for the production of each successive pulse, one would also expect much less v a r i a t i o n i n pulse amplitude for strong breakdown than for weak breakdown. — H 4 5 K V / m K - ' F i g u r e 3 Frequency ( f g ) of the o p t i c a l p u l s e s v e r s u s a p p l i e d f i e l d a m p l i t u d e ( E A ) f o r 25mm diameter b u l b . a) High p r e s s u r e neon (10 T o r r ) b) Low p r e s s u r e neon (1 T o r r ) 25 3.3.2 Weak Breakdown In weak breakdowns, E-j- i s not r e s e t t o z e r o by a breakdown. Hence the change i n the a p p l i e d f i e l d r e q u i r e d t o make the i n t e r n a l f i e l d (E-j-) e q u a l t o the breakdown f i e l d i s s m a l l e r than E Q . Because o f the s t a t i s t i c a l f l u c t u a t i o n s i n E-j- from one o p t i c a l p u l s e t o the next, i t i s to be e x p e c t e d t h a t the s t e p e x h i b i t e d i n F i g u r e 3 w i l l be smoothed out, and t h a t t o a good a p p r o x i m a t i o n f B w i l l depend l i n e a r l y on E^ ( d o t t e d l i n e i n F i g u r e 3 ) . I t i s a l s o t o be a n t i c i p a t e d t h a t the change i n E A between o p t i c a l p u l s e s w i l l be s m a l l e r , the weaker the p u l s e s . T h i s e f f e c t i s i l l u s t r a t e d i n F i g u r e 4 which shows the o p t i c a l p u l s e s superposed on the a p p l i e d f i e l d - w a v e f o r m f o r weak breakdown. 3.3.3 S t a r t - u p C o n d i t i o n s The minimum f i e l d s t r e n g t h r e q u i r e d t o m a i n t a i n s t e a d y s t a t e o p e r a t i o n o f the b u l b i s E^ = E Q/2 ( e q u a t i o n 2 ) . I t would appear t h a t a f i e l d of s t r e n g t h E^ = E Q/2 i s too s m a l l t o i n t i a t e t h i s regime because i t has been assumed t h a t the gas has a breakdown f i e l d o f E Q . However i f one w a i t s l o n g enough the s t a t i s t i c a l p r o p e r t i e s o f the phenomenon ensure t h a t the f i r s t breakdown o c c u r s even when E^ i s l e s s than E Q . T h i s i s p a r t i c u l a r l y t r u e i n the weak breakdown regime. 3.3.4 The Pennin g E f f e c t The v a l u e o f the breakdown f i e l d E 0 i s governed by the Paschen law. In the case o f sim p l e i n e r t gases such as Ar and Ne the t h r e s h o l d i s s e t O p t i c a l p u l s e s superposed on a p p l i e d f i e l d wave form ( E A ) f o r 25mm diameter n e o n - f i l l e d b u l b a t 1 T o r r (weak breakdown c a s e ) . E i = f i e l d a m p l i t ude. 27 by V Q / d . One of the drawbacks of GEM i s t h a t E Q , f o r a g i v e n gas, can o n l y be d e c r e a s e d by making l a r g e r and l a r g e r s e n s o r s (which i s not d e s i r a b l e ) Thus i t i s i m p o r t a n t to c o n s i d e r ways of l o w e r i n g Vo. The Penning e f f e c t i s such a p o s s i b i l i t y . When a m i x t u r e of gases i s used (say Ar and Ne) and the m a j o r i t y c o n s t i t u e n t has m e t a s t a b l e s t a t e s ( 9 9 % Ne) then the t h r e s h o l d f o r breakdown can be lowered. T h i s happens because the m e t a s t a b l e s r e t a i n the energy put i n t o them and i o n i z e more of the m i n o r i t y gas atoms p r o v i d e d the e x c i t a t i o n energy of the gas m e t a s t a b l e l e v e l exceeds the i o n i z a t i o n energy of the m i n o r i t y component of the gas m i x t u r e . The net e f f e c t i s t h a t more e l e c t r o n s are used f o r i o n i z a t i o n ( i e . c>< i s r a i s e d ) . Other p o s s i b i l i t i e s f o r l o w e r i n g the t h r e s h o l d i n c l u d e doping the i n e r t gas w i t h a s m a l l p e r c e n t a g e of r a d i o a c t i v e gas (say t r i t i u m ) . The r a d i o a c t i v e e m i s s i o n s c o l l i d e w i t h the atoms and p r o v i d e more "seed" e l e c t r o n s from which a v a l a n c h e s can grow. 3 . 3 . 5 F i n i t e C o n d u c t i v i t y E f f e c t s The models p r e s e n t e d above depend on the assumption t h a t the b u l b has z e r o e l e c t r i c a l c o n d u c t i v i t y . I f the c o n d u c t i v i t y i s f i n i t e , then the gas i s exposed t o a f i e l d E T , which i s d i f f e r e n t from E A (see Appendix I * ) . However, the model remains v a l i d p r o v i d e d E A i s r e p l a c e d by E i p . A t low f r e q u e n c i e s s e v e r e s c r e e n i n g o c c u r s and the b u l b ceases to produce o p t i c a l p u l s e s . A t h i g h e r f r e q u e n c i e s the f i e l d ( E T ) which * Appendix I was d e v e l o p e d by F.L. C u r z o n . p e n e t r a t e s the g l a s s i s phase s h i f t e d b u t not a t t e n u a t e d s i g n i f i c a n t l y . T h i s means t h a t graphs of E A v e r s u s time can be c o n v e r t e d i n t o p l o t s o f E T v e r s u s time by s l i g h t l y d i s p l a c i n g the c u r v e s to the l e f t . D r a s t i c i n c r e a s e s i n e l e c t r i c a l c o n d u c t i v i t y can be produced by h i g h h u m i d i t y o: h i g h temperature. T h e r e f o r e i n these c o n d i t i o n s the b u l b may cease to o p e r a t e . 29 3.4 Breakdown gu i d e d by the V e s s e l W a l l s ( l i n e a r f i e l d s ) 1 6 3.4.1 I n t r o d u c t i o n So f a r the d i s c u s s i o n has d e a l t w i t h v e s s e l s whose w a l l s do n o t i n t e r a c t w i t h the e v o l v i n g d i s c h a r g e . The w a l l s o n l y p r o v i d e photo- e l e c t r o n s , s t o p the d i s c h a r g e and t r a p the c h a r g e s . T h i s p i c t u r e i s a c c u r a t e as l o n g as the v e s s e l i s s p h e r i c a l or c y l i n d r i c a l and a l i g n e d w i t h the f i e l d d i r e c t i o n . However i n a t t e m p t i n g t o b u i l d a c y l i n d r i c a l s e n s o r ( f o r those a p p l i c a t i o n s which r e q u i r e the measurement o f e l e c t r i c f i e l d components i e . d i r e c t i o n a l s e n s i t i v i t y ) one must t r e a t the case i n which the d i s c h a r g e i n t e r a c t s w i t h the c o n t a i n e r w a l l s as i t t r a v e l s from one end o f the v e s s e l t o the o t h e r . T h i s i n t e r a c t i o n must o c c u r when the c y l i n d r i c a l v e s s e l i s a t an a n g l e t o the e l e c t r i c f i e l d . A s i m i l a r i n t e r a c t i o n must a l s o o c c u r i n a l l v e s s e l s when complex r o t a t i n g f i e l d s a r e p r e s e n t . In the f o l l o w i n g s u b s e c t i o n s the p h y s i c a l model i s e v o l v e d f o r the ca s e d e p i c t e d i n F i g u r e 5. A c y l i n d r i c a l v e s s e l a t an angle 9 w i t h the a p p l i e d f i e l d . F i r s t two p o s s i b l e p h y s i c a l e f f e c t s t h a t may govern the d i s c h a r g e a r e i n t r o d u c e d . Then each e f f e c t i s d i s c u s s e d i n d e p e n d e n t l y . 3.4.2 P o s s i b l e E f f e c t s F r e e e l e c t r o n s which i n t i a t e d i s c h a r g e s move a l o n g e l e c t r i c f i e l d l i n e s . As seen i n F i g u r e 5 these l i n e s are v e r t i c a l ( d i r e c t i o n o f E f t) and thus one would e x p e c t the e l e c t r o n s t o move i n a v e r t i c a l d i r e c t i o n 30 F i g u r e 5 Geometry of a p p l i e d f i e l d and breakdown d i s c h a r g e . E A = a p p l i e d f i e l d ; E p = f i e l d a l o n g t u b e - a x i s ; E N = f i e l d normal t o t u b e - a x i s ; 9 = a n g l e between tube a x i s and v e r t i c a l ( z ) . Shaded r e g i o n - breakdown d i s c h a r g e w i t h i n t e r n a l f i e l d a l o n g z. 31 u n t i l they h i t the v e s s e l w a l l s . I f the e l e c t r o n s s t i c k t o the w a l l as soon as they h i t i t then t h e r e w i l l be no breakdown, a t l e a s t i n the case o f v e r y narrow t u b e s . However, H a r r i e s , based on h i s work, b e l i e v e s t h a t many c o l l i s i o n s are r e q u i r e d b e f o r e e l e c t r o n s w i l l s t i c k . Thus one p o s s i b l e mechanism i s f o r the tube w a l l s t o " g uide" the d i s c h a r g e from one end of the tube (C i n F i g u r e 5) t o the o t h e r ( A ) . Another way i n which the e l e c t r o n s may t r a v e l from one end o f the tube to the o t h e r i s i f the i n t e r n a l f i e l d ( i n s i d e the v e s s e l ) i s p a r a l l e l t o the tube a x i s . The a p p l i e d f i e l d , E A , has two components (see F i g u r e 5 ) : E N , normal to the tube and Ep, p a r a l l e l t o the tube. Suppose t h a t E^ i s s c r e e n e d by some mechanism but E p i s not, then the d i s c h a r g e w i l l p r o c e e d a l o n g Ep. In c o n c l u s i o n , i f a d i s c h a r g e i s t o proceed from one end of the tube t o the o t h e r e i t h e r the normal component o f the a p p l i e d f i e l d i s s c r e e n e d or the tube w a l l s guide the d i s c h a r g e . The next two s u b s e c t i o n s d e s c r i b e t h e s e models. 3.4.2.1 F i n i t e C o n d u c t i v i t y S c r e e n i n g The s c r e e n i n g due t o the f i n i t e c o n d u c t i v i t y of the g l a s s v a r i e s i n v e r s e l y p r o p o r t i o n a l ( f o r a g i v e n g l a s s s u r f a c e c o n d i t i o n ) w i t h the v e s s e l d i m e n s i o n . A s m a l l v e s s e l w i l l s c r e e n the f i e l d more than a l a r g e v e s s e l . Thus i t may be p o s s i b l e i n a narrow tube f o r t o be s c r e e n e d w h i l e Ep i s n o t . I f t h i s i s the case then the gas w i l l behave as i f i t were immersed i n an a x i a l f i e l d o f s t r e n g t h Ep = E A cos 9 32 and w i l l y i e l d a c o r r e s p o n d i n g v a l u e o f breakdown f r e q u e n c y ( i e . e q u a t i o n 2 w i t h r e p l a c e d by E p ) . C l e a r l y when 9 i s 90° t h e r e are no p u l s e s . 3 . 4.2.2 W a l l C o l l i s i o n Guidance Now assume t h a t f i n i t e c o n d u c t i v i t y i s not r e s p o n s i b l e f o r the e f f e c t . Furthermore assume t h a t e l e c t r o n s a r e a c c e l e r a t e d towards the v e s s e l w a l l s bounce o f f and are a g a i n a c c e l e r a t e d . I f d u r i n g the w a l l c o l l i s i o n a l m o s t no energy i s l o s t then the e l e c t r o n s w i l l be guided t o the o t h e r end of the tube. The e f f e c t i v e energy g a i n e d by then w i l l be E A L cos 9 where L i s the l e n g t h o f the tube (assuming the w i d t h d i r e c t i o n i s v e r y s m a l l ) . T h i s e x p r e s s i o n i s s i m p l y the p o t e n t i a l d i f f e r e n c e between the ends o f an i n c l i n e d t ube. Thus an i n c l i n e d tube i s e q u i v a l e n t t o a tube a l i g n e d w i t h the f i e l d o f l e n g t h L e (assuming the tube i s i n f i n i t e l y t h i n ) L e = L cos 9 (3) E x p e r i m e n t a l r e s u l t s p r e s e n t e d i n S e c t i o n 5 show t h a t t h i s model i s c o r r e c t . Thus the f r e q u e n c y of breakdown i n e l e c t r o d e l e s s tubes i s governed by the p o t e n t i a l d i f f e r e n c e between the ends o f the tube, p r o v i d e d the d i s c h a r g e i s gu i d e d by the tube w a l l s . A t h i n c y l i n d r i c a l tube t h e r e f o r e measures the component o f the e l e c t r i c f i e l d a l o n g i t s a x i s ( i e . produces a maximum r e a d i n g when a l i g n e d w i t h the f i e l d ) . 33 3.4.2.3 E x t e n s i o n t o C y l i n d e r s o f F i n i t e Width I f the c y l i n d r i c a l tube has a w i d t h which i s a s i g n i f i c a n t f r a c t i o n o f i t s l e n g t h , then the model i s c o m p l i c a t e d . B a s i c a l l y , the e f f e c t i v e l e n g t h i s no l o n g e r L cos 6 but i n c l u d e s a term p r o p o r t i o n a l t o the w i d t h o f the tube. Furthermore the maximum r e a d i n g i s most l i k e l y a t a s m a l l a n g l e from a l i g n m e n t w i t h the f i e l d ( i e . a l o n g the l a r g e s t path or d i a g o n a l of the t u b e ) . " F a t " tubes a r e not t r e a t e d i n any more d e t a i l because they do not have any obvious a p p l i c a t i o n . 34 3.5 Breakdown In P l a n a r R o t a t i n g F i e l d s 1 ^ 3.5.1 I n t r o d u c t i o n The p h y s i c a l model d e v e l o p e d so f a r has c o n s i d e r e d o n l y s p a t i a l l y u n i f o r m e l e c t r i c f i e l d s of f i x e d d i r e c t i o n ( l i n e a r f i e l d s ) . The model, however, i s a p p l i c a b l e t o non u n i f o r m f i e l d s p r o v i d e d the sens o r dimensions a r e s m a l l e r than the l e n g t h s c a l e o f the non u n i f o r m i t y . T h i s i s u s u a l l y the c a s e . E n v i r o n m e n t a l e l e c t r i c f i e l d s are t y p i c a l l y p l a n a r r o t a t i n g ( i n an e l l i p t i c a l envelope, see S e c t i o n 1.1) and thus the p h y s i c a l model must be extended. However, i t s h o u l d be mentioned t h a t i n many a p p l i c a t i o n s the e l e c t r i c f i e l d meter i s p l a c e d near a l a r g e ' c o n d u c t i n g o b j e c t ( i e . p e r s o n , c r a n e , e t c . ) where to a good a p p r o x i m a t i o n , the f i e l d i s i n a f i x e d d i r e c t i o n ( i e . p e r p e n d i c u l a r t o the o b j e c t s s u r f a c e ) and u n i f o r m over a s c a l e a few times s m a l l e r than the o b j e c t s r a d i u s o f c u r v a t u r e . The f o l l o w i n g s u b s e c t i o n s w i l l d i s c u s s the model f o r s p h e r i c a l and c y l i n d r i c a l v e s s e l s i n p l a n e c i r c u l a r l y p o l a r i z e d f i e l d s . The r e s u l t s w i l l then be extended t o p l a n e e l l i p t i c a l l y p o l a r i z e d f i e l d s . 3.5.2 Breakdown i n C i r c u l a r l y P o l a r i z e d F i e l d s A c i r c u l a r l y p o l a r i z e d f i e l d has a c o n s t a n t magnitude E A but r o t a t e s a t a c o n s t a n t r a t e (60 Hz) i n the p l a n e o f p o l a r i z a t i o n . Any p r o j e c t i o n o f t h i s f i e l d i s the f a m i l i a r s p a t i a l l y u n i f o r m f i e l d i n a f i x e d d i r e c t i o n . 35 3.5.2.1 C y l i n d r i c a l Tubes The e x t e n s i o n o f the p h y s i c a l model f o r c y l i n d r i c a l tubes t o p l a n a r r o t a t i n g f i e l d s i s s i m p l e . C y l i n d r i c a l tubes measure m a i n l y the component of the e l e c t r i c f i e l d a l o n g the tube a x i s (see S e c t i o n 2.4.2.2) i r r e s p e c t i v e of whether the f i e l d i s p l a n a r r o t a t i n g or not. Due t o the f i n i t e a s p e c t r a t i o o f the c y l i n d e r , the d i r e c t i o n o f the measured component i s i n e r r o r by r o u g h l y + 2 ° f o r a t y p i c a l tube ( l e n g t h , 5 5 mm, di a m e t e r 4 mm). The c y l i n d e r responds o n l y t o E (the component of the f i e l d a l o n g the t u b e ) , i f the f o l l o w i n g c o n d i t i o n i s s a t i s f i e d E N < ( L / D ) E Q where i s the f i e l d component p e r p e n d i c u l a r t o the c y l i n d e r a x i s , L i s l e n g t h o f the c y l i n d e r and D i t s d i a m e t e r . T h i s c o n d i t i o n means t h a t EJQ i s n ot s t r o n g enough t o cause a breakdown a c r o s s the tube. F o r a t y p i c a l c y l i n d e r (L = 50 mm, D = 4 mm), E^ can be twe l v e times as l a r g e as the a x i a l breakdown f i e l d b e f o r e i t i n f l u e n c e s the response of the b u l b . I f E N exceeds LE 0/D, then i t i s c o n v e n i e n t t o d e f i n e a r e j e c t i o n r a t i o r , by the e x p r e s s i o n r = 1 - E p / E N where E p and E N a r e the r e s p e c t i v e p a r a l l e l and p e r p e n d i c u l a r f i e l d components r e q u i r e d t o produce a g i v e n f B . i t i s r e a d i l y shown t h a t 36 r = 1 - (D/L), which f o r a t y p i c a l c y l i n d r i c a l b u l b , has the v a l u e , 0.9. Thus i n a c i r c u l a r l y p o l a r i z e d f i e l d o f magnitude ( E A < L/D E Q ) the c y l i n d r i c a l tube measures E A as i n e q u a t i o n 2, where E Q i s governed by the f u l l l e n g t h (L) of the c y l i n d e r . The measurement i s the same i r r e s p e c t i v e of o r i e n t a t i o n . 3.5.2.2 S p h e r i c a l Bulbs F o r s p h e r i c a l b u l b s the e x t e n s i o n of the p h y s i c a l model i s more complex. Assuming the b a s i c phenomena ( S e c t i o n 2.3) remains the same and t h a t the i n t e r n a l f i e l d E B i n the b u l b caused by the charge s e p a r a t i o n i s u n i f o r m ( t h i s assumption w i l l be d i s c u s s e d l a t e r ) , then a new breakdown s t i l l o c c u r s e v e r y time the a p p l i e d f i e l d changes by E Q , however the a d d i t i o n of the f i e l d s i s now v e c t o r i a l . For breakdown one must have I - A + _ B I = E 0 where the s t r a i g h t b r a c k e t s i n d i c i a t e s the magnitude of the v e c t o r i a l sum. In the s p e c i a l case where i s c i r c u l a r l y p o l a r i z e d and E_g i s u n i f o r m i t i s e a s i l y seen from F i g u r e 6 t h a t f o r the f i e l d t o change by E Q , E A must r o t a t e through an a n g l e 9 g i v e n by 2 2 E Q = E A [(1 - cos 9 ) 2 + s i n 9 2 ] or 2 c o s 9 = 1 - JL E o 2 2 E A 37 F i g u r e 6 Geometry of R o t a t i n g A p p l i e d F i e l d . E A ( 1 ) - a p p l i e d f i e l d a t 1 s t breakdown. E R ( 1 ) - " w a l l charge" f i e l d , 1 s t breakdown. E a ( 2 ) - a p p l i e d f i e l d a t 2nd breakdown. E n = breakdown f i e l d 38 Thus e q u a t i o n 2 which e x p r e s s e d the f r e q u e n c y of breakdown becomes f B = f A 2 J T = f A 2 TT ( 4 ) 9 cos-1/1 - 1 Eo^ Y " I 2 E A 2 ) Two i m p o r t a n t o b s e r v a t i o n s can be made c o n c e r n i n g the c a l i b r a t i o n curve ( f B v e r s u s E A ) : 1 . the s t e p s a s s o c i a t e d w i t h s t r o n g breakdown have d i s a p p e a r e d . T h i s e f f e c t can be u n d e r s t o o d i n t u i t i v e l y . The s t e p s i n the p r e v i o u s case were due t o the f a c t t h a t the change i n changes i n s i g n . In one c y c l e E^ i n c r e a s e s and then d e c r e a s e s . S t e p s o c c u r because any i n c r e a s e i n E A of l e s s than E Q f o l l o w e d by a d e c r e a s e w i l l not l e a d t o a p u l s e ( i e . near the peaks of the s i n e wave). In t h i s case E A i s c o n s t a n t and changes i n E^ due to r o t a t i o n are always of the same s i g n and t h e r e f o r e t h e r e a r e no d i s c o n t i n u i t i e s ( i n t e g e r b r a c k e t s ) i n e q u a t i o n 4 . 2 . The number of p u l s e s per second f B f o r a g i v e n f i e l d a m p l i t u d e E A i s i n c r e a s e d by r o t a t i n g the f i e l d e x c e p t a t t h r e s h o l d . T h i s i s a p p a r e n t from e q u a t i o n 4 . A t t h r e s h o l d , E A = Eo , 2 the number of p u l s e s i s f A as i s a l s o o b t a i n e d w i t h e q u a t i o n 2 . However, f o r E A l a r g e r than t h r e s h o l d e q u a t i o n 4 always g i v e s - a b i g g e r v a l u e . For example f o r E A much l a r g e r than E Q ( w e l l above t h r e s h o l d ) 9 approaches E Q / E A and e q u a t i o n 4 becomes 39 f A = 1 7L 2__A 2 E Q Thus ( n e g l e c t i n g the p r e s e n c e of s t e p s ) the number of p u l s e s i s enchanced by "TT/2 (1/4 the c i r c u m f e r e n c e of the c i r c l e ) . The a c t u a l enhancement f a c t o r v a r i e s s l o w l y from 1 t o TT/2 as E A i s i n c r e a s e d . Now c o n s i d e r the case of Eg not u n i f o r m . C l e a r l y t h i s i s the case i n r e a l l i f e s i n c e Eg i s formed by l o c a l i z e d charges a t the ends of the b u l b . However i n the case where and Eg a r e a l i g n e d the non u n i f o r m i t y o f Eg s h o u l d have l i t t l e e f f e c t on the p h y s i c a l model. T h i s i s why the non u n i f o r m i t y of Eg was n e g l e c t e d up to t h i s s e c t i o n . When Eg i s not u n i f o r m the c o n d i t i o n f o r breakdown becomes where the a n g u l a r b r a c k e t s i n d i c a t e some form of s p a t i a l average of the v e c t o r sum over the volume of the b u l b . The average i s r e q u i r e d because the breakdown c o n d i t i o n depends on the f i e l d geometry ( s i n c e Eg i s not u n i f o r m ) . E q u a t i o n 5 i s v e r y complex t o s o l v e and w i l l not be t r e a t e d f u r t h e r h e r e . Q u a l i t i v e l y two o b s e r v a t i o n s can be made 1. f o r l a r g e a p p l i e d f i e l d ^E A much g r e a t e r than E ^ the non u n i f o r m i t y o f Eg can be n e g l e c t e d and p r e v i o u s r e s u l t s a p p l y . T h i s i s because i n t h i s regime E A and Eg are a p p r o x i m a t e l y a l i g n e d ; 2. f o r E A near t h r e s h o l d the non u n i f o r m i t y of Eg can p l a y a s i g n i f i c a n t r o l e and must change the count r a t e g i v e n by e q u a t i o n 4. 40 3.5.3 E x t e n s i o n t o E l l i p t i c a l l y P o l a r i z e d F i e l d s E l l i p t i c a l l y p o l a r i z e d f i e l d s are l i k e c i r c u l a r l y p o l a r i z e d f i e l d s e x c e p t t h a t as E^ r o t a t e s i t changes magnitude. A t one extreme ( c o n s t a n t E f t) these f i e l d s reduce t o the c i r c u l a r l y p o l a r i z e d f i e l d s , and the other extreme ( v e r y narrow e l l i p s e ) they reduce to the u n i f o r m f i e l d s i n a s i n g l e d i r e c t i o n . Thus the e f f e c t s expected f o r e l l i p t i c a l l y p o l a r i z e d f i e l d s l i e somewhere i n between. In t h i s s e c t i o n the symbol E A w i l l be used t o denote the semimajor a x i s o f the e l l i p s e . T h i s non s t a n d a r d use of E A i s d e s i r a b l e because the e q u a t i o n s f o r the l i n e a r f i e l d s case c a r r y over t o t h i s e l l i p t i c a l c a s e . In the case of c y l i n d r i c a l tubes t h e r e i s no change. The tubes measure m a i n l y the component of the f i e l d and w i l l thus g i v e a maximum r e a d i n g when a l i g n e d w i t h the semi major a x i s o f the e l l i p s e . In the case of s p h e r i c a l b u l b s c i r c u l a r l y p o l a r i z e d f i e l d s cause two changes t o the p h y s i c a l model: enchancement i n the number of p u l s e s per second f o r a g i v e n f i e l d magnitude and smoothing o u t of the s t e p s . These e f f e c t s a l s o o c c u r i n e l l i p t i c a l l y p o l a r i z e d f i e l d s . The d e r i v a t i o n o f t h e p u l s e f r e q u e n c y f Q i n e l l i p t i c a l l y p o l a r i z e d f i e l d s proceeds i n a s i m i l a r manner as the d e r i v a t i o n o f e q u a t i o n 4 (assuming E B i s uniform) and i s t r e a t e d i n d e t a i l i n Appendix I I * . By analogy the r e s u l t y i e l d s two e f f e c t s 1. an enhancement** i n the p u l s e f r e q u e n c y r a n g i n g from 1 a t * Appendix I I was d e v e l o p e d by F.L. Curzon ** The enhancement i s d e f i n e d as the r a t i o o f f B i n an e l l i p t i c a l f i e l d of semimajor a x i s E f t t o f B i n a l i n e a r f i e l d , E A 41 t h r e s h o l d t o 1/4 the c i r c u m f e r e n c e o f the e l l i p s e d i v i d e d by E A a t f i e l d s h i g h above t h r e s h o l d i e . e = E G / E A and f g = f& c i r c u m f e r e n c e e 2 . A d i s a p p e a r a n c e o f the s t e p s o f f B vs E A a t f i e l d s h i g h above t h r e s h o l d . In the c i r c u l a r l y p o l a r i z e d f i e l d s the s t e p s d i s a p p e a r because the change i n E^ i s always o f the same s i g n . T h i s i s s t i l l the case i n e l l i p t i c a l l y p o l a r i z e d f i e l d s i f the semiminor a x i s f i e l d o f the e l l i p s e i s l a r g e r than breakdown f i e l d . O therwise s t e p s w i l l s t i l l be v i s i b l e . Thus v e r y q u a l i t a t i v e l y the b u l b behaves as i n a u n i f o r m f i e l d when E Q i s l a r g e r than the semi minor a x i s and as i n a c i r c u l a r l y p o l a r i z e d f i e l d ( e x c e p t t h a t the enhancement i s 1/4 the c i r c u m f e r e n c e o f the e l l i p s e d i v i d e d by E A i e . l e s s than TT/2) when the semi minor a x i s f i e l d i s g r e a t e r than E Q . Non u n i f o r m e f f e c t s of Eg a l s o change the count r a t e f o r f i e l d s near t h r e s h o l d . In c o n c l u s i o n : 1. C y l i n d r i c a l b u l b s measure the component o f a p l a n a r r o t a t i n g f i e l d s a l o n g the c y l i n d e r a x i s . As such they can be used, f u l l y t o c h a r a c t e r i z e the f i e l d i n space and time p r o v i d e d one o r i e n t s the b u l b i n d i f f e r e n t d i r e c t i o n s and maps the f i e l d . 42 S p h e r i c a l b u l b s measure an "average" o f the p l a n a r r o t a t i n g f i e l d . T h i s average i s h e a v i l y dependent on the f i e l d geometry and magnitude. Because of t h i s and s p a t i a l l y non u n i f o r m Eg e f f e c t s the b u l b must be c a l i b r a t e d f o r any g i v e n f i e l d geometry i f a c c u r a t e measurements a r e t o be made ( t h i s i s a l s o the case w i t h e x i s t i n g m e t e r s ) . On the o t h e r hand the measurement i s independent o f the o r i e n t a t i o n o f the b u l b w i t h r e s p e c t t o the f i e l d . 43 3.6 Summary When an e l e c t r o d e l e s s gas f i l l e d b u l b i s immersed i n a s p a t i a l l y u n i f o r m f i e l d of f i x e d d i r e c t i o n and magnitude ( E A ) l i g h t p u l s e s are emmited w i t h a f r e q u e n c y f B = 2[2 E A / E Q ] f A (2) where f A i s the f r e q u e n c y o f the a p p l i e d f i e l d and E Q i s the breakdown f i e l d o f the gas. E Q i s g i v e n by the Paschen Law E Q = 0 (pd) (1 ) d where i s a f u n c t i o n w i t h a broad minimum, p i s the gas p r e s s u r e and d the dimension o f the v e s s e l (the diameter f o r a sphere and the l e n g t h f o r a c y l i n d e r ) . E 0 can be c o n t r o l l e d by the gas type and the dimension o f the v e s s e l . A c y l i n d r i c a l tube immersed a t an an g l e 9 t o the f i e l d s t i l l obeys (2) however the e f f e c t i v e f i e l d i s E A cos 9 n o t E A . In p l a n a r r o t a t i n g f i e l d s e q u a t i o n 2 must be m o d i f i e d . For c y l i n d r i c a l tubes E A must be r e p l a c e d by the component o f the f i e l d a l o n g the tube a x i s . For s p h e r i c a l b u l b s the count r a t e i s always l a r g e r than or e q u a l t o f B g i v e n by e q u a t i o n 2 . The enhancement a t v e r y l a r g e f i e l d s i s e q u a l t o 1/4 o f the c i r c u m f e r e n c e o f the e l l i p s e d i v i d e d by E A . A l s o a t h i g h f i e l d s (or lower f i e l d s f o r f a t e l l i p s e s ) the s t e p s ( i n t e g e r b r a c k e t s i n e q u a t i o n 4) d i s a p p e a r . 44 The p h y s i c a l m o d e l p r e s e n t e d s o f a r h a s d e a l t w i t h t h e b a s i c i d e a l p h y s i c s o f t h e s e n s o r i n a c o n t r o l l e d l a b o r a t o r y e n v i r o n m e n t . The n e x t s e c t i o n d e s c r i b e s t h e f u l l m e t e r ( s e n s o r and d e t e c t o r ) a n d d i s c u s s e s e n v i r o n m e n t a l e f f e c t s e s p e c i a l l y on t h e s e n s o r . E x p e r i m e n t s c o n f i r m i n g t h e p r e d i c t i o n s i n t h i s s e c t i o n a n d t h e f o l l o w i n g a r e d i s c u s s e d i n S e c t i o n 5 . 45 4.0 E l e c t r i c F i e l d Meter D e s i g n 4.1 I n t r o d u c t i o n T h i s s e c t i o n d e s c r i b e s a complete meter f o r measuring low f r e q u e n c y e l e c t r i c f i e l d s , s p e c i f i c a l l y t y p i c a l f i e l d s e n c o untered under h i g h v o l t a g e t r a n s m i s s i o n l i n e s and i n s u b s t a t i o n s . The meter i s based on the a p p l i c a t i o n o f the breakdown o f gases i n d i e l e c t r i c e n v e l o p e s . A g l a s s b u l b f i l l e d w i t h a gas (the s e n s o r ) emits p u l s e s of l i g h t when exposed to the e l e c t r i c f i e l d . The number of p u l s e s per second i s a measure of the e l e c t r i c f i e l d . S i n c e the d e v i c e depends on e l e c t r i c a l breakdown o f a gas, i t i s r e f e r r e d to below by the acronym GEM (gaseous e l e c t r i c f i e l d m e t e r ) . The s e n s o r has the f o l l o w i n g i m p o r t a n t p r o p e r t i e s . I t c o n t a i n s no metal p a r t s and can be made to have an i s o t r o p i c or d i r e c t i o n a l s e n s i t i v i t y . I t p r o v i d e s i n f o r m a t i o n about the f i e l d s t r e n g t h by e m i t t i n g l i g h t which i s e a s i l y t r a n s p o r t e d to a d e t e c t o r w i t h o u t the use of m e t a l . S i n c e the i n f o r m a t i o n i s c o n t a i n e d i n the number of p u l s e s e m i t t e d r a t h e r than i n t h e i r h e i g h t the o u t p u t i s i d e a l l y s u i t e d to d i g i t a l p r o c e s s i n g . The above c h a r a c t e r i s t i c s c o n f e r the f o l l o w i n g b e n e f i t s on t h e f i e l d measuring meter. The meter i s s m a l l , rugged and r e a d i l y p o r t a b l e . I t i s a c c u r a t e ( b e t t e r than 5%) and remains c a l i b r a t e d f o r p e r i o d s o f s e v e r a l months. Due t o the o u t p u t of l i g h t p u l s e s from the s e n s o r , the meter ( d e t e c t o r + s e n s o r ) i s e a s i l y s h i e l d e d and o p e r a t e s w e l l i n n o i s y e l e c t r i c a l e n v ironments. I t i s a l s o s a f e t o use and has good time r e s o l u t i o n (~1 s e c ) . 46 In what f o l l o w s , the GEM and i t s p a r t s are d e s c r i b e d . The d e s c r i p t i o n c o v e r s d e s i g n a s p e c t s as w e l l as p r a c t i c a l c o n s i d e r a t i o n s f o r meter o p e r a t i o n i n e n v i r o n m e n t a l e l e c t r i c f i e l d s . 4.2 F u n c t i o n a l D e s c r i p t i o n o f the GEM 1 9 The GEM i s composed of t h r e e p a r t s (see F i g u r e 7 ) : the senso r (lower l e f t i n F i g u r e 7 ) , which senses the e l e c t r i c f i e l d and gen e r a t e s p u l s e s of l i g h t whose f r e q u e n c y i s p r o p o r t i o n a l t o the f i e l d ' s magnitude; the o p t i c a l f i b r e (upper p a r t i n F i g u r e 7 ) , which t r a n s m i t s the p u l s e s o f l i g h t t o the d e t e c t o r , the d e t e c t o r (lower r i g h t i n F i g u r e 7 ) , which c o u n t s the p u l s e s of l i g h t and gen e r a t e s a d i g i t a l r e a d i n g p r o p o r t i o n a l to the e l e c t r i c f i e l d magnitude. The o u t p u t r e a d i n g can be e a s i l y d i s p l a y e d t o the u s e r s or s t o r e d f o r l a t e r r e t r i e v a l . The use of a l o n g o p t i c a l f i b r e ( a t l e a s t 2 m^) p e r m i t s e l e c t r i c a l f i e l d s t o be measured u n d i s t o r t e d by the pres e n c e o f the pe r s o n making the o b s e r v a t i o n . A compact v e r s i o n o f the GEM can be made by u s i n g a s h o r t f i b r e , or by mounting the b u l b d i r e c t l y on the d e t e c t o r . However w i t h such a system the f i e l d i s d i s t u r b e d by the user and by the presence of the d e t e c t o r . The f o l l o w i n g s u b s e c t i o n s d e s c r i b e each of the t h r e e p a r t s i n d e t a i l . 4.2.1 The Sensor The s e n s o r i s a gas f i l l e d g l a s s b u l b e n c l o s e d i n a p r o t e c t i v e d i e l e c t r i c c a s e . F i g u r e 7 P h o t o s h o w i n g p r o t o t y p e o f i n s t a n t a n e o u s d i s p l a y u n i t (66% o f r e a l s i z e ) . 48 4.2.1.1 The B u l b The b u l b i s most e a s i l y made of pyrex (see S e c t i o n 3.2) and f i l l e d w i t h neon to a t y p i c a l p r e s s u r e o f 1 t o r r f o r a b u l b o f 25 mm d i a m e t e r ( i e . Paschen minimum f o r neon). The shape of the b u l b d i c t a t e s i t s d i r e c t i o n a l s e n s i t i v i t y . A s p h e r i c a l b u l b (see F i g u r e 8) i s i s o t r o p i c i . e . , i t g i v e s the same r e a d i n g i r r e s p e c t i v e o f the f i e l d d i r e c t i o n . A narrow c y l i n d r i c a l b u l b , on the o t h e r hand, g i v e s a maximum r e a d i n g when the f i e l d d i r e c t i o n i s a l i g n e d w i t h the a x i s of the c y l i n d e r . The b u l b ' s e l e c t r i c f i e l d t h r e s h o l d i s a f u n c t i o n of the p r e s s u r e and the l e n g t h s c a l e ( L ) . F o r a s p h e r i c a l b u l b L becomes the diameter w h i l e f o r a c y l i n d r i c a l b u l b , L becomes the l e n g t h of the c y l i n d e r a x i s . For a f i x e d p r o d u c t of p r e s s u r e and l e n g t h s c a l e a t the Paschen minimum the t h r e s h o l d f i e l d s t r e n g t h i s i n v e r s e l y p r o p o r t i o n a l t o L. For example, a 25 mm b u l b f i l l e d t o 1 T o r r has a t h r e s h o l d of 15 kV/m w h i l e a 38 mm b u l b f i l l e d t o 0.66 T o r r has a t h r e s h o l d o f 10 kV/m. In t h e weak breakdown regime the o u t p u t p u l s e f r e q u e n c y i s d i r e c t l y p r o p o r t i o n a l t o the e l e c t r i c f i e l d magnitude over broad ranges (10 t o 60 kV/m) f o r low p r e s s u r e s ( 1 T o r r ) . The s e n s o r has a s l i g h t s t a r t u p h y s t e r i s i s . A f i e l d g r e a t e r than t h r e s h o l d ( t y p i c a l l y 5% more (see S e c t i o n 3.3)) i s r e q u i r e d to s t a r t i t . However, once s t a r t e d , a r e l i a b l e r e a d i n g can be o b t a i n e d i n f i e l d s e q u a l or h i g h e r than t h r e s h o l d . 4.2.2.1.1 Bulb Manufacture To f i l l the b u l b s w i t h gas they are pumped down on a copper vacuum system by a 100 mm a p e r t u r e d i f f u s i o n pump equipped w i t h a l i q u i d F i g u r e 8 P h o t o s h o w i n g t y p i c a l b u l b s . 50 n i t r o g e n t r a p ( B a l z e r ' s D i f f 170, pumping speed 170 l i t r e s / s e c ) . The d i f f u s i o n pump i s backed by a S e r g e n t Welch r o t a r y o i l pump (Model Duoseal 1402, pumping speed 160 l i t r e s / m i n ) . The complete system has a base p r e s s u r e o f l e s s than 5 x 1 0 ~ 6 T o r r . A f t e r pumping f o r 24 hours, and b a k i n g a t 280°C f o r 4 hours, the b u l b s are f i l l e d t o a s u i t a b l e p r e s s u r e w i t h r e a g e n t grade gases, s e a l e d , and then baked a g a i n a t 280°C f o r a n o t h e r 4 h o u r s . The second b a k i n g improves the performance o f the b u l b s , but i t i s not y e t known why the improvement o c c u r s . Most of the b u l b s have been made u s i n g neon (99.999% pure) and argon (99.9995% pure) o b t a i n e d from the Matheson Co. In the neon, the c h i e f i m p u r i t i e s a r e h e l i u m ( 8ppm) and n i t r o g e n ( 2ppm). In argon some i m p u r i t i e s a r e p r e s e n t a t the l e v e l o f 0.1 ppm. Some b u l b s have a l s o been f i l l e d w i t h c o m m e r c i a l l y a v a i l a b l e d r y a i r or n i t r o g e n . The b u l b s of the d e s i r e d d i a m e t e r , d, are f i l l e d t o a p r e s s u r e , p, such t h a t pd i s a t t h e Paschen minimum (determined e x p e r i m e n t a l l y ) . 4.2.1.2 The Ho l d e r The b u l b i s p l a c e d i n a p r o t e c t i v e d i e l e c t r i c case, the h o l d e r . The h o l d e r s e r v e s t h r e e f u n c t i o n s : 1 . t o p r o t e c t the b u l b from e n v i r o n m e n t a l c o n d i t i o n s and h a n d l i n g ; 2. t o h o l d the o p t i c a l f i b e r c l o s e t o the b u l b and a l l o w the c o u p l i n g o f l i g h t p u l s e s t o the f i b e r and 3. t o p r e v e n t most of the ambient l i g h t from e n t e r i n g the f i b e r . The l a s t two c r i t e r i a can be met by most p l a s t i c s . However the f i r s t c r i t e r i o n i s more s t r i n g e n t . G l a s s i n c r e a s e s i t s c o n d u c t i v i t y by 51 o r d e r s of magnitude when hand l e d or exposed t o l a r g e r e l a t i v e h u m i d i t y 2 0 because mo b i l e a l k a l i i o n s can d i s s o l v e i n adsorbed water. Thus f o r r e l i a b l e o p e r a t i o n the b u l b must be e n c l o s e d i n a c o n t a i n e r whose c o n d u c t i v i t y i s not i n f l u e n c e d by these e f f e c t s . T e f l o n ( p o l y t e t r a f l u o r e t h y l e n e ) which i s h y d r o p h o b i c i s the i d e a l m a t e r i a l and has worked w e l l i n v e r y humid environments. For more t y p i c a l c o n d i t i o n s ( h u m i d i t y under 70%) p l e x i g l a s s ( p o l y m e t h y l a c r y l a t e ) i s adequate. With these h o l d e r s b u l b s can be dropped from 2 m onto c o n c r e t e f l o o r s w i t h no damage. 4.2.1.2.1 H o l d e r Manufacture F o r r e s e a r c h and t e s t purposes h o l d e r s are made by d r i l l i n g out c y l i n d r i c a l s t o c k of the d e s i r e d m a t e r i a l . The r e s u l t i n g c y l i n d e r w i t h one c l o s e d end i s capped w i t h a screw-on cap of the same m a t e r i a l . The f i b e r i s f i t t e d t o the cap. In t h i s manner the h o l d e r can be opened to r e p l a c e the b u l b . Ambient l i g h t i s reduced ( i t does not need t o be e l i m i n a t e d because the e l e c t r o n i c s i s s e n s i t i v e t o p u l s e s not D.C. s i g n a l s ) by p a i n t i n g the h o l d e r b l a c k ( i n s i d e ) or by p u t t i n g b l a c k e l e c t r i c i a n s tape o u t s i d e . In p r o d u c t i o n the h o l d e r w i l l most l i k e l y be made by d i p p i n g the b u l b ( w i t h the f i b e r g l u e d t o i t ) i n t o molten h o l d e r m a t e r i a l (say t e f l o n ) . 52 4.2.1.3 E n g i n e e r i n g C o n s i d e r a t i o n s There a r e two types o f p r a c t i c a l c o n s i d e r a t i o n s t h a t a p p l y t o the se n s o r when o p e r a t i n g i n e n v i r o n m e n t a l f i e l d s : e n v i r o n m e n t a l e f f e c t s (temperature and h u m i d i t y ) and the e f f e c t s o f f i e l d h armonics. 4.2.1.3.1 Humidity As mentioned e a r l i e r h u m i d i t y i n c r e a s e s the c o n d u c t i v i t y o f the g l a s s and under extreme c o n d i t i o n s can c o m p l e t e l y s h i e l d the gas from the f i e l d . The e f f e c t can be e l i m i n a t e d by p r o t e c t i n g the b u l b w i t h a s u i t a b l e h o l d e r . 4.2.1.3.2 Temperature Temperature can have both d i r e c t and i n d i r e c t e f f e c t s on the performance of the s e n s o r . D i r e c t e f f e c t s o ccur because the c o n d u c t i v i t y of the g l a s s i s i n c r e a s e d w i t h temperature and because the gas breakdown parameter changes w i t h temperature. Both o f these e f f e c t s are minimal between -40°C and 40°C the t y p i c a l o p e r a t i o n range. I n d i r e c t e f f e c t s are caused because temperature i n f l u e n c e s the c o n c e n t r a t i o n o f m a t e r i a l s a d h e r i n g t o the b u l b s i n t e r i o r and e x t e r i o r s u r f a c e . The m a t e r i a l s may be p h y s i c a l l y absorbed or c h e m i c a l l y combined. On v a r y i n g the temperature p h y s i c a l a b s o r p t i o n i s r e v e r s i b l e whereas " c h e m i s o r p t i o n " i s n o t . A b s o r p t i o n of m a t e r i a l on the b u l b s u r f a c e has two e f f e c t s : 1 . i t can i n f l u e n c e g l a s s p r o p e r t i e s such as c o n d u c t i v i t y or the secondary e m i s s i o n c o e f f i c i e n t X and 53 2. i t can change the gas c o m p o s i t i o n and t h e r e f o r e i t s breakdown p r o p e r t i e s . To minimize these e f f e c t s one must keep the b u l b e x t e r i o r c l e a n and p r o t e c t e d (by u s i n g the h o l d e r ) and minimize the i m p u r i t i e s i n s i d e the b u l b . T h i s i s the main m o t i v a t i o n f o r p r o p e r l y b a k i n g and pumping the b u l b s d u r i n g p r o d u c t i o n as w e l l as f o r u s i n g v e r y pure g a s e s . The e x t e n t of temperature e f f e c t s must be e v a l u a t e d e x p e r i m e n t a l l y . R e s u l t s are p r e s e n t e d i n S e c t i o n 5. 4.2.1.3.3 H a r m o n i c s 1 8 The e f f e c t harmonics have on s e n s o r o p e r a t i o n depends on the shape of the s e n s o r . a) C y l i n d r i c a l Tubes S i n c e a c y l i n d r i c a l tube responds o n l y t o the component of the f i e l d a l o n g i t s a x i s , i t i s o n l y n e c e s s a r y t o c o n s i d e r the e f f e c t of a l i n e a r l y p o l a r i z e d f i e l d . The f i e l d w i l l n o r m a l l y c o n t a i n two major harmonic components: the fundamental ( f r e q u e n c y , f A , a m p l i t u d e E A ) and the t h i r d harmonic ( f r e q u e n c y 3 f A , a m p l i t u d e EJJ) . T y p i c a l l y E H / E A < 0 . 1 . I f E j j / E A ^ 1 / 9 then the number of extrema i n the f i e l d per c y c l e of the fundamental i s not a f f e c t e d by the v a l u e o f E H . The number of counts i s t h e r e f o r e s t i l l d e termined by the l a r g e s t v a l u e of the f i e l d s t r e n g t h , i r r e s p e c t i v e o f the harmonic c o n t e n t . 54 b) S p h e r i c a l Bulbs The a n a l y s i s f o r l i n e a r l y p o l a r i z e d f i e l d s i s i d e n t i c a l t o t h a t g i v e n above. Hence the number of breakdowns per c y c l e o f the fundamental component of the a p p l i e d f i e l d i s determined by the g r e a t e s t f i e l d s t r e n g t h i n the c y c l e . For e l l i p t i c a l l y p o l a r i z e d f i e l d s a t y p i c a l phasor i s shown i n F i g u r e 9. The enhancement i n counts caused by the r o t a t i n g f i e l d w i l l be p r o p o r t i o n a l t o the c i r c u m f e r e n c e of the phasor, which, t o f i r s t o r d e r i n E H / E A i s e q u a l t o the c i r c u m f e r e n c e of the phasor assuming no harmonic d i s t o r t i o n ( E H = 0 ) . Thus as a f i r s t a p p r o x i m a t i o n the s p h e r i c a l b u l b o n l y responds t o the fundamental component of the a p p l i e d f i e l d . The l a r g e s t f i e l d s t r e n g t h which can o c c u r d u r i n g the o s c i l l a t i o n p e r i o d o f the fundamental i s E A + E H . T h i s f i e l d i s t h e r e f o r e measured w i t h a maximum f r a c t i o n a l e r r o r o f E H / E A . 4.2.2 The F i b e r The o p t i c a l f i b r e i s a commercial p r o d u c t purchased from W e l c h - A l l y n . The p r o t o t y p e GEM uses a v i n y l c l a d g l a s s f i b r e bundle 2mm i n d i a meter and 2 m l o n g . S h o r t e r or l o n g e r f i b r e s can be used. S i n c e the s e n s o r l i g h t o u t p u t i s s t r o n g , a narrow f i b r e can be used and no s p e c i a l r e f l e c t o r s a r e needed t o enhance the c o l l e c t i o n o f l i g h t a t the i n p u t t o the f i b r e . The most i m p o r t a n t p r o p e r t y of the f i b r e c a b l e i s i t s c o n d u c t i v i t y which must be v e r y low ( < 1 0 - 1 2 mho m~1 f o r t y p i c a l f i b r e s i n f i e l d s o f 60 Hz f r e q u e n c y ) . T h i s p r o p e r t y ensures t h a t the f i e l d t o which the s e n s o r i s exposed i s not changed s i g n i f i c a n t l y by the f i b r e . Low 55 F i g u r e 9 Phasors f o r A p p l i e d F i e l d , E A d o t t e d c u r v e - e l l i p t i c a l p o l a r i z a t i o n - no harmonic d i s t o r t i o n , s o l i d curve - e l l i p t i c a l p o l a r i z a t i o n w i t h 3 r d harmonic d i s t o r i t i o n , E H . 56 c o n d u c t i v i t y a l s o reduces the c o u p l i n g o f e l e c t r i c a l n o i s e from the f i b r e to the d e t e c t o r e l e c t r o n i c s . Many commercial f i b r e bundles are p r o t e c t e d w i t h a metal s h i e l d . These are o b v i o u s l y u n s u i t a b l e . However, many of the f i b r e s w i t h no metal are c o a t e d w i t h a c o n d u c t i v e r e s i n . In f a c t two s e t s of f i b r e s s u p p l i e d by the same manufacturer over a p e r i o d of f o u r months had w i d e l y d i f f e r e n t c o n d u c t i v i t i e s . The e a r l i e r s e t was s a t i s f a c t o r y but the second s e t had u n a c c e p t a b l y h i g h c o n d u c t i v i t y . Another i m p o r t a n t p r o p e r t y of f i b r e bundle f o r t h i s a p p l i c a t i o n i s r o b u s t n e s s . Continuous use of the f i b r e i n the f i e l d can e v e n t u a l l y l e a d t o the b r e a k i n g of some or a l l f i b r e s i n the b u n d l e . T h i s i s o n l y a problem near the end caps of the bundle where the f l e x i b l e f i b r e bundle meets an i n f l e x i b l e cap. To overcome t h i s problem the commercial f i b r e b u n dles have been r e - i n f o r c e d (near the caps o n l y ) w i t h graded heat s h r i n k t u b i n g . T h i s has the same e f f e c t as s p r i n g s near the j u n c t i o n s between w i r e s and e l e c t r i c a l a p p l i a n c e s . 4.2.3 The D e t e c t o r The d e t e c t o r i s a s m a l l p l a s t i c box c o n t a i n i n g the e l e c t r o n i c s which are s h i e l d e d by an i n t e r n a l m e t a l s h e e t l i n e r . The f u n c t i o n of the d e t e c t o r i s t o measure the f r e q u e n c y of occurence of l i g h t p u l s e s s i n c e t h i s f r e q u e n c y i s p r o p o r t i o n a l t o the e l e c t r i c f i e l d . A s i m p l i f i e d b l o c k diagram of the d e t e c t o r i s shown i n F i g u r e 10. The p h o t o d e t e c t o r d e t e c t s the weak o p t i c a l p u l s e s which are then a m p l i f i e d and shaped. D u r i n g t h i s p r o c e s s a l l p u l s e s whose amplitude i s h i g h e r than a c e r t a i n t h r e s h o l d ( s e t above the n o i s e l e v e l ) are c o n v e r t e d t o s t a n d a r d p u l s e s (same am p l i t u d e and d u r a t i o n ) . These p u l s e s are then LIGHT PHOTO DETECTOR PULSE AMPLIFIER AND SHAPER DIGITAL COUNTER DIGITAL DISPLAY DIGITAL STORAGE F i g u r e 10 B l o c k D i a g r a m o f D e t e c t o r a n d D i s p l a y S y s t e m . 58 counted over a s h o r t time span (At). A f t e r e v e r y p e r i o d o f l e n g t h A.t the c o u n t e r i s r e s e t t o z e r o and a l l o w e d t o count a g a i n . The l o n g e r A t , the more a c c u r a t e the r e a d i n g , p r o v i d e d o f cou r s e t h a t the f i e l d does not change d u r i n g t h i s time span. The s h o r t e r At, the b e t t e r the time r e s o l u t i o n of the meter. A good compromise f o r t y p i c a l a l t e r n a t i n g e n v i r o n m e n t a l f i e l d s i s a time i n t e r v a l of one second. The f l u c t u a t i o n i n the number of counts i n each i n t e r v a l At tends t o be c o n s t a n t , i r r e s p e c t i v e of the f i e l d s t r e n g t h . A t t h r e s h o l d the e r r o r i s t y p i c a l l y 5% and c o r r e s p o n d i n g l y lower f o r s t r o n g e r f i e l d s . The counts which are d i r e c t l y p r o p o r t i o n a l t o the e l e c t r i c f i e l d can be e i t h e r d i s p l a y e d t o the u s e r or s t o r e d f o r l a t e r r e t r i e v a l . One l a b p r o t o t y p e employs a l i q u i d c r y s t a l d i s p l a y and has a r e c h a r g e a b l e b a t t e r y . The d e t e c t o r measures 110 x 60 x 30 mm and the b a t t e r i e s l a s t f o r a t l e a s t 10 hours (see F i g u r e 7 ) . Another l a b p r o t o t y p e c o n t a i n s a m i c r o p r o c e s s o r and some memory i n s t e a d o f the d i s p l a y . T h i s p r o t o t y p e i s s l i g h t l y l a r g e r . In t h i s case, f o r example, the minimum, maximum and average e l e c t r i c f i e l d r e a d i n g e v e r y f i v e minutes can be s t o r e d over an e i g h t hour p e r i o d . The e l e c t r o n i c s o f both d e t e c t o r s are amenable t o f u r t h e r m i n a t u r i z a t i o n . 4.2.3.1 E l e c t r i c a l D e s i g n The d e t e c t o r e l e c t r o n i c s can be s e p a r a t e d i n t o two d i s t i n c t p a r t s (and b o a r d s ) . The f i r s t p a r t which b a s i c a l l y d e t e c t s , a m p l i f i e s and shapes the p u l s e s i s s p e c i f i c t o t h i s a p p l i c a t i o n . The second p a r t which c o n s i s t s of a d i g i t a l c o u n t e r and d i s p l a y system i s a v e r y s t a n d a r d o component of most d i g i t a l d e v i c e s and w i l l not be d e s c r i b e d f u r t h e r . 59 The c i r c u i t diagram f o r the p h o t o d e t e c t o r , a m p l i f i e r and p u l s e shaper i s shown i n F i g u r e 11. The c i r c u i t i s made w i t h s t a n d a r d o f f the s h e l f components (which can o p e r a t e between -30°C and 40°C) . L i g h t i s d e t e c t e d by a p h o t o t r a n s i s t o r (FPT100). The s i g n a l i s f e d through a low n o i s e FET t o a s e r i e s o f f o u r o p e r a t i o n a l a m p l i f i e r s ( a l l w i t h i n a c h i p ) f o r a m p l i f i c a t i o n . The t o t a l g a i n i s about 30,000. Each a m p l i f i e r stage i s o n l y A.C. c o u p l e d t o the next s t a g e ( i e . c a p a c i t o r f o l l o w e d by r e s i s t o r a t the i n p u t of each o p e r a t i o n a l a m p l i f i e r ) i n o r d e r t o reduce low f r e q u e n c y n o i s e . F i n a l l y the a m p l i f i e d v o l t a g e p u l s e i s put through a comparator and c o n v e r t e d t o a s t a n d a r d h e i g h t p u l s e . The t h r e s h o l d f o r the comparator can be a d j u s t e d w i t h a v a r i a b l e r e s i s t o r (R21) so as to d e t e c t o n l y those s i g n a l s which are above the n o i s e l e v e l ( i e . when the e l e c t r i c f i e l d i s o f ) . To reduce the 60 Hz n o i s e f u r t h e r the power s u p p l y i s d e c o u p l e d as shown i n the lower r i g h t o f F i g u r e 11. The above d e s i g n and the c o u n t e r and d i s p l a y e l e c t r o n i c s can o p e r a t e from a 9v r e c h a r g e a b l e b a t t e r y f o r 10 c o n t i n u o u s h o u r s . I t i s p o s s i b l e t o reduce the s i z e o f the d e t e c t o r i f r e q u i r e d f o r c e r t a i n a p p l i c a t i o n s . For example the p r e s e n t c i r c u i t f o r the p h o t o d e t e c t o r , a m p l i f i e r and p u l s e shaper uses about f o u r I C s . These c o u l d be combined i n t o one c h i p . I f d e s i r e d the whole d e t e c t o r c o u l d be packaged i n a 50 x 50 mm board ( a t a setup c o s t of l e s s than $50,000). 4.2.4 O v e r a l l E n g i n e e r i n g C o n s i d e r a t i o n s Most of the e n g i n e e r i n g c o n s i d e r a t i o n s a p p l y t o the sensor and have been d i s c u s s e d i n S e c t i o n 4.2.1.3. E l e c t r i c a l n o i s e a f f e c t s the d e t e c t o r -TEST Ftoi K/T 4 ojrfKT To D/CrrAL •r =r c/i Rio loo F i g u r e 11 C i r c u i t Diagram f o r P h o t o d e t e c t o r A m p l i f i e r and P u l s e Shaper. o 61 but can be v i r t u a l l y e l i m i n a t e d by s h i e l d i n g the e l e c t r o n i c s and by u s i n g a low pass f i l t e r . In c o n s i d e r i n g the meter as a whole the major c o n s i d e r a t i o n s are the e f f e c t s o f nearby c o n d u c t i n g o b j e c t s (or people) and the s e p a r a t i o n between the s e n s o r and the d e t e c t o r . E x p e r i m e n t s ^ have r e v e a l e d t h a t when a p e r s o n e n t e r s an e l e c t r i c f i e l d the f i e l d i s d i s t o r t e d a t d i s t a n c e s up t o 2 m away from the p e r s o n . Thus i n u s i n g the meter f o r making u n d i s t u r b e d measurements car e s h o u l d be taken i n m a i n t a i n i n g a r e a s o n a b l e d i s t a n c e ( o r d e r o f meters) from l a r g e c o n d u c t i n g o b j e c t s . On the o t h e r hand i f one wishes t o measure the enhanced f i e l d near a c o n d u c t i n g o b j e c t (say the head o f a person) then the senso r can be brought v e r y c l o s e . However, the senso r s h o u l d be m a i n t a i n e d about 10 mm away from p o i n t e d m e t a l l i c o b j e c t s . T h i s i s r e q u i r e d because p o i n t e d o b j e c t s have such a l a r g e enhancing e f f e c t t h a t they can l e a d t o corona d i s c h a r g e i n the a i r and i n the gas i n s i d e the s e n s o r . The corona d i s c h a r g e produces l i g h t and i n v a l i d a t e s the t r u e r e a d i n g o f the meter. In a l l measurements the d e t e c t o r s h o u l d be m a i n t a i n e d a t l e a s t 0.5m ( t h i s d i s t a n c e i s d e r i v e d e x p e r i m e n t a l l y ) from the sensor i n o r d e r t h a t i t s m e t a l l i c p a r t s do not i n f l u e n c e the r e a d i n g . I f a compact d e s i g n o f GEM i s r e q u i r e d i n which the s e n s o r and d e t e c t o r are one u n i t , then the meter has t o be c a l i b r a t e d as one u n i t and can o n l y be used r e l i a b l y i n u n i f o r m f i e l d s . 62 4.3 Summary An e l e c t r i c f i e l d meter based on the breakdown o f gas i n an i n s u l a t i n g v e s s e l i s c o n s t r u c t e d of t h r e e p a r t s : the s e n s o r , the f i b e r and the d e t e c t o r . The s e n s o r produces l i g h t p u l s e s whose number i s p r o p o r t i o n a l t o the e l e c t r i c f i e l d . The f i b r e conveys these p u l s e s to the d e t e c t o r where they ar e counted and d i s p l a y e d . The s p e c i f i c a t i o n s f o r the meter are summarized i n T a b l e 2. The e n t r y f o r temperature i s d e r i v e d e x p e r i m e n t a l l y and f u r t h e r d i s c u s s e d i n S e c t i o n 5. The l a s t two s e c t i o n s have p r e s e n t e d the p h y s i c a l model f o r the s e n s o r and the d e s i g n of the meter. These s e c t i o n s are p a r t l y based on a s e r i e s o f experiments performed d u r i n g the development of GEM and i t s p h y s i c a l model. These experiments are the s u b j e c t of S e c t i o n 5. 63 T a b l e 2 GEM P r e l i m i n a r y S p e c i f i c a t i o n s A c c u r a c y C a l i b r a t i o n S i z e Sensor ( i n c l u d i n g h o l d e r , s p h e r i c a l bulb) F i b r e D e t e c t o r Power T h r e s h o l d E n v i r o n m e n t a l e f f e c t s H u m i d i t y Above room temperature Below room temperature b e t t e r than 5% seldom r e q u i r e d ( e v e r y few months) 45mm diameter x 55mm l e n g t h 2mm diameter x 2m* l e n g t h 110 x 60 x 30mm 9v r e c h a r g e a b l e b a t t e r y (10 hours o p e r a t i o n ) 14 kV/m (can be reduced by i n c r e a s i n g sensor dimensions) no e f f e c t no e f f e c t unknown * v a r i a b l e 6 4 5.0 E x p e r i m e n t a l R e s u l t s 5.1 I n t r o d u c t i o n The purpose of t h i s s e c t i o n i s t o d e s c r i b e i n d e t a i l the experiments performed d u r i n g the development of GEM and i t s p h y s i c a l model. Most experiments are performed i n the l a b o r a t o r y where e l e c t r i c f i e l d s are g e n e r a t e d between l a r g e p a r a l l e l p l a t e s . Some experiments are performed j u s t o u t s i d e the l a b o r a t o r y i n o r d e r to study e n v i r o n m e n t a l e f f e c t s . Furthermore, some f i e l d t e s t s both under t y p i c a l t r a n s m i s s i o n l i n e s and i n s u b s t a t i o n s have been c a r r i e d o u t . Experiments o u t s i d e and f i e l d t e s t s are performed w i t h a f u l l p r o t o t y p e meter. However, i n the l a b o r a t o r y i t i s more d e s i r a b l e to r e p l a c e the d e t e c t o r w i t h a more s e n s i t i v e p h o t o m u l t i p l i e r . The p h o t o m u l t i p l i e r s i g n a l can be viewed d i r e c t l y on the o s c i l l o s c o p e ( i e . the d e t e c t o r s i g n a l would have t o be a m p l i f i e d and t h e r e f o r e d i s t o r t e d ) and/or counted w i t h a d i g i t a l c o u n t e r . Thus i n the l a b o r a t o r y o n l y the s e n s o r ( o f t e n w i t h o u t the h o l d e r , s i n c e the environment i s c o n t r o l l e d ) and the f i b r e of the meter are used. Four major s e r i e s of experiments c a r r i e d out i n the l a b o r a t o r y and i t s immediate v i c i n i t y are d e s c r i b e d . The f i r s t s e r i e s d e a l s w i t h the b a s i c phenomena of p u l s e e m i s s i o n i n u n i f o r m f i e l d s o f f i x e d d i r e c i t o n . T h i s s e t of experiments e s t a b l i s h e d , the v a l i d i t y o f the b a s i c p h y s i c a l model ( S e c t i o n s 3.2 and 3.3). The second s e r i e s o f experiments d e a l s w i t h the s e n s o r shape. T h i s s e t of experiments p r o v i d e d the b a c k i n g f o r e x t e n d i n g the p h y s i c a l model t o the case when the breakdown i s g u i d e d by the v e s s e l w a l l s . The o p e r a t i o n of a d i r e c t i o n a l l y s e n s i t i v e s e n s o r i s 65 based on these experiments (see S e c t i o n 3.4). The t h i r d s e t of experiments d e a l s w i t h the o p e r a t i o n o f s p h e r i c a l and c y l i n d r i c a l s e n s o r s i n p l a n a r r o t a t i n g f i e l d s . T h i s s e t o f experiments p r o v i d e s an u n d e r s t a n d i n g of what the s e n s o r measures when exposed t o e n v i r o n m e n t a l f i e l d s which are not p e r t u r b e d by a c o n d u c t i n g o b j e c t (see S e c t i o n 3.5 and 4.2). F i n a l l y , the l a s t s e t of experiments d e a l w i t h e n g i n e e r i n g c o n s i d e r a t i o n s such as s t a b i l i t y o f c a l i b r a t i o n , a c c u r a c y , temperature and h u m i d i t y e f f e c t s and f i e l d p e r t u r b a t i o n (by the s e n s o r ) . I t s h o u l d be noted t h a t the d e t a i l s of the e x p e r i m e n t a l r e s u l t s p r e s e n t e d i n a g i v e n s e c t i o n a r e f o r the s p e c i f i c s e n s o r d e s c r i b e d i n the s e c t i o n . However, most experiments have been performed w i t h many d i f f e r e n t b u l b s and s i m i l a r r e s u l t s have been o b t a i n e d . In what f o l l o w s the l a b o r a t o r y e x p e r i m e n t a l s e t up i s d e s c r i b e d i n d e t a i l . Then the p r o c e d u r e and r e s u l t s f o r each s e t o f experiments are p r e s e n t e d . F i n a l l y , r e s u l t s from f i e l d t e s t s are g i v e n . 66 5.2 E x p e r i m e n t a l Apparatus 5.2.1 I n t r o d u c t i o n The e x p e r i m e n t a l apparatus c o n s i s t s of two main p a r t s : a d e v i c e f o r g e n e r a t i n g e l e c t r i c f i e l d s and equipment f o r m o n i t o r i n g and s t u d y i n g the l i g h t p u l s e s . There are two d e v i c e s f o r p r o d u c i n g e l e c t r i c f i e l d s . One gen e r a t e s u n i f o r m f i e l d i n a f i x e d d i r e c t i o n the o t h e r a p l a n a r r o t a t i n g f i e l d of v a r i a b l e e l l i p t i c i t y . The d e v i c e t h a t g e n e r a t e s u n i f o r m f i e l d s and the p u l s e m o n i t o r i n g equipment w i l l be d e s c r i b e d f i r s t , f o l l o w e d by a d e s c r i p t i o n of the d e v i c e f o r g e n e r a t i n g p l a n a r r o t a t i n g f i e l d s . 5.2.2 Apparatus f o r G e n e r a t i n g U n i f o r m F i e l d s i n a F i x e d D i r e c t i o n and S t u d y i n g P u l s e E m i s s i o n . The e l e c t r i c f i e l d i s gen e r a t e d between the p l a t e s o f a p a r a l l e l p l a t e c a p a c i t o r , as shown i n F i g u r e 12. The p l a t e s c o n s i s t o f 600mm x 600mm x 3mm square s h e e t s of p o l i s h e d aluminum w i t h b e v e l l e d edges and rounded c o r n e r s (radiusf\f 150mm) which s e r v e t o reduce corona a t h i g h f i e l d s t r e n g t h s . They a r e mounted h o r i z o n t a l l y on l u c i t e sheets ( t h i c k n e s s 10mm) which are quipped w i t h h i n g e d l u c i t e doors f o r s a f e t y p u r p o s e s . The upper p l a t e i s sup p o r t e d by t h r e e 25mm diameter t h r e a d e d l u c i t e rods which can be screwed i n t o the upper l u c i t e s heet and p e r m i t t h i s p l a t e t o be a l i g n e d c o r r e c t l y w i t h r e s p e c t t o the lower one. The p l a t e s a r e p a r a l l e l t o each o t h e r (+ 1mm) and the v a r i a b l e s p a c i n g i s n o r m a l l y s e t a t a d i s t a n c e o f 150mm. The senso r i s mounted on a l u c i t e 67 o BULB OPTICAL FIBRE 'A PHOTO MULTIPLIER ~ ~ T " LUCITE ^ ALUMINUM 5 0 : 1 TRANSFORMER AUTO TRANSFORMER Figure 1 2 Experimental Set Up 68 b r a c k e t a t the c e n t e r of the f i e l d g e n e r a t e d by the p l a t e s , so t h a t i t does not i n f l u e n c e the charge d i s t r i b u t i o n on the p l a t e s 2 1 . T h i s l o c a t i o n of the senso r a l s o ensures t h a t the a p p l i e d f i e l d i s s p a t i a l l y u n i f o r m . The c a p a c i t o r i s n o r m a l l y powered by a 50:1 t r a n s f o r m e r f e d by an a u t o t r a n s f o r m e r ('Variac') such t h a t a maximum p o t e n t i a l d i f f e r e n c e o f 6kV (rms) o c c u r s between the p l a t e s . T h i s c o r r e s p o n d s t o a f i e l d s t r e n g t h o f 40kV m - 1 (rms) a t a f r e q u e n c y o f 60Hz. In some experiments a programmable power s u p p l y i s used i n s t e a d (Kepco, Model OPS 5000, g a i n 1000, maximum o u t p u t 5kV). T h i s f a c i l i t a t e s s t u d i e s o f the e f f e c t of f r e q u e n c y and wave form shape on the sensor r e s p o n s e . The f r e q u e n c y range employed extends from a few Hz up t o 600 Hz. The v o l t a g e a p p l i e d t o the p l a t e s i s measured d i r e c t l y w i t h a h i g h v o l t a g e p o t e n t i a l d i v i d e r . The f l a s h e s o f l i g h t e m i t t e d by the b u l b s are conveyed t o an RCA 931A p h o t o m u l t i p l i e r through the v i n y l c l a d g l a s s f i b r e bundle (1.5m l o n g , 2mm i n diameter) o b t a i n e d from W e l c h - A l l y n Co. The p h o t o m u l t i p l i e r i s o p e r a t e d w i t h i t s cathode a t -1kV w i t h r e s p e c t t o ground. The s i g n a l s a r e r e c o r d e d p h o t o g r a p h i c a l l y u s i n g a s t o r a g e o s c i l l o s c o p e ( T e k t r o n i x Inc . , Model 549 w i t h a Model CA p l u g - i n a m p l i f i e r ) . The o u t p u t from the o s c i l l o s c o p e a m p l i f i e r (which c o n t a i n s a r e p l i c a o f the s i g n a l d i s p l a y e d on the o s c i l l o s c o p e ) i s f e d to the coun t e r (Advance In s t r u m e n t s , Model TC9A, 32 mHz). The c o u n t i n g i n t e r v a l i s u s u a l l y i n the range o f one to ten s e c o n d s . A l l experiments ( e x c e p t f o r those i n v o l v i n g the stu d y o f e n v i r o n m e n t a l e f f e c t s ) are c a r r i e d o ut a t 293 + 4 K e l v i n and l e s s than 70% r e l a t i v e h u m i d i t y . 69 5.2.3 Apparatus f o r G e n e r a t i n g P l a n a r R o t a t i n g F i e l d s ' ^ The 60 Hz p l a n a r r o t a t i n g f i e l d s are produced by f o u r v e r t i c a l m e t a l p l a t e s . The p l a t e s c o n s i s t o f 260mm x 760mm x 3mm sheet s o f p o l i s h e d aluminum w i t h b e v e l l e d edges and rounded c o r n e r s . They are mounted v e r t i c a l l y on l u c i t e s h e e t s and form a square a r r a y when viewed from the top (see F i g u r e 13). The sensor i s mounted i n the c e n t e r o f the d e v i c e on a v e r t i c a l h o l l o w l u c i t e r o d . The o p t i c a l f i b r e i s mounted i n s i d e the l u c i t e r o d and emerges from the apparatus a t the bottom (see F i g u r e 13). The f o u r p l a t e c a p a c i t o r i s powered by two p a r a l l e l c i r c u i t s each powered by a 50:1 t r a n s f o r m e r f e d by a " V a r i a c " . The two t r a n s f o r m e r s a r e d r i v e n by s i g n a l s 180° out of phase. Each c i r c u i t c o n s i s t s of a b r i d g e network which g e n e r a t e s two v o l t a g e s (V-j , V 2 and V-j, V 4 ) 90° out of phase w i t h each o t h e r (see F i g u r e 14). By c o n n e c t i n g each o f these v o l t a g e s (V-j , V 3 and V 4 ) t o a d j a c e n t p l a t e s o f the c a p a c i t o r an e l l i p t i c a l l y p o l a r i z e d f i e l d can be g e n e r a t e d . The e l l i p t i c i t y o f the f i e l d can be v a r i e d by changing the r e s i s t a n c e v a l u e s i n the b r i d g e network (see R21» F i g u r e 1 4 ) . The magnitude of the f i e l d can be v a r i e d by changing V 0 . The apparatus i s capa b l e o f g e n e r a t i n g f i e l d s o f up to 30 kV/m. The f l a s h e s o f l i g h t are conveyed t o the same apparatus d e s c r i b e d p r e v i o u s l y . F i g u r e 15 shows a p i c t u r e o f the a p p a r a t u s . 70 71 F i g u r e 14 E l e c t r i c C i r c u i t f o r G e n e r a t i n g R o t a t i n g F i e l d s F i g u r e 15 Photo showing p l a t e s f o r p r o d u c t i o n o f 60Hz r o t a t i n g e l e c t r i c f i e l d s ( r i g h t ) , 5 c h a n n e l p h o t o m u l t i p l i e r ^ v a r i a c s f o r d r i v i n g the t r a n s f o r m e r s ( l e f t ) , and e l e c t r o n i c s (bottom) 73 5.3 Study of the B a s i c Phenomenom The experiments performed t o i n v e s t i g a t e the b a s i c phenomena a r e d e s c r i b e d i n two sub s e c t i o n s . The f i r s t d e a l s w i t h a l l major f e a t u r e s of the s t a n d a r d phenomena, the second d e a l s w i t h attempts t o reduce the t h r e s h o l d by u s i n g d i f f e r e n t gas c o m p o s i t i o n s . 5.3.1 S t a n d a r d Phenomena 5.3.1.1 G e n e r a l P u l s e E m i s s i o n T h i s experiment i s performed w i t h a pyrex b u l b of 25mm diameter f i l l e d w i t h neon t o a p r e s s u r e o f 10 T o r r . The a p p l i e d f i e l d i s a 60Hz waveform whose magnitude i s v a r i e d . The r e l a t i o n between p u l s e e m i s s i o n and the phase o f the a p p l i e d f i e l d i s o b s e r v e d by add i n g the measured v o l t a g e a c r o s s the p l a t e s and the p h o t o - m u l t i p l i e r o u t p u t . The o s c i l l o s c o p e d i s p l a y o f t h i s s i g n a l shows the l i g h t p u l s e s superposed on the a p p l i e d f i e l d . A t y p i c a l o s c i l l o s c o p e t r a c e t r i g g e r e d on the a p p l i e d f i e l d has been t r a c e d i n F i g u r e 16. The p u l s e s of l i g h t appear on the r i s i n g and f a l l i n g s l o p e s of the s i n e wave. From many such p i c t u r e s i t i s observed t h a t the l i g h t p u l s e s on a g i v e n s l o p e o f the a p p l i e d f i e l d are spaced so t h a t the change i n the magnitude of the a p p l i e d f i e l d i s approximately, c o n s t a n t . T h i s change E D i n the a p p l i e d f i e l d r e q u i r e d t o produce a new p u l s e i s a p p r o x i m a t e l y the same on r i s i n g and f a l l i n g s l o p e s . However the change i n f i e l d between the l a s t p u l s e i n a r i s i n g / f a l l i n g s l o p e and the f i r s t p u l s e of a f a l l i n g / r i s i n g s l o p e i s u s u a l l y s l i g h t l y l e s s than E 0 . T h i s 74 F i g u r e 16 O p t i c a l p u l s e s superposed on a p p l i e d f i e l d wave form ( E A ) f o r 25mm di a m e t e r n e o n - f i l l e d b u l b a t 10 T o r r ( s t r o n g breakdown c a s e ) . E A = f i e l d a m p l i t u d e . 75 d i f f e r e n c e i s c o n s i s t a n t w i t h the e f f e c t o f f i n i t e c o n d u c t i v i t y which causes the f i e l d i n s i d e the b u l b ( E T t o be phase s h i f t e d w i t h r e s p e c t t o the a p p l i e d f i e l d ( E A ) (see S e c t i o n 3.3.5). In f a c t , i f the graph o f E A v e r s u s time i s d i s p l a c e d s l i g h t l y t o the l e f t (becoming E T v e r s u s time) p u l s e s a and c ( i n F i g u r e 16) w i l l o c c u r a t i d e n t i a l f i e l d s t r e n g t h . When the peak t o peak v a l u e , 2 E A , of the a p p l i e d f i e l d i s l e s s than the t h r e s h o l d f i e l d , E D , no l i g h t p u l s e s are seen. As E A i s i n c r e a s e d above E Q/2 the b u l b s t a r t s t o emit p u l s e s . For E A c l o s e t o E 0/2 i t can take a v e r y long time f o r the f i r s t p u l s e t o appear. For a f i e l d , E A , h i g h e r than E Q the b u l b " s t a r t s up" almost i m m e d i a t e l y . As the a p p l i e d f i e l d i s d e c r e a s e d p u l s e s are observed u n t i l E A d e c r e a s e s below E Q / 2 . Thus t h e r e i s a s t a r t up h y s t e r e s i s . These r e s u l t s are c o n s i s t e n t w i t h the s t r o n g breakdown model. 5.3.1.2 Rate o f P u l s e E m i s s i o n as a F u n c t i o n o f A p p l i e d F i e l d Magnitude T h i s experiment i s performed w i t h the same equipment as the p r e v i o u s one b u t the b u l b p r e s s u r e i s a l s o v a r i e d . For f i e l d s above t h r e s h o l d and p r e s s u r e s h i g h e r than 10 T o r r the s i t u a t i o n i s as d e p i c t e d i n F i g u r e 16. The number of p u l s e s e m i t t e d per c y c l e o f the a p p l i e d f i e l d depends on the i n t e g e r number of times t h a t E Q f i t s i n t o 2 E A . An i l l u s t r a t i v e p l o t o f t h i s e f f e c t i s o b t a i n e d by measuring f B , the number of p u l s e s per second (averaged over one second) v e r s u s E A . Such a p l o t c o n s i s t s of s t e p s o f s i z e E D/2 i n a p p l i e d f i e l d and of s i z e 120 Hz i n f B ( i e . twice f A , the f r e q u e n c y o f E A ) . F i g u r e 17 shows such a p l o t . The same e f f e c t i s observed f o r h i g h e r b u l b p r e s s u r e s , e x c e p t t h a t the s t e p s become s h a r p e r . However f o r the lower 76 500 400 300 200 100 F i g u r e 17 10 20 30 40 50 E A* ( kv m " 1 , Rns ) P u l s e e m i s s i o n f r e q u e n c y ( f B ) a s a f u n c t i o n o f a p p l i e d f i e l d ( E A ) f o r d i f f e r e n t b u l b p r e s s u r e s . B u l b d i a m e t e r was 25mm. 77 p r e s s u r e s the s t e p s are g r a d u a l l y smeared out u n t i l a t 1 T o r r the r e l a t i o n s h i p between p u l s e f r e q u e n c y and a p p l i e d f i e l d i s l i n e a r (see F i g u r e 1 8 ) . Another e f f e c t of low p r e s s u r e s i s t h a t the t h r e s h o l d i s not s h a r p l y d e f i n e d a t E Q/2 and p u l s e s are e m i t t e d f o r E A < E Q / 2 . However r e s u l t s a r e not r e l i a b l e i n t h i s regime. For b u l b s f i l l e d t o p r e s s u r e s o f 10 T o r r or g r e a t e r , the change i n a p p l i e d f i e l d (_\E A) between p u l s e s i n e f f e c t i v e l y c o n s t a n t . However a t p r e s s u r e s l e s s t h a t 10 T o r r _ E A b e g i n s t o f l u c t u a t e . I t commonly f l u c t u a t e s by a f a c t o r of two a t p r e s s u r e s l e s s than 1 T o r r f o r a c o n s t a n t amplitude ( E A ) o f the a p p l i e d f i e l d . F i g u r e 4 shows a t y p i c a l waveform f o r a low p r e s s u r e b u l b , o b t a i n e d i n the same way as the one shown i n F i g u r e 16. In comparison, a t p r e s s u r e s e x c e e d i n g 10 T o r r E A f l u c t u a t e s by l e s s than 15%. C l e a r l y , i t i s the v a r i a t i o n s i n A E a t h a t smear out the s t e p s i n F i g u r e 17. I t i s a l s o c l e a r from F i g u r e 4 t h a t _ _ E A depend on the p u l s e h e i g h t . The p u l s e h e i g h t i s a measure of the amount of l i g h t e m i t t e d or the s t r e n g t h o f the breakdown a v a l a n c h e . P u l s e h e i g h t v a r i e s a t a l l p r e s s u r e s . The v a r i a t i o n a t a g i v e n p r e s s u r e depends on the d i f f e r e n t b u l b s . A t h i g h p r e s s u r e s the v a r i a t i o n i s much l e s s than a t low p r e s s u r e s . A t lower p r e s s u r e s , the f i e l d i ncrement between s u c c e s s i v e p u l s e s i n c r e a s e s w i t h the h e i g h t o f the f i r s t p u l s e as can be seen i n F i g u r e 4. These r e s u l t s i n d i c a t e t h a t weak breakdown o c c u r s a t the lower p r e s s u r e s . A t these p r e s s u r e s i t i s ha r d e r t o r e s e t E j c o m p l e t e l y t o 0. I t i s i n t h i s regime t h a t one needs to o p e r a t e GEM i f a l i n e a r r e l a t i o n between E f i e l d and count r a t e i s d e s i r e d . 78 F i n a l l y , i t must be observed t h a t p u l s e shape i s remarkably c o n s t a n t ( f o r an example see i n s e r t i n F i g u r e 16) f o r the p r e s s u r e s t e s t e d from 1 t o 30 T o r r . Furthermore the t o t a l d u r a t i o n o f the p u l s e i s independant of i t s h e i g h t . The ar e a under the p u l s e p r o f i l e ( i e . the amount o f l i g h t e m i t t e d ) i s t h e r e f o r e p r o p o r t i o n a l t o i t s h e i g h t as has been assumed above. 5.3.1.3 D e t e r m i n a t i o n o f E Q T h i s experiment i s performed w i t h a s e r i e s o f bu l b s whose diameter ranges from 12.5mm t o 38.1mm and whose p r e s s u r e ranges from 0.66 T o r r t o 30 T o r r . The a p p l i e d v o l t a g e i s a 60Hz waveform whose magnitude i s v a r i e d . We have seen t h a t E Q i s a w e l l d e f i n e d q u a n t i t y f o r h i g h p r e s s u r e s . I t i s the change i n the a p p l i e d f i e l d n e c e s s a r y t o o b t a i n a new p u l s e (averaged over f a l l i n g and r i s i n g s l o p e s ) . A t low p r e s s u r e s the change i n a p p l i e d f i e l d r e q u i r e d t o o b t a i n a new p u l s e f l u c t u a t e s w i d e l y . However d e f i n i n g E Q as the change i n the a p p l i e d f i e l d r e q u i r e d t o change the p u l s e f r e q u e n c y by 240Hz a l l o w s comparison of low p r e s s u r e and h i g h p r e s s u r e r e s u l t s . A l s o E A = E Q/2 i s d e f i n e d as the t h r e s h o l d f o r b u l b o p e r a t i o n (assuming the b u l b i s s t a r t e d and the v o l t a g e lowered u n t i l 120 p u l s e s / s e c a r e o b s e r v e d ) . A t h i g h p r e s s u r e s t h i s i s a t r u e t h r e s h o l d , b u t a t low p r e s s u r e s , p u l s e s are observed w i t h E A < E Q / 2 . However, r e l i a b l e r e s u l t s a r e o n l y o b t a i n e d f o r count r a t e s g r e a t e r than 120Hz. The p u l s e r a t e was p l o t t e d a g a i n s t the a p p l i e d f i e l d magnitude f o r a s e r i e s o f 25mm diameter b u l b s f i l l e d w i t h neon a t p r e s s u r e s o f 30, 10, 5 79 and 1 T o r r p r e s s u r e . R e s u l t s i n d i c a t e d t h a t the 10, 5 and 1 T o r r b u l b s had an E Q of around 42+3 kV/m. T h i s i s w i t h i n the r e p r o d u c i b i l i t y o f the b u l b c h a r a c t e r i s t i c s . However the 30 T o r r b u l b had an E Q o f 59kV/m. T h i s r e s u l t i n d i c a t e d t h a t the o p t i m a l t h r e s h o l d i s r e l a t i v e l y i ndependent of p r e s s u r e f o r p r e s s u r e s below 10 T o r r . The r e s u l t s are c o n s i s t e n t w i t h the Paschen's law w i t h a broad minimum i n , 0, (see E q u a t i o n 1 ). The v a l i d i t y of t h i s law was a l s o checked f o r t h r e e b u l b s h a v i n g a pd of 25mm T o r r ( d i a m e t e r s 12.5mm, 25mm and 38mm). The r e s u l t s i n d i c a t e d the t h r e s h o l d f i e l d v a r i e s i n v e r s e l y w i t h b u l b diameter f o r f i x e d pd. E Q d was found t o be about 1000 v o l t s (see F i g u r e 1 8 ) . By s e t t i n g pd near the Paschen minimum, the t h r e s h o l d , E Q / 2 , can be reduced by i n c r e a s i n g the b u l b d i a m e t e r s i n c e E Q = 1000/d. F o r t u n a t e l y the Paschen minimum i s s u f f i c i e n t l y broad t o encompass a p r e s s u r e o f 1 T o r r w i t h b u l b s of c o n v e n i e n t s i z e , such t h a t p u l s e f r e q u e n c y i s p r o p o r t i o n a l to the a p p l i e d f i e l d . The s e t o f experiments d e s c r i b e d so f a r e s t a b l i s h the b a s i c p h y s i c a l model f o r u n i f o r m f i e l d s of f i x e d d i r e c t i o n and f o r b u l b s whose w a l l s do not i n t e r a c t w i t h the d i s c h a r g e . However, another s e t o f experiments i n which the wave p r o f i l e and f r e q u e n c y o f the a p p l i e d f i e l d i s v a r i e d was performed t o f u r t h e r check the b a s i c assumption of c o n s t a n t E Q and the e f f e c t o f f i n i t e c o n d u c t i v i t y . 5.3.1.4 P u l s e E m i s s i o n : Dependence on the A p p l i e d Waveform T h i s i n v e s t i g a t i o n i s performed w i t h a pyrex b u l b o f 25mm dia m e t e r f i l l e d w i t h neon t o a pressure, o f 10 T o r r . The a p p l i e d f i e l d i s a programmable waveform whose f r e q u e n c y can be v a r i e d from 1 Hz t o 600Hz. F i g u r e 18 P u l s e e m i s s i o n f r e q u e n c y a s a f u n c t i o n o f a p p l i e d f i e l d f o r d i f f e r e n t b u l b d i a m e t e r s . B u l b p d was 25mm T o r r 81 D i f f e r e n t Frequencies Varying the frequency of the applied f i e l d does not change the phenomenon. The number of pulses emitted per cycle depends only on the f i e l d magnitude (1Hz to 600Hz) not on the rate of change of the f i e l d . Pictures of the pulses superposed on the applied f i e l d f o r 6Hz, 60Hz and 600Hz frequencies are very s i m i l a r provided the appropriate time scales (50 ms/d, 5ms/d,0.5ms/d) are chosen. However at 600Hz i t i s common to see r e l i a b l e operations for 2E A = E Q with one pulse on the maximum and one on the minimum of the sine wave. At this frequency the screening e f f e c t s due to the bulb conductivity are n e g l i g i b l e . Operation above 600Hz has not been investigated due to the lack of equipment for generating the f i e l d s . However, i t i s clear that the e f f e c t w i l l be s i m i l a r u n t i l the frequency approaches the inverse of the pulse width (kHz region). For frequencies below 1 Hz the bulb operation i s u n r e l i a b l e : pulse emission i s sporadic even f or f i e l d s high above threshold. For time independent f i e l d s , pulses are observed when the f i e l d i s turned o f f or on. Maintaining a very high s t a t i c f i e l d w i l l , at times ( e s p e c i a l l y for bulbs at low pressure), cause pulses to be emitted • separated by a few seconds. However th i s i s only observed for very low gas pressures (0.65 To r r ) . D i f f e r e n t Waveforms The above experiments can be repeated with various waveform shapes. Of s p e c i a l i n t e r e s t i s a square wave since i t contains large time 82 i n t e r v a l s of c o n s t a n t f i e l d , where p u l s e s s h o u l d not appear. F i g u r e 19 shows such a wave, p u l s e s are o b s e r v e d o n l y when the f i e l d changes. T r i a n g u l a r waves f o r which each s l o p e i s c o n s t a n t are u s e f u l i n c o n f i r m i n g the p u l s e e m i s s i o n a t c o n s t a n t E Q by c h e c k i n g f o r e m i s s i o n a t c o n s t a n t time i n t e r v a l s d u r i n g each r i s i n g / f a l l i n g s l o p e of the f i e l d . These experiments c o n f i r m the v a l i d i t y o f e q u a t i o n 2 f o r a broad range of f r e q u e n c y (1 *C f A < 600Hz). Furthermore, they c o n f i r m t h a t waveform shape i s not i m p o r t a n t t o the phenomena, o n l y peak to peak f i e l d magnitude m a t t e r s . 5.3.2 P o s s i b l e Mechanisms to Reduce the T h r e s h o l d The most p r o m i s i n g mechanism to reduce the t h r e s h o l d i s the Penning e f f e c t . T h i s experiment i s performed w i t h a 25mm pyrex b u l b f i l l e d w i t h a Penning mixture of Ne and 0.3% Ar to a p r e s s u r e o f 18 T o r r . The a p p l i e d f i e l d i s a 60Hz wave form whose am p l i t u d e i s v a r i e d . A t y p i c a l o s c i l l o s c o p e t r a c e f o r t h i s Penning b u l b i s shown i n F i g u r e 20. As can be seen the e f f e c t i s d i f f e r e n t . I n s t e a d of the u n i f o r m h e i g h t p u l s e s e q u a l l y spaced, t h e r e i s one l a r g e p u l s e f o l l o w e d by a s e r i e s o f s m a l l p u l s e s g e t t i n g p r o g r e s s i v e l y c l o s e r . The t h r e s h o l d f o r the f i r s t p u l s e i s v e r y c l o s e ( i e . w i t h i n 1kV/m) t o the t h r e s h o l d f o r non Penning m i x t u r e s . However as can be deduced from F i g u r e 21 the t h r e s h o l d f o r subsequent p u l s e s i s much lower (2 t o 10 t i m e s ) . Thus a graph of the f r e q u e n c y ( f g ) of o p t i c a l p u l s e s v e r s u s a p p l i e d f i e l d i s s i m i l a r t o normal b u l b s but has a much l a r g e r s l o p e as shown by the d o t t e d l i n e i n F i g u r e 18. F i g u r e 19 O p t i c a l p u l s e superposed on a p p l i e d waveform ( E A ) CO - p u l s e s occur o n l y where E A v a r i e s i n time. 84 10 ms/div 5 ms/div 2 ms/div F i g u r e 20 O p t i c a l p u l s e s superposed on the a p p l i e d wave form E A f o r a Penning mixture b u l b (time runs l e f t ) 85 S i m i l a r r e s u l t s are o b t a i n e d w i t h o t h e r Penning m i x t u r e s . C l e a r l y the e x p e c t e d Penning mechanism works o n l y f o r the p u l s e s f o l l o w i n g the f i r s t one and i s t h e r e f o r e not u s e f u l i n r e d u c i n g the t h r e s h o l d of the s e n s o r . Another a l t e r n a t i v e t o reduce the t h r e s h o l d i s the a d d i t i o n o f t r i t i u m t o a gas. S i n c e i m p u r i t i e s a f f e c t breakdown i r r e s p e c t i v e o f whether they are r a d i o a c t i v e or not experiments were c a r r i e d out to determine what e f f e c t s H 2 would have on the breakdown c h a r a c t e r i s t i c s . I t was found t h a t even v e r y s m a l l p e r c e n t a g e s of H 2 produced b u l b w i t h c l e a r l y d e f i n e d s t e p s which are u n d e s i r a b l e . Thus the b e s t gases f o r b u l b d e s i g n are the i n e r t gases (Ne and A r ) . 86 5.4 Sensor Shape I n v e s t i g a t i o n s The main t h r u s t of these experiments i s t o u n d e r s t a n d the o p e r a t i o n of a c y l i n d r i c a l tube p l a c e d a t an a r b i t r a r y angle to a u n i f o r m e l e c t r i c f i e l d of f i x e d d i r e c t i o n . However, a v e r y simple experiment to show the i s o t r o p i c response of a s p h e r i c a l b u l b i s d e s c r i b e d f i r s t . 5.4.1 S p h e r i c a l B u l b T h i s experiment i s performed w i t h a t y p i c a l s p h e r i c a l b u l b (38mm d i a m e t e r 0.66 T o r r neon). The a p p l i e d f i e l d i s a 60Hz waveform whose magnitude i s v a r i e d . The b u l b i s f i r s t p l a c e d w i t h i t s n i p p l e (see F i g u r e 8) h o r i z o n t a l ( i e . p a r a l l e l t o an e q u i p o t e n t i a l s u r f a c e ) and the t h r e s h o l d f i e l d i s measured f o r d i f f e r e n t r o t a t i o n s of the b u l b about a v e r t i c a l l i n e ( p e r p e n d i c u l a r to the c a p a c i t o r p l a t e s ) . The v a r i a t i o n i n t h r e s h o l d o b s e r v e d i s l e s s t h a t 1%. Thus the response i s i s o t r o p i c . Next the b u l b i s r o t a t e d about a h o r i z o n t a l l i n e w i t h the d i ameter through the n i p p l e p e r p e n d i c u l a r t o the l i n e . The t h r e s h o l d i n c r e a s e s as the n i p p l e d e v i a t e s from p o i n t i n g h o r i z o n t a l l y t o p o i n t i n g v e r t i c a l l y . The change ob s e r v e d f o r t h i s b u l b was 19%. T h i s r e s u l t i s not i n c o n s i s t a n t w i t h the e x t r a l e n g t h p r o v i d e d by the n i p p l e . C l e a r l y the i d e a l s p h e r i c a l b u l b i s i s o t r o p i c , however the i s o t r o p y of a r e a l b u l b depends on how w e l l i t i s s e a l e d o f f a f t e r f i l l i n g . 87 5.4.2 C y l i n d r i c a l Tubes T h i s experiment i s performed w i t h two c y l i n d r i c a l gas f i l l e d t u b e s . The f a t tube i s 12.5mm (diameter, D) by 55mm ( l e n g t h , L) and i s f i l l e d t o , 0.5 T o r r Ar ( t h i s i s w i t h i n the Paschen minimum f o r both d i m e n s i o n s ) . The t h i n tube i s 6mm (D) by 55mm (L) and i s a l s o f i l l e d t o 0.5 T o r r o f A r . The tube under i n v e s t i g a t i o n i s a t t a c h e d a t i t s mid p o i n t t o a h o r i z o n t a l i n s u l a t i n g rod made o f l u c i t e . The r o d , which i s 75mm above the lower p l a t e (see F i g u r e 12), r o t a t e s the tube i n a v e r t i c a l p l a n e . The a n g l e , 0, between the tube a x i s and the v e r t i c a l ( i e . the d i r e c t i o n o f E A ) can be s e t t o an a c c u r a c y o f ±1° by means o f the p r o t r a c t o r f i t t e d to the r o d . To measure f B , l i g h t has t o be c o l l e c t e d from both s i d e s o f the tube ( F i g u r e 5 AD and BC). The f i b r e a x i s t h e r e f o r e i s n o r m a l l y s e t a l o n g the tube r a d i u s i n the pl a n e of F i g u r e 5. The l i g h t e m i s s i o n was a l s o observed by s e t t i n g the end of the f i b r e a t v a r i o u s p o i n t s a l o n g AD or BC, w i t h the f i b r e - a x i s p e r p e n d i c u l a r t o the plane o f F i g u r e 5. The l i g h t e m i s s i o n was a l s o o b served by s e t t i n g the end of the f i b r e a t v a r i o u s p o i n t s a l o n g AD o r BC, w i t h the f i b r e - a x i s p e r p e n d i c u l a r t o the pl a n e of F i g u r e 5. These s t u d i e s showed t h a t the i n t e r n a l f i e l d (E-j-) i s d i r e c t e d away from the s i d e of the tube from which the l i g h t i s e m i t t e d . T i m e - i n t e g r a t e d photographs a l s o show t h a t the breakdown d i s c h a r g e takes t h e form o f the shaded r e g i o n i n F i g u r e 5 when E j i s a l o n g the +z_ d i r e c t i o n . When E j i s a l o n g the -z d i r e c t i o n the breakdown d i s c h a r g e goes from C t o D t o A and i s t h i c k e s t a t A. For the sake o f c l a r i t y t h i s p o r t i o n of the d i s c h a r g e i s o m i t t e d from F i g u r e 5 b u t can be seen i n the 88 photograph ( F i g u r e 2 1 ) . The o b s e r v a t i o n s are c o n s i s t e n t w i t h the i d e a t h a t the breakdown d i s c h a r g e i s m a i n t a i n e d by e l e c t r o n s a c c e l e r a t e d by the i n t e r n a l f i e l d E j and i s guided by the tube w a l l s . The tubes o p e r a t e r e l i a b l y i f f B > 1 2 0 H z . T h r e s h o l d c o n d i t i o n s are t h e r e f o r e d e f i n e d t o be those a t which f B = 120Hz. The t h r e s h o l d f i e l d f o r breakdown was measured f o r the wide tube (D = 12.5mm) w i t h the tube a x i s a l o n g the a p p l i e d f i e l d , or p e r p e n d i c u l a r t o i t . I t was found t h a t E o p / E O N = 12.4kVm- 1/50kVm- 1 - D/L where E Q p and E Q N a r e the r e s p e c t i v e t h r e s h o l d f i e l d s f o r the tube a x i s p a r a l l e l or normal t o the a p p l i e d f i e l d . I t t h e r e f o r e f o l l o w s from the above e q u a t i o n t h a t V(0) = V(90) where V(0) = E 0 p L i s the p o t e n t i a l d i f f e r e n c e a t t h r e s h o l d between the tube ends w i t h 9 = 0 ° , and V(90) = E Q N D i s the p o t e n t i a l d i f f e r e n c e a t t h r e s h o l d a c r o s s the v e r t i c a l d i a meter w i t h 9 = 9 0 ° . T h i s e q u a t i o n i s c o n s i s t e n t w i t h the n o t i o n t h a t the breakdown c o n d i t i o n s s a t i s f y the Paschen r u l e ( E q u a t i o n 1 ) w i t h 0 near i t s broad minimum. The r e s u l t s a l s o show t h a t the f i e l d s a r e not s i g n i f i c a n t l y a t t e n u a t e d by the f l o w o f c o n d u c t i o n c u r r e n t s i n the g l a s s ( i e . has the same s t r e n g t h both o u t s i d e and i n s i d e the t u b e ) . F i g u r e 21 Time i n t e g r a t e d photographs showing the breakdown d i s c h a r g e i n c y l i n d r i c a l t u b e s . The e l e c t r i c f i e l d i s v e r t i c a l . 90 To minimize the e f f e c t of the " f r e e t r a v e l " p a r t o f the d i s c h a r g e , (AB and CD i n F i g u r e 5) f u r t h e r s t u d i e s were done w i t h the narrow tube (D = 6mm, L = 55mm). With the tube v e r t i c a l (9 = 0 ) , the f r e q u e n c y of breakdown (fg) was measured as a f u n c t i o n o f the p o t e n t i a l d i f f e r e n c e V A = E A L a l ° n g the tube. f B s a t i s f i e s the c a l i b r a t i o n e q u a t i o n (see F i g u r e 22, c u r v e a) f B = 380(V A - 0.54) f o r V f t ^ 0.85kV = 0 V A < 0.85kV where V A i s i n kV and f B i n Hz. To maximize the f r e q u e n c y of breakdown f B ( 9 ) , i n a tube i n c l i n e d a t an a n g l e , 9, t o the a p p l i e d f i e l d , the amplitude of the f i e l d i n the gap was m a i n t a i n e d a t i t s l a r g e s t v a l u e of E A m = ^ 3 k V m 1« f B^®^ was then measured f o r 0 < 9-? 7 5 ° . T h i s c u r v e i s shown i n F i g u r e 22 (curve b ) . I f the breakdown d i s c h a r g e does not l o s e s i g n i f i c a n t amounts of energy i n i t s i n t e r a c t i o n w i t h the w a l l , then the f r e q u e n c y of breakdown s h o u l d depend o n l y on the p o t e n t i a l d i f f e r e n c e between the two ends of the tube, i r r e s p e c t i v e o f the angle 9. To check t h i s h y p o t h e s i s we d e f i n e E A as the f i e l d a m p litude which pro d u c e s a p a r t i c u l a r v a l u e of the breakdown f r e q u e n c y , f B , w i t h the tube mounted v e r t i c a l l y . The same breakdown f r e q u e n c y w i l l t h e r e f o r e o c c u r when 9 = 6 ' p r o v i d e d t h a t the p o t e n t i a l d i f f e r e n c e a c r o s s t h e tube remains unchanged, i e . t h a t E A L = E A m L cos 9' (6) Thus a p l o t of E A / E A m a g a i n s t cos9 s h o u l d be a s t r a i g h t l i n e o f u n i t 91 cosG 0 0.1 0.3 0.5 OS 1.0 VA (kv, Rms) F i g u r e 22 Frequency of breakdown as a f u n c t i o n o f p o t e n t i a l d i f f e r e n c e a l o n g the tube (V A) (c u r v e a) and as a f u n c t i o n of o r i e n t a t i o n a n g l e (cos 9) f o r f i x e d E A = 53kVm~'(curve b ) . Bulb i s 55mm x 6mm and f i l l e d w i t h 0.5 T o r r A r . 92 s l o p e , p a s s i n g through the o r i g i n . The above d i s c u s s i o n r e q u i r e s t h a t the p o t e n t i a l d i f f e r e n c e a c r o s s the tube exceeds the t h r e s h o l d v a l u e (0.85kV i n our c a s e ) , so t h a t E q u a t i o n 6 a p p l i e s p r o v i d e d cosQ 0.29. I t s h o u l d be noted t h a t E q u a t i o n 6 i s independent of the s p e c i f i c form of the c a l i b r a t i o n curve ( F i g u r e 21) p r o v i d e d t h a t f B i s a s i n g l e - v a l u e d f u n c t i o n of V A . The p l o t of E j ^ / E ^ a g a i n s t cos 9' can be o b t a i n e d g r a p h i c a l l y from F i g u r e 22 as f o l l o w s : s e l e c t any count r a t e (say 400), cos9' i s o b t a i n e d from curve b (0.58) and E A L i s o b t a i n e d from curve a (1.59kV). e A / E A I T I •*-S o b t a i n e d by d i v i d i n g E A L by 2.92kV ( i e . 53kV/m x 0.055m). F i g u r e 23 shows a p l o t of E A / E A m v e r s u s c o s 9 . The f a c t t h a t i t i s an e x c e l l e n t s t r a i g h t l i n e i n d i c a t e s t h a t E q u a t i o n 6 i s v a l i d . The above r e s u l t s show t h a t the f r e q u e n c y of breakdown i n e l e c t r o d e l e s s tubes, immersed i n low f r e q u e n c y f i e l d s , i s governed by the p o t e n t i a l d i f f e r e n c e between the ends of the tube, p r o v i d e d the d i s c h a r g e i s guided by the tube w a l l s . They a l s o show t h a t energy l o s s e s i n the w a l l - d i s c h a r g e i n t e r a c t i o n are not s i g n i f i c a n t . I f they were, then V(9) would have to be l a r g e r than the v a l u e p r e d i c t e d by the model p r e s e n t e d above. F i n a l l y the r e s u l t s demonstrate t h a t a narrow, c y l i n d r i c a l , g a s - f i l l e d , i n s u l a t i n g tube can be used t o measure the component of a low f r e q u e n c y f i e l d a l o n g the t u b e - a x i s . 93 0.1 0.2 0.3 0.4 0J5 0.6 0.7 0.8 0.9 1.0 cos 6 F i g u r e 23 N o r m a l i z e d r e s p o n s e o f c y l i n d r i c a l t u b e a s a f u n c t i o n o f t h e a n g l e b e t w e e n t h e t u b e a x i s a n d t h e f i e l d d i r e c t i o n . 94 5.5 I n v e s t i g a t i o n s of P l a n a r R o t a t i n g F i e l d s The main t h r u s t o f these experiements i s to u n d e r s t a n d the o p e r a t i o n of s p h e r i c a l b u l b s i n p l a n a r r o t a t i n g f i e l d s . However, a v e r y simple experiment i s d e s c r i b e d f i r s t t o show t h a t a c y l i n d r i c a l b u l b responds t o the component of the e l e c t r i c f i e l d a l o n g the b u l b a x i s (see S e c t i o n 3.5.2.1). These experiments a r e performed w i t h the apparatus d e s c r i b e d i n S e c t i o n 5.3.2. 5.5.1 C y l i n d r i c a l Tube T h i s experiment i s performed w i t h the t h i n c y l i n d r i c a l tube ( S e c t i o n 5.4.2). A c i r c u l a r l y p o l a r i z e d f i e l d r o t a t i n g i n a h o r i z o n t a l plane a t 60Hz i s g e n e r a t e d by the equipment d e s c r i b e d i n S e c t i o n 5.2.3. The c y l i n d r i c a l tube i s p l a c e d i n v a r i o u s d i r e c t i o n s i n the plane of the f i e l d . The measurement (fg) remains c o n s t a n t i r r e s p e c t i v e o f tube o r i e n t a t i o n and i s e q u i v a l e n t t o the measurement o b t a i n e d when the tube i s a l i g n e d w i t h a u n i f o r m f i e l d i n a f i x e d d i r e c t i o n of the same magni tude. 5.5.2 S p h e r i c a l Bulbs T h i s experiment i s performed w i t h a 38mm, 1.3 T o r r Ar l e a d g l a s s b u l b whose c a l i b r a t i o n c u r v e has v e r y d i s t i n c t s t e p s i n a u n i f o r m f i e l d of f i x e d d i r e c t i o n (see F i g u r e 24) 95 f B(Hz) 500 + 400 + 300 + 200 + 100 -+ 10 20 30 40 50 F i g u r e 24 P u l s e e m i s s i o n f r e q u e n c y a s a f u n c t i o n o f a p p l i e d f i e l d , 38mm, 1 . 5 T o r r A r , l e a d g l a s s s p h e r i c a l b u l b . 96 5.5.2.1 G e n e r a l P u l s e E m i s s i o n i n C i r c u l a r l y P o l a r i z e d F i e l d s The p u l s e e m i s s i o n observed i s s i m i l a r t o t h a t observed i n f i e l d s of f i x e d d i r e c t i o n , e x c e p t f o r one major d i f f e r e n c e . The p u l s e s are e q u a l l y spaced i n time and do not have any r e l a t i o n t o the phase of the a p p l i e d f i e l d . T h i s o b s e r v a t i o n i s c o n s i s t a n t w i t h the model t h a t a p u l s e o c c u r s e v e r y time E A r o t a t e s through a f i x e d a n g l e . 5.5.2.2 Rate o f P u l s e E m i s s i o n as a F u n c t i o n o f F i e l d Magnitude and Shape F i g u r e 25 shows a p l o t of f B v e r s u s the a p p l i e d e l e c t r i c f i e l d f o r a c i r c u l a r l y p o l a r i z e d f i e l d and a l i n e a r l y p o l a r i z e d f i e l d (which i s g e n e r a t e d by gro u n d i n g the p l a t e s t h a t would n o r m a l l y be connected t o , and V j i n F i g u r e 1 4 ) . As expected, the s t e p s observed i n the l i n e a r f i e l d d i s a p p e a r . The enhancement f a c t o r as d e f i n e d i n S e c t i o n 3.5.2.2 i s 1.67. I t i s o b t a i n e d by t a k i n g the r a t i o o f the s l o p e o f a l i n e c o n s t r u c t e d from the c a l i b r a t i o n c u r v e f o r c i r c u l a r l y p o l a r i z e d f i e l d t o one c o n s t r u c e d from the c a l i b r a t i o n curve f o r a l i n e a r l y p o l a r i z e d f i e l d . For the c i r c u l a r l y p o l a r i z e d f i e l d the l i n e i s the c a l i b r a t i o n c u r v e i t s e l f . F o r the l i n e a r l y p o l a r i z e d f i e l d i t i s the l i n e drawn through the top of the s t e p on the c a l i b r a t i o n c u r v e . The observed enhancement i s c l o s e t o the t h e o r e t i c a l v a l u e ( T T/2) expec t e d f o r E A / E 0 ^ 2 . However t h e enhancement f a c t o r i s observed f o r a l l v a l u e s o f E A above t h r e s h o l d . Furthermore, the t h r e s h o l d f i e l d i s s l i g h t l y lower i n a c i r c u l a r l y p o l a r i z e d f i e l d . T h i s d e s c r e p a n c y between experiment and t h e o r y a t the lower f i e l d s i s p r o b a b l y due t o the assumption t h a t E g i s u n i f o r m . A t h i g h f i e l d s 9 i s s m a l l and and Eg a r e a p p r o x i m a t e l y a n t i p a r a l l e l 10 20 30 40 50 EA' < k v nf1, Rms) F i g u r e 25 P u l s e e m i s s i o n f r e q u e n c y as a f u n c t i o n of a p p l i e d f i e l d (a) l i n e a r f i e l d (b) c i r c u l a r l y p o l a r i z e d f i e l d . 38mm, 1 .5 T o r r Ar^ l e a d g l a s s b u l b . 98 (± a few d e g r e e s ) . F o r t h i s s i t u a t i o n the non u n i f o r m i t y of E B s h o u l d have l i t t l e e f f e c t . However as 9 becomes l a r g e r than 90° (say 120°) n o n - u n i f o r m i t i e s i n Eg can l e a d t o breakdown paths which are not s t r a i g h t and which may be more f a v o u r a b l e than a p a t h a t 9 = 180°. S i n c e t h e r e a r e 180 p u l s e s / s e c a t t h r e s h o l d 9 must be 120 which i s c o n s i s t e n t w i t h the above e x p l a n a t i o n . The same experiment i s r e p e a t e d w i t h p l a n a r r o t a t i n g f i e l d s o f d i f f e r e n t a s p e c t r a t i o s o r r a t i o of semimajor a x i s f i e l d t o semiminor a x i s f i e l d . These f i e l d s are g e n e r a t e d by v a r y i n g the r a t i o o f R 2 i t o R 2 i n the e l e c t r i c c i r c u i t shown i n F i g u r e 14. F i g u r e 26 i s a p l o t o f the r e s u l t s . A g a i n the s t e p s d i s a p p e a r ( p r o g r e s s i v e l y e a r l i e r f o r more rounded e l l i p s e s ) and the h i g h f i e l d enhancement i s v e r y c l o s e t o the t h e o r y ( t h e e l l i p s e c i r c u m f e r e n c e d i v i d e d by 4 E A * ) . However, the s t e p s d i s a p p e a r e a r l i e r than e x p e c t e d from a t h e o r y w i t h u n i f o r m E B (see Appendix I I ) . F o r example, f o r an e l l i p s e o f a s p e c t r a t i o 1:1/2 ( i e . of semimajor a x i s t o semiminor a x i s r a t i o o f 2 ) , the s t e p s s h o u l d d i s a p p e a r n e a r t w i c e t h e t h r e s h o l d (around 24kVm - 1) but they d i s a p p e a r a l m o s t a t t h r e s h o l d ( i e . a t 14kVm - 1). T h i s e f f e c t i s s i m i l a r t o the enhancement observed a t t h r e s h o l d i n the c i r c u l a r l y p o l a r i z e d f i e l d and i s p r o b a b l y a l s o due to the e f f e c t o f non u n i f o r m E B near t h r e s h o l d . These r e s u l t s c o n f i r m the b a s i c t h e o r y o f o p e r a t i o n of a s p h e r i c a l b u l b i n p l a n a r r o t a t i n g f i e l d s w e l l above t h r e s h o l d . They a l s o i n d i c a t e t h a t f o r f i e l d s near t h r e s h o l d the t h e o r y i s not as a c c u r a t e , an o b s e r v a t i o n which i s c o n s i s t e n t w i t h p o s s i b l e e f f e c t s due t o a s p a t i a l l y non u n i f o r m E B . r e c a l l t h a t E A r e p r e s e n t s the semimajor a x i s f i e l d o f the e l l i p t i c a l f i e l d . 99 f R ( H l ) 400 + 300 200 100 + ellipse slope aspect experiment/ ratios theory O O | 1.62/1.57 1-5/1.32 EA' semi major axis (kv m_1) Figure 26 Pulse emission frequency as a function of applied f i e l d magnitude for d i f f e r e n t e l l i p t i c a l l y p o larized f i e l d s . Slopes f o r l i n e s are normalized to the slope of the l i n e through the top of the steps of the c a l i b r a t i o n curve for the li n e a r f i e l d . 38mm, 1.5 Torr Ar lead glass bulb. 100 5.5.2.3 P l a n a r R o t a t i n g F i e l d s and Penning M i x t u r e s T h i s experiment i s i d e n t i c a l t o the p r e v i o u s e x c e p t the Penning b u l b ( S e c t i o n 5.3.2) i s used. L i n e a r f i e l d experiments ( S e c t i o n 5.3.2) r e v e a l e d t h a t a Penning b u l b e x h i b i t s a v e r y l a r g e p u l s e f o l l o w e d by p r o g r e s s i v e l y s m a l l e r and c l o s e r t o g e t h e r p u l s e s i n each s l o p e o f the f i e l d . E x t r a p o l a t i n g t h i s to a c o n t i n u o u s l y i n c r e a s i n g f i e l d ( i e . a ramp) which b a s i c a l l y r e p r e s e n t s a c i r c u l a r l y p o l a r i z e d f i e l d ( i e . the change i n E f t always has the same s i g n and o c c u r s a t a f i x e d r a t e ) one e x p e c t s a c o n t i n u o u s glow ( i e . no p u l s e s ) . The c o n t i n u o u s glow i s m a i n t a i n e d by a c o n t i n u o u s r e p l e n i s h m e n t of the m e t a s t a b l e s t a t e . T h i s r e s u l t i s e x a c t l y what i s observed; no p u l s e s j u s t a weak D.C. l i g h t s i g n a l . 101 5.6 I n v e s t i g a t i o n s of E n g i n e e r i n g Problems Four b a s i c types of experiments have been c a r r i e d out: f i e l d p e r t u r b a t i o n due t o the s e n s o r , meter c a l i b r a t i o n s t a b i l i t y , h u m i d i t y e f f e c t s and temperature e f f e c t s * . 5.6.1 F i e l d P e r t u r b a t i o n The purpose of t h i s experiment i s to f i n d out how c l o s e t o a c o n d u c t o r a b u l b can be brought b e f o r e the f i e l d i s d i s t u r b e d . The experiment i s performed w i t h the 38mm diameter b u l b f i l l e d t o 0.66 T o r r Ne. The a p p l i e d f i e l d i s a 60Hz s i n e wave of v a r i a b l e s t r e n g t h . C a l i b r a t i o n c u r v e s ( f B v e r s u s E A ) are o b t a i n e d as a f u n c t i o n of the d i s t a n c e , L, t o one of the p l a t e s used i n the c a p a c i t o r . I t i s found t h a t as the b u l b i s brought c l o s e r t o the p l a t e the count r a t e (fg) i n c r e a s e s f o r a c o n s t a n t a p p l i e d f i e l d magnitude. F i g u r e 27 shows the c o u n t r a t e as a f u n c t i o n of d i s t a n c e from the p l a t e . The maximum e f f e c t ( t o u c h i n g the p l a t e ) i s a 20% enhancement i n the count r a t e . A t d i s t a n c e s g r e a t e r than one b u l b r a d i u s t h e r e i s no e f f e c t . Thus the s e n s o r i n f l u e n c e s the measurement of the f i e l d up t o a d i s t a n c e of one r a d i u s away from a p l a n a r c o n d u c t o r . the a u t h o r d i d not p a r t i c i p a t e i n these temperature e x p e r i m e n t s . 102 F i g u r e 27 Response of Bulb Near one P l a t e o f a C a p a c i t o r . D = b u l b diameter; C Q = counts a t l a r g e d i s t a n c e from p l a t e . 103 5.6.2 Meter C a l i b r a t i o n S t a b i l i t y A s e t of s m a l l p l a t e s i d e n t i c a l t o the l a r g e ones was c o n s t r u c t e d and p l a c e d o u t s i d e the l a b o r a t o r y and s h i e l d e d from the d i r e c t r a i n . The c o u n t r a t e f o r d i f f e r e n t b u l b s a t f i x e d f i e l d was monitored over the summer months. A t y p i c a l r e s u l t i s shown i n F i g u r e 28. The v a r i a t i o n i s l e s s than 3%. Experiments such as these demonstrate t h a t b u l b s produce r e l i a b l e r e s u l t s and remain c a l i b r a t e d f o r months. 5.6.3 Humidity E f f e c t s T h i s experiment i s performed w i t h a 25mm diameter pyrex b u l b f i l l e d w i t h .5 T o r r of neon. The a p p l i e d f i e l d i s a 60Hz waveform whose am p l i t u d e can be v a r i e d . R e l a t i v e h u m i d i t i e s up t o v a l u e s of 95% can be o b t a i n e d by d i r e c t i n g the a i r from a h u m i d i f i e r on to the b u l b . Higher r e l a t i v e h u m i d i t i e s can be o b t a i n e d by b r e a t h i n g on the b u l b . The o p e r a t i o n of the s e n s o r r e q u i r e s t h a t the a p p l i e d f i e l d p e n e t r a t e the b u l b m a t e r i a l . As the h u m i d i t y i n c r e a s e s the g l a s s c o n d u c t i v i t y i n c r e a s e s and the a p p l i e d f i e l d i s s c r e e n e d from the gas. The e f f e c t can e a s i l y be observed by m o n i t o r i n g p u l s e e m i s s i o n w h i l e b r e a t h i n g on the b u l b . A soon as the g l a s s s u r f a c e i s fogged w i t h the condensed water vapour, p u l s e e m i s s i o n s t o p s a l t o g e t h e r ; as the water e v a p o r a t e s p u l s e e m i s s i o n r e t u r n s t o normal. C o n t r o l l e d experiments are h a r d t o do i n the l a b o r a t o r y due t o water c o n d e n s a t i o n on the p l a t e s . However two c o n c l u s i o n can be r e a c h e d : (1) As the h u m i d i t y i n c r e a s e s the p u l s e r a t e d e c r e a s e s s l o w l y a t f i r s t and then r a p i d l y f o r r e l a t i v e  105 h u m i d i t i e s above 95%; the e f f e c t i s s t r o n g l y dependent- on how c l e a n the b u l b i s . (2) Encasement of the b u l b i n a h y d r o p h o b i c m a t e r i a l such as t e f l o n p r o v i d e s f o r r e l i a b l e o p e r a t i o n a t r e l a t i v e h u m i d i t i e s o f a t l e a s t 95% B r e a t h i n g on the t e f l o n h o l d e r has no e f f e c t . F i e l d t r i a l s i n l i g h t r a i n were a l s o s a t i s f a c t o r y . 5.6.4 Temperature E f f e c t s ( P r e l i m i n a r y R e s u l t s ) T h i s experiment i s performed by f l o w i n g d r y a i r o f v a r y i n g temperature (-40°C t o 40°C) around the b u l b . The b u l b i s mounted i n a p l e x i g l a s s box to p r e v e n t c o n d e n s a t i o n from o c c u r r i n g a t low t e m p e r a t u r e s . From 0°C t o 40°C no e f f e c t i s o b s e r v e d . However from -40° t o 0° d r a m a t i c changes are seen. Furthermore these changes are d i f f e r e n t f o r d i f f e r e n t b u l b s . R e s u l t s i n t h i s range a r e t h e r e f o r e s t i l l v e r y i n c o n c l u s i v e and f u r t h e r work i s r e q u i r e d . 106 5.7 F i e l d T e s t s The meter has been f i e l d t e s t e d a t v a r i o u s times under t r a n s m i s s i o n l i n e s (265kV t o 765kV), i n s u b s t a t i o n s and i n h i g h v o l t a g e l a b o r a t o r i e s . The purpose of these t e s t s has been t o c o n f i r m meter o p e r a t i o n (mainly the d e t e c t o r ) i n nois y e l e c t r i c a l e n v ironments. A l t h o u g h i n i t i a l p r o t o t y p e s r e v e a l e d problems w i t h d e t e c t o r s h i e l d i n g and o p t i c a l f i b r e c o n d u c t i v i t y - the p r o t o t y p e d e s c r i b e d here has worked v e r y w e l l . Because o f the s m a l l f i e l d p e r t u r b a t i o n and s i z e o f the GEM senso r i t has been p o s s i b l e t o measure enhancement f a c t o r s around the human body. The enhancement i s about two when the senso r i s p l a c e d i n the s h i r t p o c k e t and the pe r s o n stands e r e c t . When the senso r i s p l a c e d on the head the enhancement ranges between 6 and 10 depending on the person and the e x a c t p o s i t i o n . These r e s u l t s are c o n s i s t a n t w i t h Deno's 8 t h e o r e t i c a l c a l c u l a t i o n s . O t h e r measurements have i n c l u d e d the mapping o f the e l e c t r i c f i e l d under t r a n s m i s s i o n l i n e s . F i g u r e 29 shows a p l o t o b t a i n e d by w a l k i n g under a 500kV t r a n s m i s s i o n l i n e w i t h a GEM f i t t e d w i t h a 100mm l o n g c y l i n d r i c a l s e n s o r . The g e n e r a l shape o f the curve i s as e x p e c t e d . A c t u a l f i e l d magnitudes a r e har d t o compare t o t h e o r y because the h e i g h t of t h e c o n d u c t o r s v a r i e s g r e a t l y from p l a c e t o p l a c e and the b u l b r e a d i n g i s enhanced by the o b s e r v e r . 107 10 20 30 40 distance (m) F i g u r e 29 E l e c t r i c F i e l d Under a T y p i c a l 500kV L i n e O b t a i n e d With a 100mm Long C y l i n d r i c a l Sensor About 2m Above the Ground. 108 5.8 Summary T h i s s e c t i o n has summarized experiments performed to d e v e l o p GEM and the p h y s i c a l model f o r i t s s e n s o r . R e s u l t s show t h a t the p h y s i c a l model makes p r e d i c t i o n s which are i n good agreement w i t h e x p e r i m e n t a l r e s u l t s . However, some d i s c r e p a n c i e s are observed i n p l a n a r r o t a t i n g f i e l d s near t h r e s h o l d . These d i s c r e p a n c e i s are l i k e l y due t o the erroneous b a s i c assumption o f a u n i f o r m Eg. R e s u l t s a l s o shows the GEM meter o p e r a t e s r e l i a b l y i n t y p i c a l e n v i r o n m e n t a l e l e c t r i c f i e l d s . However temperature e f f e c t s o b s e r v e d below f r e e z i n g are not u n d e r s t o o d . 109 6.0 Summary and C o n c l u s i o n s Adverse h e a l t h e f f e c t s r e s u l t i n g from exposure t o h i g h e l e c t r i c f i e l d s i n the v i c i n i t y o f overhead t r a n s m i s s i o n l i n e s and s w i t c h y a r d s are now a matter of c o n s i d e r a b l e p u b l i c c o n c e r n . In a d d i t i o n d i r e c t a c c i d e n t s r e s u l t i n g from heavy equipment ( i e . c r anes) coming i n t o c o n t a c t w i t h power l i n e s are on the i n c r e a s e . E x i s t i n g e l e c t r i c f i e l d meters are not i d e a l f o r s t u d y i n g and m o n i t o r i n g e n v i r o n m e n t a l e l e c t r i c f i e l d s . The meters measure the i n d u c e d charge or c u r r e n t between metal e l e c t r o d e s . As such the s e n s o r s a r e m e t a l l i c , d i r e c t i o n a l l y s e n s i t i v e and i n most cases (where the e l e c t r o n i c s i s housed w i t h the e l e c t r o d e s ) l a r g e and heavy. T h i s t h e s i s d e s c r i b e s an e l e c t r i c f i e l d meter based on a d i f f e r e n t p r i n c i p l e : e l e c t r o d e l e s s breakdown of gases i n i n s u l a t i n g v e s s e l s . T h i s p r i n c i p l e a l l o w s the c o n s t r u c t i o n of a meter w i t h the sensor and d e t e c t o r s e p a r a t e d by an o p t i c a l f i b r e . The s e n s o r c o n s i s t s of a g l a s s s h e l l f i l l e d w i t h gas. As such the meter has the f o l l o w i n g advantages: 1. i t i s v e r y l i g h t and s m a l l ; 2. i t i s s p a t i a l l y i s o t r o p i c or d i r e c t i o n a l l y s e n s i t i v e (as d e s i r e d ) ; 3. i t has no m e t a l or c o n d u c t i n g p a r t s w i t h i n an a r b i t r a r y d i s t a n c e from the s e n s o r . The d i s t a n c e i s s e t by the o p t i c a l f i b r e l e n g t h and 4. i t remains c a l i b r a t e d f o r l o n g p e r i o d s of time ( e s s e n t i a l l y because i t uses a d i g i t a l s i g n a l ) and i s not a d v e r s e l y a f f e c t e d by e l e c t r i c a l n o i s e . 110 Two types of se n s o r s have been s t u d i e d i n d e t a i l : a s p h e r i c a l b u l b and a c y l i n d r i c a l tube. The s p h e r i c a l b u l b has a s p a t i a l l y i s o t r o p i c s e n s i t i v i t y t o the e l e c t r i c f i e l d s . For s u f f i c i e n t l y s m a l l harmonic d i s t o r t i o n , i t responds o n l y t o the fundamental f r e q u e n c y component of the a p p l i e d f i e l d . The response a l s o depends on the p o l a r i z a t i o n of the f i e l d . A c y l i n d r i c a l b u l b responds t o the component o f the e l e c t r i c f i e l d a l o n g i t s a x i s i r r e s p e c t i v e of f i e l d p o l a r i z a t i o n , and measures the g r e a t e s t f i e l d s t r e n g t h a l o n g t h a t d i r e c t i o n . By s u i t a b l y s e l e c t i n g the t h r e s h o l d f i e l d of the tube, one can d i s c r i m i n a t e e n t i r e l y a g a i n s t f i e l d components which are normal t o the t u b e - a x i s . A meter f i t t e d w i t h a c y l i n d r i c a l tube i s most s u i t a b l e f o r a c c u r a t e l y measuring components of the e l e c t r i c f i e l d and thus f u l l y c h a r a c t e r i z i n g the f i e l d . A meter f i t t e d w i t h a s p h e r i c a l b u l b , on the o t h e r hand, i s b e s t s u i t e d f o r l e s s a c c u r a t e a p p l i c a t i o n s i n which c a r e f u l o r i e n t a t i o n o f the sensor i s not p o s s i b l e . The measurement p r o v i d e d by the s p h e r i c a l b u l b i s dependent both on the magnitude and the geometry of the f i e l d . Thus i t does not u n i q u e l y c h a r a c t e r i z e the f i e l d . For e i t h e r type o f senso r t h e r e i s a t h r e s h o l d below which the e l e c t r i c f i e l d cannot be d e t e c t e d . The t h r e s h o l d depends on the type of gas and the b u l b d i m e n s i o n . F o r a g i v e n type o f gas a l a r g e r b u l b has a lower t h r e s h o l d . T h i s p r o p e r t y , a l t h o u g h v e r y advantageous f o r t h r e s h o l d c o n t r o l , i s the main l i m i t a t i o n on the meter. A r e a s o n a b l e s i z e sensor (40mm b u l b d i a m e t e r ) has a t h r e s h o l d o f about 10kV/m. T h i s f i e l d i s s l i g h t l y h i g h e r than t y p i c a l u n p e r t u r b e d f i e l d s found a t ground l e v e l near power l i n e s and s w i t c h y a r d s . Thus t o measure these low f i e l d s a l a r g e r s e n s o r i s r e q u i r e d . On the o t h e r hand v e r y s m a l l b u l b s (a few m i l l i m e t e r s ) can be made and even though they have a v e r y l a r g e t h r e s h o l d they can be used i n unique s i t u a t i o n s as d i s c u s s e d l a t e r on. 111 A l t h o u g h the b a s i c t h e o r y of e l e c t r o d e l e s s breakdown was de v e l o p e d by H a r r i e s and von E n g e l based on Townsend's work t h i s t h e s i s extends t h e i r work both i n t o more b a s i c p h y s i c s and e n g i n e e r i n g a s p e c t s . The s t u d y of e l e c t r o d e l e s s breakdown had been c o n f i n e d t o the b a s i c e x p l a n a t i o n of why c u r r e n t p u l s e s o c c u r when the gas i s exposed to an e l e c t r i c f i e l d . T h i s t h e s i s extends the b a s i c p h y s i c s t o a t h e o r e t i c a l e x p l a n a t i o n o f : 1. the r e l a t i o n between the f r e q u e n c y of l i g h t p u l s e s (fg) and the a p p l i e d f i e l d ( E A ) ; 2. the e f f e c t of gas p r e s s u r e on f B v e r s u s E A ; 3 . the dependence o f f B on E A i n e l l i p t i c a l l y p o l a r i z e d f i e l d s r a n g i n g from l i n e a r t o c i r c u l a r p o l a r i z a t i o n ; 4 . the e f f e c t s o f the geometry and o r i e n t a t i o n of b u l b s on the dependence of f B on E A ; 5 . the e f f e c t s o f harmonics on f B v e r s u s E A and 6. the breakdown e f f e c t i n Penning m i x t u r e s . T h i s t h e s i s a l s o p r e s e n t s many new e n g i n e e r i n g r e s u l t s : 1. e n v i r o n m e n t a l e f f e c t s on the performance of the s e n s o r and meter; 2. l i f e t i m e and s t a b i l i t y of the meter; 3 . meter d e s i g n to reduce e l e c t r i c a l n o i s e and p r o t e c t the sensor a g a i n s t e n v i r o n m e n t a l e f f e c t s and h a n d l i n g and 4 . the i n f l u e n c e o f the s e n s o r on the f i e l d b e i n g measured. Even w i t h a l l these e x t e n s i o n s much work remains t o be done both on the s t u d y of the phenomena and on meter d e s i g n . There a r e a t l e a s t t h r e e a r e a s o f the b a s i c phenomena t h a t r e q u i r e f u r t h e r s t u d y : b u l b c o n s t r u c t i o n , temperature e f f e c t s and o p e r a t i o n i n 112 p l a n a r r o t a t i n g f i e l d s . The f i r s t two a r e a s a r e most l i k e l y r e l a t e d and by f a r the most i m p o r t a n t from a p r a c t i c a l p o i n t of view. A l t h o u g h s i m i l a r b u l b s can be made r o u t i n e l y w i t h a y i e l d r a t e o f about 70% no two b u l b s a r e a l i k e . They have s l i g h t l y d i f f e r e n t t h r e s h o l d s and c a l i b r a t i o n c u r v e s . T h i s i s a problem i n the commercial e x p l o i t a t i o n o f the meter, because each b u l b must be c a l i b r a t e d and each d e t e c t o r a d j u s t e d t o the b u l b ( i e . the mean p u l s e h e i g h t i s d i f f e r e n t f o r d i f f e r e n t b u l b s ) . Temperature e f f e c t s below f r e e z i n g a r e a l s o n o t y e t u n d e r s t o o d . I t i s p r e s e n t l y b e l i e v e d t h a t they are due t o the a d s o r p t i o n o f i m p u r i t i e s a t low t e m p e r a t u r e s . I f t h i s i s the case r e p r o d u c i b l e b u l b s w i t h no temperature e f f e c t s c o u l d be produced by p r o p e r c o n t r o l o f i m p u r i t i e s d u r i n g manufacture. F i n a l l y , a r i g o r o u s q u a n t i t a t i v e model f o r the o p e r a t i o n o f s p h e r i c a l b u l b s i n p l a n n a r r o t a t i n g f i e l d s c o u l d be d e v e l o p e d . The main problem here i s u n d e r s t a n d i n g the form of the non u n i f o r m i n t e r n a l b u l b f i e l d Eg. However, t h i s more r i g o r o u s model may have l i t t l e p r a c t i c a l a p p l i c a t i o n . With p r e s e n t u n d e r s t a n d i n g i t i s c l e a r t h a t s p h e r i c a l b u l b s w i l l need t o be c a l i b r a t e d f o r each f i e l d geometry. F u t u r e work on meter d e s i g n w i l l be g u i d e d by market developments. The p r e s e n t p r o t o t y p e which c o n s i s t s o f a t y p i c a l s i z e s e n s o r (40mm) s e p a r a t e d by an o p t i c a l f i b r e from the d e t e c t o r i s w e l l s u i t e d f o r d e m o n s t r a t i o n s and f i e l d t e s t s . However, the d e s i g n must be m o d i f i e d f o r s p e c i f i c a p p l i c a t i o n s . The main parameters t h a t can be changed a r e : the s e n s o r dimensions ( k e e p i n g i n mind the e f f e c t on t h r e s h o l d ) and geometry, the f i b r e l e n g t h ( i n c l u d i n g z e r o l e n g t h or no f i b r e ) and the d e t e c t o r s i z e ( i f m i n i a t u r i z a t i o n i s d e s i r e d ) and t y p e . Making these changes may be h a r d e r than a p p a r e n t . For example a v e r y s m a l l sensor or a l o n g f i b r e 113 may r e q u i r e d l a r g e r g a i n i n the d e t e c t o r . T a b l e 3 summarizes p o s s i b l e a p p l i c a t i o n s f o r d i f f e r e n t f i b r e l e n g t h s and d e t e c t o r types ( i e . d i f f e r e n t e l e c t r o n i c s f o r c o n v e y i n g the e l e c t r i c f i e l d i n f o r m a t i o n ) . The major a p p l i c a t i o n s are f o r m o n i t o r i n g and warning d e v i c e s . As mentioned b e f o r e one of the major l i m i t a t i o n s i s t h r e s h o l d . However i n most a p p l i c a t i o n s i n which low f i e l d s need t o be measured ( i e . p e r s o n a l warning or m o n i t o r i n g ) the u n p e r t u r b e d (low) f i e l d i s enhanced g r e a t l y by the presence of c o n d u c t i n g o b j e c t s and t h e r e f o r e u s u a l l y d e t e c t a b l e . Other t o t a l l y d i f f e r e n t a p p l i c a t i o n s have r e c e n t l y appeared. These a p p l i c a t i o n s encompass ar e a s of e l e c t r i c f i e l d measurement which cannot be done a t a l l w i t h e x i s t i n g d e v i c e s . These a p p l i c a t i o n s r e q u i r e a v e r y s m a l l (few m i l l i m e t e r s ) non m e t a l l i c sensor ( i e . i n o t h e r a p p l i c a t i o n s m e t a l i s dangerous but can be u s e d ) . One example i s measurement of e l e c t r i c f i e l d s i n s i d e b e e h i v e s (the f i r s t d e v i c e s o l d was f o r t h i s a p p l i c a t i o n ) . Another more p r o m i s i n g a p p l i c a t i o n i s the measurementof f i e l d s i n s i d e ( i n o i l ) l a r g e t r a n s f o r m e r s . These measurements may a s s i s t i n p r e d i c t i n g p o s s i b l e f u t u r e t r a n s f o r m e r m a l f u n c t i o n s d u r i n g t r a n s f o r m e r assembly. 114 T a b l e 3 M e t e r C o n f i g u r a t i o n s a n d A p p l i c a t i o n s ^ v ^ f i b r e ^ \ l e n g t h d e t e c t o r ^ " ^ no f i b r e ( o n e p a c k a g e ) s h o r t f i b r e (< 2 m e t e r s ) l o n g f i b r e ( > 2 m e t e r s ) w a r n i n g d e v i c e P e r s o n a l w a r n i n g d e v i c e ( w orkmen) T e s t i n g v i c i n i t y o f l i v e h i g h t e n s i o n l i n e ( c r a n e o p e r a t o r s , h y d r o r e p a i r - men, e t c . ) d i r e c t r e a d o u t Hand h e l d f i e l d m e a s u r e m e n t d e v i c e ( w o r k m e n , r e s e a r c h e r s , e v a l u a t o r s ) U n p e r t u r b e d a c c u r a t e f i e l d m e a s u r e m e n t L i n e t e s t i n g ( w o r k m e n , r e s e a r c h e r s ) m i c r o - p r o c e s s o r U n s u p e r v i s e d f i e l d m o n i t o r y i n g ( o p e r a t o r s , r e s e a r c h e r s ) P e r s o n a l e x p o s u r e me t e r ( w o r k m e n ) P e r s o n a l e x p o s u r e m e t e r (workmen) U n p e r t u r b e d f i e l d m e e a s u r e m e n t s o v e r l o n g p e r i o d s o f t i m e ( r e s e a r c h e r s , e v a l - u a t o r s ) h i s t o g r a m d e v i c e same a s m i c r o p r o c e s s o r b u t o n l y comp h i s t o g r a m ( i e . no t i m e r e s o l u t i o n ) > i l e s a 115 7.0 R e f e r e n c e s 1. Kronberg H.A., Concern Overhead, EPRI j . , V o l . 5, No. 7, pp 7 - 13, J u n e / J u l y , 1977 2. New York S t a t e P u b l i c S e r v i c e Commission Cases 26529 and 26559 conducted over the p e r i o d 1975 t o 1978, New York S t a t e P u b l i c S e r v i c e Commission (Albany, N.Y.) 3. Banks, R.S., K a n n i a i n e n , CM., and C l a r k , R.D., P u b l i c H e a l t h and S a f e t y E f f e c t s of H i g h - V o l t a g e Overhead T r a n s m i s s i o n L i n e s : An A n a l y s i s f o r the Mines o t a E n v i r o n m e n t a l Q u a l i t y Board, Minesota Department o f H e a l t h , October 1977 4. Presman, A.S., E l e c t r o m a g n e t i c F i e l d s L i f e ( t r a n s l a t e d from R u s s i a n ) 1970, Plenum P r e s s , New York 5. J a n i s c h e w s k y j , W. and Stopps, J.G., "An E p i d e m i o l o g i c a l Study o f P e r s o n n e l Working on ac T r a n s m i s s i o n L i n e s " , Conference o f the CEA, Vancouver, B.C., March, 1979 6. K o t l e r , R.F., M i s o k i a n , M., AC T r a n s m i s s i o n L i n e s F i e l d Measurements, I n s t i t u t e f o r B a s i c S t a n d a r d s , NBS, Washington, D . C , November, 1977 7. " E v a l u a t i o n of P r o x i m i t y Warning D e v i c e s , South-West Research I n s t i t u t e , F e b r u a r y 22, 1980 8. Deno, W.D., C u r r e n t s Induced i n the Human Body by High V o l t a g e T r a n s m i s s i o n L i n e s E l e c t r i c F i e l d - Measurement and C a l c u l a t i o n of D i s t r i b u t i o n and Dose, IEEE T r a n s a c t i o n s on Power Apparatus and Systems, V o l . PAS-96, N.J. September/ Oct o b e r , 1977 9. S p e i g e l , R.J., Kerns, D.R., Cooper, E.H. and Bronaugh, E.L., "A S m a l l , A c c u r a t e , O p t i c a l l y I s o l a t e d E l e c t r i c F i e l d Probe", IEEE PES Summer meeting, Vancouver, B.C., J u l y 15-20, 1979 10. F r a n c i s , G., " I o n i z a t i o n Phenomena i n Gas", B u t t e r w o r t h P u b l i c a t i o n s (London) 1960, pp. 137-172 11. H a r r i e s , W.L. and von E n g e l , A., P r o c . Phys. Soc. B64, 915 (1951) 12. H a r r i e s , W.L., P r o c . I.E.E.E. IIA, 100, 132 - 7 (1953) 13. H a r r i e s , W.L. and von E n g e l , A., P r o c . Roy. Soc. (London) A-222, 490 (1954) 14. Dutton, J . , i n E l e c t r i c a l Breakdown i n Gases, e d i t e d by J.M. Meek and J.D. Craggs (Wiley, New York, 1978), pp. 209-318 116 15. Friedmann, D., Curzon, F.L. and Young, J.F.^ A New E l e c t r i c a l Breakdown Phenomenon! i n g a s - f i l l e d I n s u l a t i n g B u l b s , A p p l . Phys. L e t t 3 8 ( 6 ) , 15 March 1981. (Erratum App. Phys. L e t t . 3 9 ( 8 ) , 15 O c t o b e r 1981) 16. Curzon, F.L., Friedmann, D.E., F e e l e y , M. O r i e n t a t i o n - Dependent E l e c t r o d e l e s s Breakdown o f Gas i n G l a s s Tubes, J o u r n a l of A p p l i e d P h y s i c s (In PressDecember 1982) 17. Friedmann, D.E., Curzon, F.L., F e e l e y , M., Young, J.F., and A u c h i n l e c k , G., An E l e c t r i c F i e l d Meter Based on the Breakdown of Gases, Rev. S c i . I n s t r . 53_, 1273 - 1277, 1982 18. Friedmann, D.E., Curzon, F.L., and F e e l e y , M. E l e c t r o d e l e s s Breadown of Gas i n R o t a t i n g E l e c t r i c F i e l d s a t 60Hz, i n p r e p a r a t i o n f o r s u b m i s s i o n to the Canadian J o u r n a l o f P h y s i c s 19. Young, J . and Friedmann, D.E., E l e c t r i c F i e l d D e t e c t o r , U.S. P a t e n t A p p l i c a t i o n No. 06/142, 815, f i l e d 22 A p r i l 1980 20. Holloway, D.G., "The P h y s i c a l P r o p e r t i e s of G l a s s " , Wykeham P u b l i c a t i o n s (London) L t d . , 1973, C h a p t e r 3. 21. L o r r a i n , P., and Corson, P., " E l e c t r o m a g n e t i c F i e l d s and Waves", San F r a n c i s c o , W.H. Freeman and Co., C h a p t e r 4, F i g u r e 4.22, pg. 169, 1970 22. Da S i l v a , L., "A R o t a t i n g E l e c t r i c F i e l d D e v i c e " , 4 t h year e n g i n e e r i n g p h y s i c s p r o j e c t , UBC, J a n u a r y 4, 1982 23. Jackson, J.D., " C l a s s i c a l E l e c t r o d y n a m i c s " , New York, John W i l e y and Sons, 1975, C h a p t e r 2, pp. 60 - 62 117 APPENDIX I E f f e c t s o f F i n i t e C o n d u c t i v i t y F o r the purposes of t h i s Appendix i t i s assumed t h a t the s p h e r i c a l s h e l l c o n s i s t s of g l a s s i n which s u r f a c e c o n d u c t i o n of l e a c h e d i o n s i n absorbed water i s the main charge t r a n s p o r t m echanism 2 0. T h i s means t h a t the i o n s move as a r e s u l t of the e l e c t r i c f i e l d ( E Q ) t a n g e n t i a l t o the s u r f a c e of the s p h e r e . I t i s a l s o assumed t h a t ohmic c o n d u c t i o n o c c u r s ( i e . v = jj Eg, where v i s the i o n v e l o c i t y , and, g , the m o b i l i t y ) , and t h a t t h e r e i s no net charge on the s p h e re. P o s i t i o n s on the sphere are d e s c r i b e d by r i g h t hand s p h e r i c a l p o l a r c o - o r d i n a t e s a, 9 and <j>. A s p a t i a l l y u n i f o r m o s c i l l a t i n g f i e l d , E A e J ^ t ^ s a p p l i e d a l o n g the z - a x i s of the s h e l l , as shown i n F i g u r e 1^ where ^ i s the f r e q u e n c y , t the time and j 2 = -1. The p o l o i d a l f i e l d EQ i s t h e r e f o r e g i v e n by the e x p r e s s i o n . E e = - E ^ e J ^ t s i n 9 - (1/a)( i v / d 9 ) (1-1) where the p o t e n t i a l V a t p o i n t P ( F i g u r e I ) r e s u l t s from motion o f c h a r g e s o v e r the s u r f a c e of the s h e l l . These charges have a d e n s i t y of S coulombs _ n m so t h a t V s a t i s f i e s the e q u a t i o n , er d £ / 4 T T £ Q R ( I - 2 ) where d £ i s an element of a r e a a t Q ( F i g u r e I ^ R i s the d i s t a n c e from Q to P, and £ 0 i s the p e r m i t t i v i t y of f r e e s p a c e . The domain of i n t e g r a t i o n i s 118 F i g u r e I Geometry of the c o n d u c t i n g s h e l l i n the a p p l i e d f i e l d ( E A ) . 119 s u r f a c e of the s h e l l . Change c o n s e r v a t i o n r e q u i r e s t h a t s a t i s f y the f o l l o w i n g e q u a t i o n : a - T L s i n 9(2<s/£t) + o ( E Q s i n 9)/^9 = 0 (1-3) where -TL. i s the r e s i s t a n c e of the s h e l l i n ohms per s q u a r e . U s i n g t h i s e q u a t i o n t o e l i m i n a t e 6 from (1-2) en a b l e s V t o be e x p r e s s e d i n terms o f Eg as f o l l o w s : "cW/^t = - ^(4TT6 0aR JL s i n 9 ) _ 1 $ E e s i n 9/<j9)d£ (1-4) By d i f f e r e n t i a t i n g (1-1 ) w i t h r e s p e c t t o time ^ V / ^ t can be e l i m i n a t e d from (1-1) and (1-4) t o y i e l d an i n t e g r a l e q u a t i o n f o r Eg, namely; ^E e/3t = - j W E A e J w t s i n 9 + (a/^Q) ( K d £ (1-5) where K = (4TTa 2£ 0 R j l s i n 9 ) _ 1 ( o E e s i n 9 / ^ 9 ) (1-6) To s o l v e the e q u a t i o n we w r i t e E 9 = ^ . E A ( s i n 9 ) e 3 ^ t ( I _ 7 ) where ^ i s a c o n s t a n t which i s t o be de t e r m i n e d . With t h i s assumption e q u a t i o n (1-5) becomes s i n 9 = - s i n 9 + ^ 9 \ (2TJjwa 2c" 0 R J U - 1 cV(cos 9 ) d £ (1-8) 120 The i n t e g r a l can be e v a l u a t e d by n o t i n g t h a t i t i s r e l a t e d t o the p o l o i d a l f i e l d E Q produced by a s u r f a c e charge d e n s i t y fi' o f the f o r m s ' = c cos 9. From J a c k s o n ' s s o l u t i o n 2 3 t o the problem r e g a r d i n g the e f f e c t o f a c o n s t a n t f i e l d a p p l i e d to a c o n d u c t i n g sphere, i t i s r e a d i l y shown t h a t E Q = -(^/<J9) j c ( c o s 9) d£/4TT£ 0 aR = (c sin9/3£. Q) (1-9) Comparing the i n t e g r a n d s i n (1-8) and (1-9) i t i s apparent t h a t e q u a t i o n (1-8) can be w r i t t e n as otsin 9= -sine -2<*. s i n 9 / 3 j a f Q JL (1-10) Hence o( = -1 [1 - j (w'/V)]- 1 (1-11 ) where v/v' = 2 / ( 3 a f 0 A ) (1-12) U s i n g (1-17), (1-11) and (1-3) shows t h a t the s u r f a c e charge d e n s i t y £> i s g i v e n by the e q u a t i o n , € = 2 E A ( c o s 9) eJwt/laSL ( j v ^ + v V ) ] (1-13) Again h a v i n g r e c o u r s e t o Jackson's book i t i s r e a d i l y shown t h a t t h i s s u r f a c e charge produces a u n i f o r m f i e l d E Q i n i t s i n t e r i o r g i v e n by the e q u a t i o n s : 121 Ej, = -w,EAke>t/(Jw+w') (1-14) where k i s a u n i t v e c t o r a l o n g the z - a x i s ( F i g u r e 1-1). Hence the t o t a l a p p l i e d f i e l d i n s i d e the s h e l l ( E T ) i s E T = Ec + E^ = j v j E A e D w t + jw) (1-15) F o r a f i e l d of s t r e n g t h E A , s w i t c h e d on a t time t = 0 i t i s r e a d i l y shown t h a t f o r t > 0 E T = E A e ~ w ' t (1-16) E q u a t i o n (1-15) shows t h a t i s a t t e n u a t e d by the c o n d u c t i n g s h e l l ( E T = E A [ 1 + ( W ' / w ) 2 ] - 1 / 2 ) and i s a l s o phase s h i f t e d by an angle <J> where t a n <)>= (VJ'/W). F o r the case of i n t e r e s t ( w ' « w ) , o n l y the phase s h i f t i s s i g n i f i c a n t . 122 APPENDIX I I Rate of P u l s e E m i s s i o n i n P l a n a r R o t a t i n g F i e l d s In a l i n e a r l y p o l a r i z e d f i e l d (an e l l i p t i c a l f i e l d w i t h u n i t e c c e n t r i c i t y ) the b u l b obeys r e l a t i o n ( 2 ) . In e l l i p t i c a l or c i r c u l a r l y p o l a r i z e d f i e l d however i t i s n e c e s s a r y t o extend the b a s i c t h e o r y . Assuming the b a s i c phenomena remains the same and t h a t the i n t e r n a l f i e l d (Eg) i n the b u l b caused by charge s e p a r a t i o n i s u n i f o r m , then a new breakdown s t i l l o c c u r s e v e r y time the a p p l i e d f i e l d changes by E Q , however the a d d i t i o n o f the f i e l d s i s now v e c t o r i a l . For breakdown one must have \EA + Egl = E 0 ( I I - 2 ) To p r o c e e d f u r t h e r i t i s assumed t h a t the phasors f o r E A (and Eg) are e l l i p t i c a l w i t h major a x i s E A = Y E 0 , ( I I - 3 ) and minor a x i s , e E A = e Y E 0 (n-4) where Y i s a f a c t o r which determines the r a t i o o f the a p p l i e d f i e l d a m p l i t u d e E A , t o the breakdown v o l t a g e of the gas. F o r V O , breakdown does not o c c u r . With these v a r i a b l e s the f i e l d s can be d e s c r i b e d p a r a m e t r i c a l l y by the a n g l e s , A and B and the e q u a t i o n s 123 Eft. = ^ E D ( ( & s i n A) i _ + (cos A ) _ ) — ( I I - 5 ) E__= - r E Q ( ( f s i n B)_i + (cos B)j_ where i i s a u n i t v e c t o r a l o n g the minor a x i s of the phasor and j i s a u n i t v e c t o r a l o n g the c o r r e s p o n d i n g major a x i s . I t f o l l o w s from e q u a t i o n s ( I I -5) t h a t the magnitude of E A + Eg i s g i v e n by the e x p r e s s i o n |EA + E R | 2 = y 2 E 2 (£ 2(sin A - s i n B ) 2 + (cos A - cos B ) 2 ) S i n c e | E ^ + E_| = E Q a t breakdown i t f o l l o w s from the above r e s u l t t h a t , a t breakdown, B s a t i s f i e s the e q u a t i o n (1/2r) = s i n ( (A-B)/2) [ (1-£ 2)sin ( ( A + B ) /2 ) + £ 2 ] 1 / 2 ( I I - 6 ) F o r B =0 ( l i n e a r l y p o l a r i z e d f i e l d s ) the above e q u a t i o n reduces t o the form 1=V(cos B - cos A ) , or E A ( c o s B - cos A ) = E Q ( I I - 7 ) S i n c e E A cos B and E A cos A are the components of the f i e l d s a l o n g the major a x i s o f the e l l i p s e , e q u a t i o n ( I I - 7 ) i s c o n s i s t e n t w i t h e q u a t i o n ( I I - 1 ) . For p, =1 ( c i r c u l a r p o l a r i z a t i o n ) e q u a t i o n ( I I - 6 ) a l s o has the s i m p l e s o l u t i o n (1/2V) = s i n ( ( A - B ) / 2 ) ( I I - 8 ) The a n g l e through which E A r o t a t e s between s u c c e s s i v e breakdowns i s however, A - B . I t t h e r e f o r e f o l l o w s t h a t the number of breakdowns per c y c l e , f g , i s 2 T T f A / ( A - B ) o r , 124 f B = T T f A / s i n 1 (1/2Y) " ( I I - 9 ) Comparing t h i s r e s u l t w i t h e q u a t i o n (Il - I) shows t h a t , f o r s t r o n g f i e l d s (Y>̂ > 1) the breakdowns per c y c l e i s i n c r e a s e d i n g o i n g from l i n e a r t o c i r c u l a r p o l a r i z a t i o n f o r f i e l d s o f c o n s t a n t a m p l i t u d e . In f a c t , as Y a o f c / f L - + T T / 2 (11-10) where f c and f L and the r e s p e c t i v e number of breakdowns per second i n c i r c u l a r l y and l i n e a r l y p o l a r i z e d f i e l d s a t the same f i e l d s t r e n g t h s . F o r more g e n e r a l cases of e l l i p t i c a l p o l a r i z a t i o n , e q u a t i o n ( I I - 6 ) can be reduced to a q u a r t i c e q u a t i o n f o r B, and has e i t h e r 0, 2 o r 4 r e a l s o l u t i o n s . For s u c c e s s i v e breakdowns B i s the s m a l l e s t r e a l s o l u t i o n which exceeds A. In the l i m i t i n g case of v e r y s t r o n g f i e l d s E Q becomes an element o f a r c l e n g t h of the ph a s o r . Hence the enhancement r a t i o r e s u l t i n g from the e l l i p t i c a l f i e l d becomes f E / f L = E(m., ) (II-11 ) where f E i s the f r e q u e n c y o f breakdown i n the e l l i p t i c a l l y p o l a r i z e d f i e l d and E(m 1) i s an e l l i p t i c i n t e g r a l o f the second k i n d , w i t h m̂  = 1 - £ 2 # T h i s r e s u l t i s analogous t o t h a t o b t a i n e d f o r a c i r c u l a r l y p o l a r i z e d f i e l d . For an e l l i p s e E(m 1) i s the l e n g t h o f a quadrant o f the e l l i p t i c a l p h asor. For the c i r c u l a r p o l a r i z a t i o n m1 = 1 and E(1) =T/2 as e x p e c t e d . To g a i n more i n s i g h t c o n c e r n i n g the e f f e c t s o f cha n g i n g the p o l a r i z a t i o n o f the f i e l d e q u a t i o n (lI-6) has been s o l v e d i t e r a t i v e l y w i t h a computer. The c a l c u l a t i o n i s begun by s e l e c t i n g £ , Y a n d an i n i t i a l v a l u e o f B (B 1 s a y ) . A s o l u t i o n i s 125 o b t a i n e d f o r A (A-|), and B i s then r e s e t t o the v a l u e A-| . .The p r o c e s s i s r e p e a t e d f o r twenty s u c c e s s i v e breakdowns. The angle (j>(n) between the b u l b f i e l d ( E B ) and the _ - a x i s ( F i g u r e 6) i s then e v a l u a t e d on the nth i t e r a t i o n u s i n g the r e s u l t <)>(n) = a r c t a n (p t a n B n) (11-12) The average angle <}> through which Ej^ r o t a t e s f o r breakdown i s then g i v e n by $ = (<}>(20)-A1 )/20 (11-13) and the s t a n d a r d d e v i a t i o n , 6 , by the e x p r e s s i o n 20 g- 2 = ^ (<|)(n)-$) 2/20. (11-14) j=1 (The s t a n d a r d d e v i a t i o n g i v e s i n f o r m a t i o n on the v a r i a b i l i t y i n the time i n t e r v a l between breakdowns). The number o f breakdowns per second i s thus f B = 2Trf A/<f (11-15) F i g u r e I I shows graphs o f f B f o r 0 = 1, .66, .5, 0.33 and 0. The s i g n i f i c a n t f e a t u r e s of the response c u r v e s are as f o l l o w s : 1. a s p i n c r e a s e s from 0 ( l i n e a r p o l a r i z a t i o n ) t o 1 ( c i r c u l a r p o l a r i z a t i o n ) the graphs become p r o g r e s s i v e l y l e s s "stepped" i n form; 2. the graphs become smoother a t s u c c e s s i v e l y l a r g e r v a l u e s of as p i s d e c r e a s e d ; 126 400 300 I 200 100 1 H \- 1 2 F i g u r e I I P u l s e E m i s s i o n as a F u n c t i o n of N o r m a l i z e d A p p l i e d F i e l d Magnitude f o r D i f f e r e n t E l l i p t i c a l l y P o l a r i z e d F i e l d s . T h e o r e t i c a l R e s u l t Assuming Eg i s u n i f o r m . Compare w i t h F i g u r e 26. 127 3. t h e c u r v e s a l l s t a r t a t t h e same p o i n t ( i e . t h e t h r e s h o l d f i e l d s t r e n g t h , a n d b r e a k d o w n f r e q u e n c y a t t h r e s h o l d a r e i n d e p e n d e n t o f p o l a r i z a t i o n ) ; 4 . a t h i g h f i e l d s t r e n g t h s , t h e r e s p o n s e c u r v e s a r e a s y m p t o t i c t o s t r a i g h t l i n e s w h i c h p a s s t h r o u g h t h e o r i g i n s . The m o d e l a l s o shows t h a t t h e b r e a k d o w n f r e q u e n c y a t l a r g e f i e l d s i s e n h a n c e d b y a f a c t o r f E / f L , i n c o m p a r i s o n t o t h e v a l u e o f f g o b s e r v e d w i t h l i n e a r l y p o l a r i z e d f i e l d s . The l a r g e s t e n h a n c e m e n t i s TJ/2 a n d o c c u r s i n t h e c a s e o f c i r c u l a r p o l a r i z a t i o n .

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