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The design and operation of a completely automated liquid hydrogen or deuterium target Currie-Johnson, Murray Allen 1976

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TBI DESIGN AND CPEEAIICN CP A COMPLETELY AUTOMATED LIQUID EYEBOGEN CE DEUTERIUM TARGET By MUREAY ALLEN CURRIE-JOHNSON B . S c , U n i v e r s i t y o f E r i t i s h C c l u m b i a , 1973 A THESIS SUEKITTED IN PARTIAI FULFILMENT OF THE REQUIREMENTS FOB THE DEGREE CF MASTER OF SCIENCE i n t h e Department cf P h y s i c s We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d The U n i v e r s i t y o f E r i t i s h C o l u m b i a A p r i l , 1976 Hurray A l l e n Currie-Johnson-, 1976 In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th i s thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my wr i t ten permission. Department of The Univers i ty of B r i t i s h Columbia 20 75 Wesbrook Place Vancouver, Canada V6T 1W5 i i ABSTHACT A s e l f contained completely automated l i q u i d hydrogen target has been b u i l t in the TJBC Nuclear Physics Department for use i n experiments at 1BIUMF. The system i s designed f c r use i n secondary beam l i n e s . The r e f r i g e r a t i v e system u t i l i z e s a CTI Cryodyne 1020 re f r i g e r a t o r capable of 10 watts at 20 K. The fla s k assembly, with a volume of 1 l i t r e (extendable to 7 l i t r e s ) i s made cf Mylar surrounded with 20 layers of superinsulation. The vacuum system i s completely automated. The safety " system i s designed to shut the system down i n a controlled manner i n a l l types cf f a i l u r e . The cooling time of the system i s of the order of 8 hours and hydrogen l i g u i f i c a t i c n rate i s of the order of 1 l i t r e / 1 0 hours. i i i ££££1 CP CCNflNTS 1 I n t r o d u c t i o n .1 2 Choice Cf Target 3 3 Design Cf The Target .9 3.1 General Design Features ..9 3.2 Flas k Assembly ..12 3.2.1 The Flask 12 3.2.2 Some Flask C o n s t r u c t i o n Technigues ............ 16 3.2.3 F l a s k Support S t r u c t u r e .....17 3.3 The R e f r i g e r a t i o n System ...20 3.3.1 R e f r i g e r a t o r And Condensing V e s s e l .20 3.3.2 Compressor .......22 3.4 Vacuum System ......27 3.4.1 The Weed For A Vacuum; S u p e r i n s u l a t i o n ........27 3.4.2 Vacuum System Used .29 4 Safety And Alarm Devices. .........33 5 Operating C h a r a c t e r i s t i c s . 36 5.1 Operation .36 5.2 C p e r a t i c n a l C h a r a c t e r i s t i c s .......37 . E i b l i o g r a p h y .................41 iv LIST CF TABLES 3.1 System s p e c i f i c a t i o n s for the Model 1020 Cryodyne Refrigerator .......................... 25 3.2 Compressor Unit Specifications 26 3.3 M4 Diffusion Pump Specifications 32 3.a Specifications for the Alc a t e l Model 1030 Primary Vacuum Pump 32 V LIST GF FIGUBES 2.1 Geometry of H y p o t h e t i c a l Experiment 7 2.2 Comparison of p-p and p-C s c a t t e r i n g c r o s s s e c t i o n s ................................. 8 3.1 B l u e p r i n t of Target System ........................11 3.2 Y i e l d S t r e s s vs. Temperature (Mylar) .............. 18 3.3 Young's Modulus vs. Temperature (Mylar) ........... 19 3.4 Schematic of R e f r i g e r a t o r Operation .......23 3.5 Twc Stage R e f r i g e r a t o r ..24 3.6 Heat Flow vs. Pressure 31 4.1 Flask Pressure vs. Ccoldown Time ..................39 4.2 Second Stage "Temperature vs. Cccldcwn Time ........ 40 v i ACKNC8IEEGEJ3EN1S I wish to express my appreciation and thanks to my research supervisor, Dr. David A. Axen for his assistance and guidance. I also wish to thank Al Bishop, al Stevenson and Al Morgan for the assistance they rendered. 1 1 R u c t i o n Advances i n technology have greatly s i m p l i f i e d l i q u i d hydrogen (H^x) target construction in the past decade. The introduction of small r e f r i g e r a t o r s has eliminated the need for large quantities of hydrogen and permitted more compact target systems. In the following, the advantages of $.U^ targets over gaseous and composite CH^ targets i s discussed. The design and operating c h a r a c t e r i s t i c s of the l i q u i d hydrogen target b u i l t at UBC i s described. In order to observe the scattering of fundamental p a r t i c l e s from protons cr deuterons with low intensity beams in a reasonable time an &Hv. target i s e s s e n t i a l . Calculations i n chapter 2 show that using other targets the count rates are too low or background from ether material i s d i f f i c u l t tc eliminate. targets are complicated by the cooling required and the vacuum necessary for insulation. The thick walls required by the vacuum and the insulators required to reduce heat losses provide background scattering from material other than the target material. The target discused here i s very v e r s a t i l e , with e a s i l y interchangeable flaskss and i s capable of handling both £H>_ and l i q u i d Deuterium {fiE z). Modular construction i s an important feature of the system, allowing easy replacement of those components which have a short useful l i f e t i m e . Safety has a very high p r i o r i t y i n the design. Several alarm devices prevent the development of unsafe conditions. However i n the very remote p o s s i b i l i t y that the flask should 2 burst, a dump tank i s provided to contain the hydrogen u n t i l i t can be safely vented. Chapter 3 describes the component parts of the system. 14 mil Mylar i s shown to be adequate for flask construction. The flask i s kept cool and supplied with iti^ by the r e f r i g e r a t o r and condensing vessel through 3 connecting pipes. A l l cold components (the f l a s k , pipes and condensing vessel) are surrounded by high vacuum. Additional ins u l a t i o n i s provided by aluminized Mylar and a copper thermal (77*K) shield around the condensing vessel. To supply the vacuum a d i f f u s i o n pump i s used. Most of the vacuum vessel i s made of high guality aluminium. An external compressor i s used to supply helium to the r e f r i g e r a t o r . Two rotary pumps back the d i f f u s i o n pump and supply a rough vacuum. Monitoring devices and controls for the pressure actuated valves and other parts of the system are located in the pumping and remote control modules. The flow of hydrogen (deuterium) gas from the tanks i s controlled by the hydrogen reservoir module. A large a i r duct and fan vents a i r from the v i c i n i t y of the system tc roof l e v e l . Three exhaust pipes and a dump tank l i n e also go to roof l e v e l . Chapter 4 d e t a i l s the alarm and safety features used to prevent damage to equipment and tc prevent p o t e n t i a l l y dangerous situations from a r i s i n g . The f i n a l chapter describes actual operation of the system. 3 2 Choice Cf Target The c r i t e r i o n for the choice cf the target i s that count rates must be reasonably large and background events must be either capable of being eliminated or in s u f f i c i e n t l y small numbers to be neglected. Figure 2.1 shows the geometry of the experiment considered in choosing the type of target. The experiment i s basi c a l l y a t r i p l e scattering p-p experiment. A polarized beam i s produced by scattering the primary proton beam from an i n i t i a l fiH-^ target. The polarized proton beam scattered 9 i s then rescattered from the target considered here. Pol a r i z a t i o n e f f e cts are ignored in the calculations as they contribute only r e l a t i v e l y small corrections. There are primarily three choices for a hydrogen target: 1. i H v 2. Gaseous hydrogen 3. Hydrocarbon (e.g. polyethylene: CHO • A comparison of count rates between Jclk and gH,. i s made. The major advantage of ^Hx i s that elaborate cooling procedure i s not reguired; the advantage of J^HV being the greater (~103x) density, and therefore shorter runs are needed. For the purposes of t h i s comparison the calculations w i l l be done on a target consisting of a cylinder 10 cm long with a radius of 5 cm. Target 1 i s an #HX target 10 cm long. A 50 nanoamp beam at 400 MeV w i l l be considered i n the cal c u l a t i o n s . Sterj_ J i Calculation of beam incident on the target. Osing 380 BeV data 1 the d i f f e r e n t i a l cross-section at Out = 9° (@ C H =2t f ) i s ( j C ) c „ = 4 , 2 ffii3/ster o r 2 f 2 0 mb/ster. The 4 f r a c t i o n of the i n c i d e n t beam s c a t t e r e d i n t o the element of s o l i d angle dSl i s given by K = fe) N^a (2 .D ul, where IV = the number of s c a t t e r i n g c e n t r e s per cm 2 AU {2*2) and3 =density of J | H V = 0.070 g/cm 3 (\lo= Avogadros number = 6.02 1 0 2 3 p a r t i c l e s /mole. H = l e n g t h of t a r g e t = 10 cm. Atomic Weight = 1 g/mole t h e r e f o r e |\J = 4,21 I C 2 3 s.c./cm 2 and = 8.43 10"3 s t e r 1 N. B. i s the same f o r both of the (J^HjJ t a r g e t s considered here. A l s o , c ( S l = ( 5 ) 2 10* = 7.85 1(T5 s t e r . 3L;*~ 5 0 nanoamps = 3.13 1 0 1 1 protons/sec. T h e r e f o r e the c u r r e n t going i n t o the t a r g e t i s Ns Z fsJ'^dA - 2 . 0 ? ]0 5 p r o W * / r . c . 12.3) The energy of the e l a s t i c a l l y s c a t t e r e d p a r t i c l e s i s 380 MeV. S t e ^ _ 2 i C a l c u l a t i o n of the c u r r e n t i n t o a counter of angular area ol-R.after the second s c a t t e r i n g i s A counter of angular area 10 3 ster Is considered. At 20° a ( j = 48°* (^) = 4 * c mb/ster. From 2.3 and 2.4 then Therefore the time reguired f c r 10* counts (necessary f o r 11 s t a t i s t i c a l precision) i s -~2 hours, quite reasonable compared to the set-up time. The corresponding time for Q H x i s calculated i n the same manner but with the density at STP conditions given by AM This i s a factor of 790 less than for Q.BX density, so the time required f o r 10* counts i s 60 days . Clearly t h i s eliminates gfl-^  as a possible target with the beam i n t e n s i t i e s presently av a i l a b l e . The other possible target, polyethylene, l i k e $Hj_, does not have tc be cooled and better, because i t i s a s o l i d , does not need to be contained. It can be i n s t a l l e d in a few minutes. The elaborate safety precautions reguired for hydrogen are circumvented. l i t h a density 3 of about 0.95 g/cm3 there i s a free proton density of 0. 135 g/cm3, almost double the proton density in an J2HX target. Figure 2.2 compares the p-p 1 and p-C* d i f f e r e n t i a l scattering cross-sections (at 300 HeV and 2 89 MeV respectively) The p-C scattering i s strongly forward peaked and dominates u n t i l about 19° . P-p guasi-elastic scattering from protons inside the C nucleus allows many events which can be eliminated to a certain extent by kinematical reconstruction. 6 Unfortunately, p-c e l a s t i c scattering i s much mors d i f f i c u l t to i d e n t i f y than p-p scattering. Two-body kinematics are used i n analysing p-p scattering. There are 2 f i n a l momenta and 2 f i n a l energies or 4 unknowns. If the i n i t i a l monentum and energy of the system are known, the number of unknowns i s reduced to two. Only two measurements are then reguired tc analyse the scattering. Hith 3-body scatter i n g , there are three f i n a l momenta and three f i n a l energies, or six unknowns. After i n i t i a l conditions (momentum and energy) are incorporated, then four measurements are reguired. This doubles the number of measurements reguired and complicates analysis. Also, these protons inside the C-nucleus have zero momentum a f r a c t i o n of the time and these cannot be eliminated as they are indistinguishable from pure p-p events. Kinematical reconstruction i s also complicated by the f a c t that the r e c o i l proton may be stopped by the CHZ. For example, with a 300 MeV beam and p-p scattering, the minimum lab angle 2 through which a proton whose r e c o i l proton goes through 1 cm of CH can be scattered i s about 19°. The corresponding angle for 1 cm of H i s 5°, but as explained above, coincidence technigues are not essential i n 5H^ scattering. As the CHj, target must have comparable dimensions tc an J?HZ target and measurements under 20 are desired, p-p coincidence technigues w i l l net work. The above reasons, p a r t i c u l a r l y the experimental complications involved, eliminate further consideration of CH,. as a possible target, leaving 5H^ , as our choice for the target. Incident proton beam (50 namps) various angles FIG.2.1 Geometry of Hypothetical Experiment X ~\ 1 ' l 1 1 T O 5 15" xo XI Lab Angle FIG. 2.2 Comparison of p-p and p-C scattering cross-sections 9 3 2ssig.ii Of Ihe Target 3.1 General Design Features Hydrogen t a r g e t design underwent major r e v i s i o n i n the l a t e 1960's due t o the i n t r o d u c t i o n of a s m a l l recondensing r e f i g e r a t o r T h i s enabled a compact, safe system to be b u i l t by e l i m i n a t i n g the need f o r a l a r g e H^^ . r e s e r v o i r and a t r a n s p o r t system f o r the t r a n s f e r of fiHv from the r e s e r v o i r to the t a r g e t . The hydrogen t a r g e t c o n s i d e r e d here c l o s e l y f o l l o w s the design developed at E u t h e r f o r d High Energy l a b (HHEL 5). F i g u r e 3.1 i s a schematic of the system. I t c o n s i s t s of s a f e t y devices p l u s the 6 modules l i s t e d below: 1. The compressor module s u p p l i e s compressed helium to run the r e f r i g e r a t o r . 2. The t a r g e t module c o n t a i n s the t a r g e t f l a s k with a vacuum j a c k e t , the r e f r i g e r a t o r , a d i f f u s i o n pump, some of the hydrogen supply system and the rough vacuum pumping system, 3. The c o n t r o l and pumping module s u p p l i e s the roughing vacuum and c o n t r o l s the pressure actuated v a l v e s . 4. The remote c o n t r o l module has gauges and valve c o n t r o l s necessary f c r normal o p e r a t i o n of the t a r g e t . 5. The hydrogen r e s e r v o i r module s u p p l i e s the hydrogen to the system. 6. The p u r g e / f i l l module enables the hydrogen r e s e r v o i r tank to be f i l l e d s a f e l y . To e l i m i n a t e the hazard of hydrogen e x p l o s i o n , e l e c t r i c a l c o nnections are minimal. Valves cn the t a r g e t module are a l l 10 pressure actuated and e l e c t r i c a l connections are explosion proofed by encasement in a i r tight containers. 11 v v4 J . . ^ HOOF LEVEL CONTROL AND PUMPING MODULE FIG.3.1 Bl u e p r i n t of Target System 12 3.2 Flask Assembly 3.2.1 The Flask The c a l c u l a t i o n s of Chapter 2 indicate the size of the target reguired. The lew density of J2H,. compared with construction materials requires that very thin flask walls be used. Because of the very low temperatures involved, few materials are suitable: generally either Mylar or sta i n l e s s s t e e l are used. The yi e l d strength 6 of sta i n l e s s s t e e l (type 321) i s roughly constant with temperature at 60 10 3 psi or 4 times the y i e l d strength of Mylar at room temperature (and a someahat smaller r a t i o at lower temperatures), (see figure 3.2) Mylar i s superior to st e e l as far as undesirable scattering i s concerned but radiation damage may cause i t s re j e c t i o n ; therefore a t y p i c a l l i f e t i m e of Mylar i s calculated in the following where: Rfl = maximum allowable radiation acceptance of Mylar. =level at which Mylar has 751 of i t s o r i g i n a l physical properties =9 10 7 rads* = 9 102 joules/g. T M = li f e t i m e of Mylar ^ = stopping power of Mylar = 5 «eV-cm 2/g at 200 MeV (for CH ) 2 J = beam current per unit area = 1.4 1 0 1 6 amp/cm2 (from equation 2.3) 13 therefore D T M = — = 1. 3 1012 sec or about 400 centuries. Therefore Mylar i s suitable for use in secondary beams. The Mylar must be thick enough so that i t w i l l not break under operating conditions. For the operating pressure of 30 psi (as i t i s at room temperature) the flasks are designed and tested to 90 p s i . For t h i s shape of cylinder, the weakest stress i s the hoop membrane s t r e s s 8 . The end of the cylinder i s approximately hemispherical and stronger than the rest of the cylinder. The hoop stress was calculated as follows: the stress i s given by 5 = P R twhere P = pressure = 90 p s i . - radius = 2-3/16" •fc, = thickness i . e . t= PR ^ (3.1) One end of the flask i s constrained by s t a i n l e s s s t e e l . The relevent stress i s the yi e l d stress, that i s the stress at which appreciable elongation w i l l occur with a small increase i n the 14 s t r e s s , f o r Mylar, t h i s i s 15,000 p s i at room temperature and i t i s higher at lower temperatures 6 (see f i g u r e 3.2). From 3.1 then -{^  - 1.31 10-2 inches or about 13 m i l . The r e f o r e 14 mil Mylar was used (Bursting pressure-96 p s i ) . S e v e r a l were t e s t e d and burst at 98 p s i , Both a i r pressure and h y d r a u l i c pressure t e s t s y i e l d e d the same r e s u l t ; h y d r a u l i c pressure i s e a s i e r t c work with. To determine the length of the t a r g e t i n o p e r a t i n g c o n d i t i o n s , we must a l s o c a l c u l a t e the s t r a i n , the amount of s t r e t c h i n g of the c y l i n d e r . The i n c r e a s e i n diameter of the c y l i n d e r i s of no concern because the amount cf beam e n t e r i n g the t a r g e t w i l l be d e f i n e d using other techniques. The s t r a i n c ^ = ~ (£ = amount s t r e t c h e d so £ i s the f r a c t i o n of the t o t a l l e n g t h stretched) i s given by Hooke's Law: T h i s s t r e s s i s given by C . P R it where the pressure i s 15 p s i (the normal o p e r a t i n g pressure when the hydrogen has been l i q u i f i e d ) and the r a d i u s and t h i c k n e s s are as given above. Young's modulus £ (or the modulus of e l a s t i c i t y ) i s 1.7 10Vat QEZ temperatures (see f i g u r e 3.3) so the s t r a i n i s 15 or about 0.01%, which i s n e g l i g i b l e . To improve i n s u l a t i o n , 20 l a y e r s of 0.25 m i l alumi n i z e d Mylar was wrapped around the f l a s k and some of the support s t r u c t u r e . While t h i s i n c r e a s e s u n d e s i r a b l e s c a t t e r i n g , i t s absence would r e s u l t i n bubbles i n the J ^ H ^ which are very u n d e s i r a b l e (see s u b s e c t i o n 3.4.1). A 50 cm long f l a s k w i l l be used i n n-p s c a t t e r i n g . Smaller f l a s k s w i l l be used with the t a r g e t f c r p-p s c a t t e r i n g and other experiments. \ 16 3.2.2 Some Flask Construction Technigu.es The end forms of the flask are made using the following technique: the Mylar i s clamped i n a dye and the whole assembly i s heated to 250°C; the Mylar becomes p l a s t i c at t h i s high temperature and i s then 'punched' with the desired shape punch. Epoxy adhesives have proven r e l i a b l e f or joining the pieces of Mylar. A great deal of care was taken when preparing these j o i n t s . Scrupulous cleanliness was esse n t i a l for a l l steps i n the procedure. The joi n t clearances were such that the adhesive thickness i s .003" + .0005". These join t s have great sheer strength but very l i t t l e 'peel' strength so the design has only sheer j o i n t s . The procedure for making this joint was 1. Shot-blast material with Air Brasive (27 micron aluminium oxide) . 2. Bond using equal parts by weight of EPON 819 epoxy and Versamid 140 hardener. Osing a dash of Silane (a wetting agent) improves bending. 3. Allow to cure at room temperature for 24 hours. 17 3.2.3 f l a s k Suj^cort Structure In supporting the f l a s k , materials with high strength at low temperatures have been used. Niobium s t a b i l i z e d austenitic s t a i n l e s s s t e e l (type 347) i s the preferable material because of i t s good insulating properties and i t s good vacuum properties. Other s t a i n l e s s steels have f a u l t s in t h e i r c r y s t a l structure allowing more leakage. To prevent weld decay (carbon accumulation causing 'holes' in the steel) the s t e e l i s s t a b i l i z e d with niobium or titanium. However the phencnema of titanium streaking also allows seepage (niobium s t a b i l i z e d s t a i n l e s s s t e e l does not have a s i m i l a r phenonemum). However, titanium s t a b i l i z e d s t a i n l e s s s t e e l (321) was used i n the pipe construction. The f l a s k i s supported by 3 pipes connected to the vacuum vessel wall. Three pipes, the heat pipe, the b o i l off pipe and the f i l l e r pipe, connect i t to the r e f r i g e r a t o r . I f higher thermal conductivity Is reguired (e.g. i n a radiation shield) deoxidized high conductivity copper (e.g. DEE grade) i s the best material; CDA110 type was used however. Aluminium may also be used but i s more d i f f i c u l t to fabricate. FIG.3.2 Y i e l d Stress vs. Temperature (Mylar) 8 x - 2 ' 2 0.81 0.44 —I 1 I I 50 100 ISO 2.00 T0MP£RATUftE (aK) FIG. 3.3 Young's Modulus vs. Temperature (Mylar 20 3.3 The Refrigeration System 3.3.1 Befrijgerator And Condensing Vessel The c o e f f i c i e n t of performance (or effi c i e n c y ) of a re f r i g e r a t o r i s given by For an n-stage r e f r i g e r a t o r (with egual temperature differences between the stages) the e f f i c i e n c y of each stage (therefore of stages, the more e f f i c i e n t the r e f r i g e r a t o r . However, this has to be balanced with increasing complexity of equipment with more stages plus the fact we don't have i d e a l refrigerators.-The t h e o r e t i c a l e f f i c i e n c y cf a one stage r e f r i g e r a t o r cooling from 300°K to 20°K i s CP=7.1%. To increase e f f i c i e n c y , the r e f r i g e r a t o r used in the system i s a two stage r e f r i g e r a t o r . The t h e o r e t i c a l best intermediate stage temperature i s the geometric mean of the ambient and low temperatures, i . e . for a low temperature of 20°K and an ambient temperature of 3 00°K we have "Ti^fci^^ a°ont 80* K which gives a the o r e t i c a l e f f i c i e n c y of about 30%. The refrigerator considered here has an intermediate stage at 77°K. A schematic of one stage of the operation i s shewn i n _ It ~ A T and where ~Tp = f i n a l (cold) temperature AT = T**i>w -Tf the whole refrigerator) i s greater than CP. Clearly the more 21 figure 3.4. The thermodynamic cycle i s a modified S t i r l i n g cycle. If piston C i s down i n i t i a l l y with valve B closed, then C i s forced up, compressed gas i s forced into the cylinder. Valve A i s then closed and B i s opened, resulting in decompression i n the cylinder. The resultant temperature gradient causes a heat flow from the load into the cylinder. The ess e n t i a l feature of the r e f r i g e r a t o r i s the heat-exchanger or regenerator. This cools the gas coming into the piston and heats the gas leaving the cylinder. Also, as ordinary pistons reguire pressure seals to account f o r the unbalanced forces in them, a double-ended displaces i s used which allows egual pressure at both ends. The regenerator i s inside the displacer i t s e l f . Figure 3.5 shows the 2-stage r e f r i g e r a t o r . To cool the hydrogen, the gas i s passed through a pipe co i l e d around the f i r s t stage, res u l t i n g in cooling tc about 80°K. In the second stage the gas cools to about 20*K. The l i q u i d c o l l e c t s i n the condensing vessel, passing to the flask via the f i l l e r pipe. The heat_pj.£e i s used to cool the target and to keep i t cool. It i s a tube closed at the bottom and f i l l e d with porous material. The pipe enables the target flask to be kept at e s s e n t i a l l y QEX temperatures even i f the f l a s k i s emptied. This allows the f l a s k to be evacuated and r e f i l l e d with almost no cooldown period when background measurements are desired. Because of the low temperatures in the condensing vessel, helium actuated valves {rather than air) are used i n the f i l l e r and b o i l - o f f pipes. This i s accomplished through actuator cylinders which convert a i r pressure to helium pressure. 22 The condensing vessel was made from type 321 stainl e s s s t e e l . Table 3.1 gives the refrigerator s p e c i f i c a t i o n s . 3.3.2 Compressor The Compressor serves three purposes: 1. I t supplies helium to the re f r i g e r a t o r at the required pressure and rate. 2. It recompresses the helium returned from the r e f r i q e r a t c r . 3. I t removes from the helium both the heat of recompression and the heat absorbed from the ccndensor, Helium i s compressed in the compressor pump and in the process gets some o i l mixed with i t . This o i l i s necessary for proper cooling of the gas. After cooling, the gas goes through separators and the charcoal adsorbers which eliminate almost a l l of the o i l . The helium i s then supplied to the r e f r i g e r a t o r . When replacing the cover of the compressor unit care was exercised to ensure i t was correctly positioned so the cooling a i r flew was not blocked. To ensure adequate cooling, a minimum clearance around the sides, top and back of the compressor of 12-inches i s needed. A thermal switch automatically shuts down the compressor i f the outgoing gas exceeds 280 "P.... Automatic shutdown also occurs i f the compressor motor gets too hot. These switches are automatically reset when acceptable temperatures are regained. The most l i k e l y causes of overheating are either a blocked o i l o r i f i c e or a plugged o i l f i l t e r . 23 Load Regenerator F i l l i n g Cycle /\Cofrtpressor Regenerator Decompression Cycle Compressor Regerv«rat«r Emptying Cycle Compressor FIG. 3.4 Schematic of Refrigerator Operation FIG. 3.5 Two stage Refrigerator 25 SYSTEM SPECIFICATIONS POB_THE_ll2BEI^122P„£BX2DXfiI_S£ISIfiliil2S Heat C a p a c i t y REFBIG EE ATIO N STATION NO. 1 10 watts at approximately 77°K of a d d i t i o n a l r e f r i g e r a t i o n c a p a c i t y a v a i l a b l e at R e f r i g -e r a t i o n S t a t i o n No. 1. Heat C a p a c i t y REFRIGES ATION STATION NO. 2 HEAT_LGAD TE'MPEBATURE J KI I w a t t s J i l l Hzl J50 H z l 2 4 10 15 14. 2 19.0 26.0 14.0 15.5 24.0 Ambient Temperature Operating Range Net weight Approximate Dimensions (in inches) P r e v e n t i v e F i e l d Maintenance I n t e r v a l -32 C to +52 C (-25 F to +125 F) 15 Kg (33 pounds) 20-1/2 high, 9 wide, 16 long 6000 hours Power Beguirement 100w, 160V, 50 or 60 c y c l e s , s p l i t phase (sup p l i e d frcm c o n t r o l panel) Nominal Size Type Operating Pressure ambient Temperature Operating Sange Mounting Approximate Dimensions (in inches) Weight Preventive F i e l d Maintenance Interval Power Eeguirement 3 hp 2-cylinder reciprocating, o i l - l u b r i c a t e d , air-cooled Varies with temperature; 275 psig supply, 75 psig return under normal oper-ating conditions and charging pressure -4 °C to +52 °C ( + 25 °F to +125 °F) Horizontal ±10° 26-7/16 high, 26-1/4 wide, 41-1/4 long 148 Kg (425 pounds) 6000 hours (pump) 3000 hours (adsorber) 5.5 KS, 220 or 440 V 27 3.4 Vacuum System 3.4.1 The Need For J Vacuumj, Su^erinsulation Vacuum i s necessitated by insulation requirements. With pressures greater than 10"4 Torr, a i r i s a s u f f i c i e n t l y good conductor that the r e f r i g e r a t o r could not handle the heat load (see figure 3 . 6 ) . Also, a i r would condense on the flask providing a source of undesirable scattering. Thus a d i f f u s i o n pump was used to provide a good vacuum surrounding the f l a s k . The NRC pump i s capable of pressures less than 10"7 Torr, more than adequate. Even with high vacuum, there i s a great deal cf heat transferred by radiation from the vacuum vessel walls to the cooled flask, pipes and condensing vessel. Around the condensing vessel a heat sh i e l d (of copper cooled to l i q u i d nitrogen temperatures (77°K)) was i n s t a l l e d , providing some protection against r a d i a t i o n , but t h i s was not possible for the target flask as i t would cause background scattering. without any protection, the J?Hi i n the flask would form bubbles, which would make precise measurements impossible. A discussion cf 'superinsulation» w i l l follow calculations below showing the need for i t . The radiational heat flux between two bodies i s given by the Stefan Bcltzmann law as (3.2) where unit time and area from body 1 to 28 body 2, (T~ = Stefan Boltzmann constant = 5,67 1Q-5 erg cm-2 deg"* ^temperatures of the surfaces (in •K) £ =effective emissivity. E. depends on the e m i s s i v i t i e s , the shapes and the sizes of the two surfaces. The f i v e l i t r e f l a s k has an area of about 2,000 cm2. With temperatures of 300«K and 20°K for the vacuum vessel and the f l a s k respectively, and conservatively estimating P = Q < A r t a = 9.2 watts s u f f i c i e n t to evaporate about one l i t r e of l i q u i d per hour (see page 4 cf reference 9). This would amount to about 15 cm3 of gas per second being formed i n the f l a s k . Also, the capacity of the r e f r i g e r a t o r would be exceeded. To avoid these d i f f i c u l t i e s , superinsulation was used. This consists of numerous sheets of highly r e f l e c t i v e material with low conducitivity between the hot and cold surfaces. If there are H sheets, then assuming only r a d i a t i v e heat transfer and that the temperature cf the intermediate sheets is E = 0.1, the power transferred i s temperature of the th sheet from the hot surface temperature of the cold surface temperature of hot surface then 29 Thus heat l o s s i s reduced by a f a c t o r of 7^+7 . With h i g h l y r e f l e c t i v e aluminized Mylar, E i s a l s o reduced compared with eguation 3.2, reducing heat l o s s f u r t h e r . T h e r e f o r e , 20 sheets of 0.25 m i l a l u m i n i z e d Mylar surrounded a l l c o o l e d components. The sheets were c r i n k l e d to keep the area of c o n t a c t ( i . e . the conductive heat t r a n s f e r ) a minimum. 3.4.2 Vacuum System Used Some necessary o p e r a t i o n a l notes are presented below. The pump i s g u i t e simple and presented no d i f f i c u l t i e s as long as i t was p r o p e r l y t r e a t e d . I t i s e s s e n t i a l t h a t c o o l i n g water i s f l o w i n g c o n t i n u o u s l y when i n o p e r a t i o n as o v e r h e a t i n g w i l l r e s u l t i n poor performance and l o s s of f l u i d . There i s a automatic o v e r h e a t i n g c u t - o f f switch which must be manually r e s e t when t r i p p e d . The backing pressure of the pump must be maintained as low as p o s s i b l e : 0.5 Torr to 20 mTorr or b e t t e r . Before s t a r t i n g the pump, the pressure i n the vacuum v e s s e l must be below 0.5 Torr ( p r e f e r a b l y about 100 mlorr) . I t should not operate f o r l o n g p e r i o d s of time at pressures above 1 mTorr as l o s s of f l u i d and damage to the pump can occur. These c o n d i t i o n s are monitored and an alarm w i l l sound upon overheating or pcor vacuum (see Chapter 4 ) . While i t i s p o s s i b l e t h a t poor vacuum v e s s e l pressure c o u l d be caused by a f a u l t .in the pump, i t i s f a r more l i k e l y that i t i s due t c a leak . In case of poor pump performance, check the f l u i d l e v e l f o r contaminants or low f l u i d l e v e l . Some 30 s p e c i f i c a t i o n s cf the pump are given in Table 3.3. Tc back the d i f f u s i o n pump, two Alcatel rotary pumps were used, both capable of pressures of 5 10"3 Torr. One pump i s adequate for the purpose and the other i s used as a backup i n case of f a i l u r e . If f a i l u r e occurs the o i i l e v e l should be checked and o i l added i f necessary (The sight hole i s e a s i l y v i s i b l e cn the pump). These pumps produce a rough vacuum which i s adequate for a l l purposes except the vacuum vessel. These pumps are also used to rough cut the vacuum vessel, to provide a vacuum for the dump tank and the beam windows, and tc serve several ether functions which may occasionally be needed (see figure 3.1). If f c r some reason i t i s decided to evacuate the fl a s k and maintain a vacuum i n i t , care must be taken that the vacuum vessel pressure i s less than the flask pressure for the fla s k w i l l l i k e l y implode. Seme sp e c i f i c a t i o n s of the rotary pump are given i n Table 3.4. The vacuum vessel was made out of high quality materials which must be homogeneous and free from vacuum leaks. It was as thin as possible so that unwanted scattering i s kept at a minimum, especially in the immediate v i c i n i t y of the flask i t s e l f . A high strength aluminium alloy (type 6061) and st a i n l e s s s t e e l (type 321) were used in the vacuum vessel construction. FIG. 3.6 Heat Flow vs. Pressure 32 TABLE 3 A3 MJ4 CIJFOSION PUMP VABIAN VACUUM DIVISION,/NEC OPEHAIICN Cooling water Flow 0.13 GPM a 6C-80°F cr 0.049 1 per minute 2 15-27°C Power Eeguirements 120 V 1190 watts Fl u i d Capacity 250 ml TABLE 3.4 SPECIFICATIONS FOB THE ALCATEL MODEL 1030_PBIjMABI_VACUOM_PUMP Pumping Speed: 1 atmosphere " " : 1 Torr Ultimate vacuum Motor O i l Capacity Net Weight Power Eeguirement 50G 1/min. {17.7 cfm) 450 1/min. <16 cfm) 5 -lO"3 Torr 1 HP 3 l i t r e s 40 kg. (88 6b.) 1.1 Kw 3 phase 220 V (with 440 V option) 60 Hz. motor 33 Safety And Alarm Ee vices. Many precautions have been taken to prevent malfunctions from causing serious damage. Of primary importance i s the handling of hydrogen in case of leakage or flask rupture. The system was designed either to keep the hydrogen contained (away from air) or, i f released, tc be diluted to non-explosive l e v e l s . Valve VD 4 Is a pressure operated valve designed to open i f the pressure inside the vacuum vessel rises abcve 0.5 p s i . When opened i t vents the contents of the vessel to the dump tank (which consists cf one cr two 1900 l i t r e tanks, depending on the siz e of the flask used) where i t i s kept away frcm a i r . One l i t r e cf J?HZ expands to 840 l i t r e s of gas at SIP, as mixtures of a i r and hydrogen containing between 4$ and 74$ hydrogen are explosive, care must be taken to insure that i f hydrogen i s vented cut, i t has concentrations l e s s than 4$. Rotary pumps maintain the vacuum in the dump tank: i f hydrogen i s vented into the dump tank, some may go through the pump into the pump exhaust the pump exhaust i s largely nitrogen (nitrogen can be bled into the the exhaust through VP23). This should eliminate any chance of explosion in the system, but care must be taken at the outlet of the exhaust (on the roof) to prevent possible i n j u r y . A fan vents a i r from the immeditate v i c i n i t y of the target (using a 'house' on top cf the target) to the reef top. A hydrogen detector incorporated into t h i s sounds an alarm i f hydrogen i s i n the a i r . The TEIDMF safety group i n s t a l l e d and maintains the alarm. It i s Independent of the rest of the system 34 so that i f other control monitoring systems f a i l , i t would s t i l l he operative. The alarm system i s both audio and visual. vAccepting the alarm stops the 'siren' hut the l i g h t goes green only when the aaIfunction i s corrected. (There i s no reset). In case a di f f e r e n t malfunction occurs while the f i r s t malfunction (s) i s (are) being corrected, the siren w i l l sound again. The following i s a l i s t of the alarm causing malfunctions and some probable causes of the malfunctions. Nominal alarm l i m i t s are also given. 1. Diffusion pump temperature; Overheating could be caused by inadeguate flow cf cooling water cr too high backing pressure (alarm 5). If i t remains uncorrected, i t forces shutdown of the system. 2. Hydrogen gas pressure: Leakage from the tank cr in the supply system i s the probable cause of lew pressure. Alarm set for 15 ps i . Some other malfunction e.g. high vacuum vessel pressure, w i l l probably also occur i f there i s an inter n a l leak. 3. Air pressure: F a i l u r e of TBIOMF * s a i r pressure supply system may be the cause. Generally, the nitrogen backup can take over so t h i s i s net c r i t i c a l . Alarm set for 65 p s i ; must be 68 psi to reset. 4. Nitrogen pressure: need to replace the nitrogen cylinder. Alarm set for 350 p s i ; 450 psi needed to reset alarm. 5. Diffusion pump lacking pressure: May be due to f a i l u r e of rotary pump or leak i n system. Switching to the au x i l i a r y rotary pump may eliminate the problem u n t i l a more convenient time i s availab l e to check the pump. If the problem cannot be solved. 35 system shutdown i s necessary. Alarm set for 100 mTorr. 6. Vacuum vessel pressure: Failure of d i f f u s i o n pump.or leakage in either the f l a s k or vacuum vessel. System shutdown necessary i f problem cannct be corrected. Alarm set at 1 mTorr. 7. Dump tank pressure: i f due to f a i l u r e of rotary pump then can be eliminated by switching to the other rotary pump. Alarm set at 2 Terr. 8 . Extraction flow: Fan may be blocked or malfunctioning. Alarm set for 1000 cu.ft.sec" 1. 9. Dump tank l i n e valve (VD5). Closed for some reason. There are numerous e l e c t r i c a l i n terlock devices to prevent damaging situations from a r i s i n g . Some operate when the system i s i n the automatic mode; others w i l l shut off devices when damaging levels are reached. Most of the devices control the vacuum pumps and associated valves. 36 5 G^eratincj Characteristics.. 5. 1 Operation (See diagram 3.1) The following seguence must be followed to get the system f u l l y operational: 1. Ensure the vacuum was as required for good i n s u l a t i o n . 2. Deliver hydrogen to the r e f r i g e r a t o r . 3. Cool the hydrogen to l i g u i f i c a t i o n temperature. 4. Deliver the Gh\ to the f l a s k . The f i r s t step in achieving the desired vacuum was to rough out the vacuum vessel using the rotary pumps. When the vacuum vessel pressure was below 100 mlorr the d i f f u s i o n pump was started and the pressure brought to the desired l e v e l (~10"3 mlorr). The dump vacuum was maintained by the roughing pump by occasional evacuations (every day or two). The p u r g e / f i l l module was used to put ultrahigh purity hydrogen into the reservoir tank. Great care was .taken to ensure that hydrogen was not mixed with a i r except at the roof l e v e l pump exhaust and there i n very small quantities. This was accomplished by f i l l i n g the tank with helium and roughing i t out. This procedure was repeated twice. The tank was then f i l l e d with hydrogen and roughed cut a t h i r d time and f i n a l l y f i l l e d with hydrogen to 30 p s i . As the deoxc and p u r i f i e r do not remove nitrogen from the hydrogen, a molecular sieve was also used. After p u r i f i c a t i o n , the hydrogen was delivered through the f i r s t stage cooling cf the re f r i g e r a t o r tc the second stage 37 where i t was l i q u i f i e d . This l i q u i f i c a t i c n reduces the pressure to just over 16 p s i . As the l i q u i d i s forming, some goes into the f l a s k , f i r s t cooling i t , then f i l l i n g i t . To empty the fla s k , the b o i l - o f f pipe is closed, thus permitting a buildup of pressure in the flask (by evaporation of hydrogen) forcing the Ittx back through the f i l l e r pipe. The fl a s k can be emptied completely by closing the f i l l e r pipe and evacuating using the roughing vacuum. For safety, reasons, the switches are explosion proofed and most of the valves are pressure actuated, compressed a i r i s supplied from the TRIUMF compressed a i r supply system and i s cleaned before use (air pressure fluctuated from 70 to 90 p s i ) . In case of f a i l u r e of the a i r supply system, there i s compressed nitrogen as a backup. The surge tank smooths out pressure fluctuations due to valve actuations. 5.2 Operational Characteristics The results of actual operation of the target are presented i n the following pages. A one l i t r e flask was used. I t takes approximately 18 hours to f i l l the flask with !H V. A larger flask would require mere time. Before cooldc«n the dump tank pressure was at 250 mTcrr. If i t was at atmospheric pressure, i t would take about two hours to pump down to l e v e l . The vacuum vessel pressure was 5.5 10~3 mTorr. This the pump down time from atmospheric pressure was approximately 5 hours. This time was reduced to about 1 hour i f a rough vacuum was maintained. 38 Crycpumping reduced the vacuum v e s s e l pressure to 2.0 10"* T o r r within an hour a f t e r s t a r t i n g cocldown and i t g r a d u a l l y goes dean t c 1.0 10* Torr when cocldown i s complete. The helium (for the r e f r i g e r a t o r ) (supply/return) pressure s t a r t e d at (285/30) p s i g , changed t c (260/70) p s i g w i t h i n three hours, then g r a d u a l l y went to (250/70) p s i g as the ccoldcwn was completed. F i g u r e 4.1 shows the time e v o l u t i o n of pressure i n the condensing v e s s e l (as measured with gauges TH1 or TH2). When the pressure i s between 16.0 and 16.5 p s i a , valve BE5 i s a u t o m a t i c a l l y opened or c l o s e d to maintain constant condensing v e s s e l pressure ( i . e . temperature). Fi g u r e 4.2 shows the decrease i n temperature with time. The temperature i s represented both i n vapour pressure of hydrogen (from gauge GH3) and i n degrees K e l v i n (derived from hydrogen vapour p r e s s u r e ) . 14-1 - — i 1 1 1 1— 0 4 8 IX. 16 10 Cooldown Time (hours) FIG. 4.1 Flask Pressure vs. Cooldown Time 40 41 6 I i b l i c c r a £ h ^ 1. 8.N.Hess, Rev.Mod.Phys. 30 368 (1958) 2. Triumf Kinematics Handbook 3. CEC Handbook of Chemistry and Physics, 49th Edition 4. C. Chamberlain et a l . , Phys. Rev. 102 1659 (1956). 5. I.E. Mortimer and J. E. Stokoe, 1971, A l i q u i d Hydrogen or Deuterium Target System with Closed Cycle Refrigeration RHEL/R 237 Rutherford laboratory Report. Chilton Didcot Eerkshire. 6. Cryogenic Materials Data Handbook, Edited by F R. Schertzeber, The Martin Company, Denver, Colorado. 7. Effects of Radiation on Materials and Components, edited by J. F. Kircher and R* E. Bowman, New York, Rienhold 1964. 8. Formulas for Stress and Strai n , B. J . Roark, 4th ed i t i o n , New York, McGraw-Hill, 1965 9. Thermal Insulation, edited by S.D. Probert and D.B. flub, Amsterdam, New York, Elsevier , 1S68 10. Heat and Thermodynamics, K. H. Zemansky, 5th e d i t i o n , York, McGraw-Hill, 1968 


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