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RF control of the M9 separator at TRIUMF Burge, R. 1990

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RF CONTROL OF THE M9 SEPARATOR AT TRIUMF by RAYMOND STANLEY BURGE B.A.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1984  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  in THE DEPARTMENT OF ELECTRICAL ENGINEERING  We accept t h i s t h e s i s as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1990 (E)Raymond S t a n l e y B u r g e ,  1990  In presenting  this  degree at the  thesis  in  partial fulfilment  of  University of  British Columbia,  I agree  freely available for reference copying  of  department  this or  publication of  and study.  thesis for scholarly by  this  his  or  her  purposes  representatives.  fwcjMecrM^  The University of British Columbia Vancouver, Canada  Date  DE-6 (2/88)  Ktxy  HlO  that the  may be It  thesis for financial gain shall not  £7ccrVic<J  requirements  I further agree  permission.  Department of  the  that  advanced  Library shall make it  by the  understood be  an  permission for extensive  granted  is  for  that  allowed without  head  of  my  copying  or  my written  Abstract  High voltage RF systems are used to a c c e l e r a t e proton beams f o r nuclear physics experiments. The a c c e l e r a t i o n process shapes the proton beam into a t r a i n of narrow pulses w i t h the same period as the  RF. This  bunched  beam s t r u c t u r e  i d e n t i f y secondary p a r t i c l e s  i s used  to separate and  that are produced when the proton  beam i s d i r e c t e d at a "target". An RF c o n t r o l l e r f o r a system that separates secondary p a r t i c l e s was b u i l t .  Control of high power RF c a v i t i e s that operate near resonance i s discussed.  The emphasis  i s on developing a c o n t r o l  model f o r  resonant systems and b u i l d i n g a c o n t r o l system based on hardware and software modules that can be e a s i l y configured f o r d i f f e r e n t RF systems.  i i  Table of Contents  page Abstract  <  Table of Contents  i i i i i  L i s t of Figures  v  Acknowledgement  vii  1. Introduction 1.1  RF Systems and P a r t i c l e Accelerators  1  1.2  An RF Separator f o r Muon Studies  4  1.3  System Requirements  6  2. The RF C a v i t y and Power A m p l i f i e r  8  3. Transfer Functions f o r RF Regulation 3. 1  An RLC Model  12  3.2  A Simple Transfer Function f o r the Separator  13  3.3  Matrix Transfer Functions  3.4  f o r Amplitude and Phase Regulation  16  P a r t i c l e Beam Loading as System Disturbance  21  4. Closed Loop Control 4.1  Phase and Amplitude Loops  25  4.2  The C a v i t y Tuning Loop  30  iii  page 5. Hardware Implementation of the C o n t r o l l e r 5. 1  Overview  35  5.2  System Description  35  5.3  S e l f - E x c i t e d Operation  38  5.4  S e l f - E x c i t e d and Driven Tuning Systems  40  5.5  The Frequency Comparator  43  5.6  Spark Detection  44  5.7  System Hardware Configuration  45  5.7  Regulator E l e c t r o n i c s  47  6. Software 6.1  The Main Program Loop  51  6.2  Task Communication  52  6.3  Control Tasks  53  6.4  Front Panel Input  54  6.5  Auto S t a r t  57  7. Conclusions  60  8. References  61  iv  L i s t of Figures  page 1.  D r i f t - t u b e Linac s t r u c t u r e  2  2.  Time s t r u c t u r e of the TRIUMF beam  4  3.  R e l a t i v e phase o f TT, U , and e beam components  5  4.  Schematic of the separator c a v i t y s t r u c t u r e  8  5.  Push-Push model of the c a v i t y and p l a t e c i r c u i t  9  6.  Push-Pull model of the c a v i t y and p l a t e c i r c u i t  10  7.  Equivalent c i r c u i t of high Q elements  11  8.  Lumped RLC resonator  12  9.  Pulsed power t e s t s  14  10. One dimensional  t r a n s f e r f u n c t i o n f o r the RF system  15  11. L i n e a r i z e d model f o r high Q c i r c u i t elements  21  12. Beam loading seen by a s i n g l e voltage probe  22  13. Block diagram of the regulator loop  26  14. RF c o n t r o l space  27  15. Reguator s i m u l a t i o n showing cross term r e j e c t i o n  30  16. C a v i t y tuning mechanism  31  17. C a v i t y tuning loop  31  18. Tuner small s i g n a l model  33  19. Basic c o n t r o l l e r concept  35  20. Control modules  36  21. Front panel d i s p l a y  37  v  page 22. S e l f - e x c i t e d c o n f i g u r a t i o n  39  23. P r i n c i p a l system states  39  24. S e l f - e x c i t e d tuning loop  41  25. Driven tuning loop  42  26. S i g n a l switch f o r tuner and phase r e g u l a t o r  43  27. Frequency comparator  44  28. Spark detector  44  29. Separator RF c o n t r o l modules  46  30. PID Regulator and RF System Model  48  31. S i m p l i f i e d regulator schematic  49  32. D i s c r e t e component d i f f e r e n t i a t o r  49  33. D i f f e r e n t i a t o r simulation  50  vi  Acknowledgement  I am much indebted to Dr. R. Ward f o r her guidance i n my  studies  and her encouragement i n the preparation of t h i s t h e s i s .  Thanks are also due to the members of the TRIUMF RF group MR. L Durieu of CERN and MR. P. Sigg of PSI. due t o Mr. B. Chow and Mr. G. c o n t r o l system  and without  Dennision whose  p r o j e c t could not have succeeded.  vi i  who  support  Special  and t o  thanks  are  build  this  and d i l i g e n c e  this  helped  Chapter 1 Introduction  1.1  RF Systems and P a r t i c l e  Accelerators  H i g h power RF systems a r e used t o c o n t r o l and a c c e l e r a t e beams i n n u c l e a r p h y s i c s  laboratories.  In  this  process  d e s i r a b l e that the RF system be as easy to o p e r a t e supply  in  an  industrial  environment.  Before  r e q u i r e m e n t s f o r r e g u l a t i o n and c o n t r o l ,  it  particle  as  it  any  power  discussing  is useful  to  is  the  introduce  the b a s i c f u n c t i o n o f RF systems i n modern p a r t i c l e a c c e l e r a t o r s . P a r t i c l e a c c e l e r a t o r s a r e machines t h a t generate sufficient  k i n e t i c energy t o produce  nuclear  ion  beams  reactions.  Usually  the a c c e l e r a t e d p a r t i c l e s a r e e l e c t r o n s  o r p r o t o n s and the  energy o f a p a r t i c l e  electron  electron volt  is  measured  in  volts  d r o p o f one v o l t .  in  Modern p r o t o n  passing  machines  through  (eV).  An  can  a  potential  produce  particles  w i t h e n e r g i e s o f many b i l l i o n e l e c t r o n v o l t s  (GeV).  Charged p a r t i c l e s a r e a c c e l e r a t e d  by  first  kinetic  i s the energy g a i n e d by an e l e c t r o n (or any p a r t i c l e  w i t h the same magnitude c h a r g e )  The  with  high  energy  i n vacuum  accelerators  were  C o c k c r o f t - W a l t o n and Van de G r a a f f g e n e r a t o r s .  electric  DC  fields.  machines  using  These h i g h  voltage  s o u r c e s c a n produce p a r t i c l e beams w i t h e n e r g i e s o f a few  million  electron volts limit  (MeV).  The s t a t i c  fields  encountered i n DC machines  the k i n e t i c e n e r g y g a i n e d by the p a r t i c l e to  1  the  potential  energy of the system.  The  maximum  energy  of an  electrostatic  a c c e l e r a t o r i s determined by the voltage that can be maintained at the high voltage t e r m i n a l . With careful design t h i s can be as high 7  as 10  volts.  C y c l i c a c c e l e r a t o r s were developed to overcome the l i m i t s  imposed  by e l e c t r o s t a t i c f i e l d s . In these machines, p a r t i c l e s are made t o f o l l o w a path where they p e r i o d i c a l l y  encounter  voltage which i s a small f r a c t i o n of the f i n a l  an a c c e l e r a t i n g energy  gained  by  the p a r t i c l e . The p a r t i c l e t r a j e c t o r y can be s t r a i g h t as i n l i n e a r a c c e l e r a t o r s (Linacs), o r the t r a j e c t o r y  can be curved  as i n  c y c l o t r o n s and synchrotrons. High k i n e t i c energies are b u i l t up as the p a r t i c l e encounters the a c c e l e r a t i n g voltage a large number of times. T h i s process develops a gradual a c c e l e r a t i o n which  i s not  l i m i t e d by the voltage drop i n the machine.  ion source  0  RF System  o -> to target  o o o -» J  fig.  1  L  Drift-tube  Linac  structure.  Large energy gains are p o s s i b l e i n c y c l i c a c c e l e r a t o r s because the e l e c t r i c f i e l d s i n the machine change with time. Such systems are nonconservative i n that i t i s possible to f i n d a closed path along which the k i n e t i c energy gained  by the p a r t i c l e  2  i s not zero.  Figure  1  shows  demonstrates  the  that,  structure with  of  correct  a  drift  phasing  tube  of  Linac.  the  It  accelerating  voltage, one can inject charged particles at zero volts and  later  extract them into a zero volt region with a higher kinetic energy. Acceleration occurs in the gap between the drift tubes. While particle is inside the drift region and does  not  tube,  experience  it  any  is  in  change  an in  the  equipotential energy.  Cyclic  accelerators use equipotential regions, such as the drift tube, to allow the voltage time to achieve the correct amplitude and as the particle approaches the next acceleration  gap.  machines, the particle velocity quickly approaches light and,  In  the  modern  speed  in order to keep the size of the machine small,  This  minimizes  the  drift  tube  length  of  i t is  advantageous to use high frequency voltages to accelerate particles.  phase  charged  and  allows  resonant RF structures to be used as the accelerating elements.  Most  nuclear  reactions  of  interest  to  researchers  occur  relatively rare events. A great number of particles with defined kinetic energy are needed to  study  these  a  events.  as well  Since  there is an equivalence between mass and energy i t is important to know the threshold energy needed to produce the  exotic  particles  that make up the nucleus of the atom. To get meaningful statistics an experiment can run for a year at low event rates. The  task  of  the accelerator designer is to produce a machine that delivers  as  much beam as possible at a very well  defined  energy.  In  cyclic  accelerators a well defined energy occurs over a small range of RF phase and voltage. The result is that accelerators produce or buckets of particles at a given energy. This is  3  true  pulses for a l l  c y c l i c a c c e l e r a t o r s i n c l u d i n g the TRIUMF c y c l o t r o n ( f i g . 2).  2  fig.  Typical  time structure  of the TRIUMF proton beam.  C y c l i c a c c e l e r a t o r s produce beam pulses at the RF r e p e t i t i o n r a t e w i t h a t y p i c a l pulse occupying approximately 30° of an RF c y c l e . Control  of high  i n t e n s i t y , high  energy p a r t i c l e  beams r e q u i r e s  c a r e f u l c o n t r o l of the RF phase and amplitude.  1.2  An RF Separator f o r Muon S t u d i e s  P a r t i c l e s extracted  from the a c c e l e r a t o r are c o l l i m a t e d  into a  beam and are d i r e c t e d to a "target" where the desired r e a c t i o n s occur. be  nuclear  Secondary p a r t i c l e s produced i n the r e a c t i o n can  s e l e c t e d and used i n f u r t h e r s t u d i e s . One of the p a r t i c l e s  produced i s the pion, an exchange p a r t i c l e that operates i n s i d e the nucleus. positive  An atomic nucleus i s made up of protons which have a  charge and neutrons which have zero  charge.  The net  charge of the nucleus i s p o s i t i v e and since l i k e charges r e p e l , the  nucleus should  f l y apart.  The pion  i s an exchange p a r t i c l e  that transmits a short range a t t r a c t i v e force which allows s t a b l e nuclei  to exist.  This p a r t i c l e  i s one of a group o f p a r t i c l e s  c a l l e d mesons which are the object of much study at TRIUMF.  4  The name pion i s a short form of ir-meson. Outside the nucleus t h i s p a r t i c l e has a high p r o b a b i l i t y of q u i c k l y decaying to a jx-meson (muon). A f t e r a longer time, the muon w i l l decay into an e l e c t r o n . Pions are produced when a pulse of high energy h i t s a B e r y l l i u m target i n the beam  line.  protons  (>100Mev)  The groups  o f pions  l e a v i n g the target have the same time s t r u c t u r e as the high energy protons from the TRIUMF c y c l o t r o n .  A cloud o f muons w i l l begin to form around the pions as they decay i n f l i g h t from the production target. The p a r t i c l e s have and d i f f e r e n t energies such that the pions, form separate suppressed  groups  i n flight.  Pions  muons  distinct  and e l e c t r o n s  and e l e c t r o n s  can  t o l e s s than 1% of the beam by a s t a t i c magnetic  and a perpendicular  RF e l e c t r i c  field.  Particles  be  field  a r e passed  through the separator when the RF e l e c t r i c f i e l d cancels  the  vxB  f o r c e of the h o r i z o n t a l magnetic f i e l d . By a d j u s t i n g the phase o f the RF voltage, p h y s i c i s t s can l e t pions, e l e c t r o n s , or muons i n t o the  experiment.  T i m e (ns)  fig.  3  Relative  phase of n, u, and e beam components.  5  1.3  System Requirements  TRIUMF i s s i m i l a r t o a u t i l i t y company that d e l i v e r s  high  energy  protons t o i t s customers and a l s o provides t e c h n i c a l s e r v i c e s t o the various experiments that use the proton  beam.  A device t o  separate decay products i n f l i g h t from the pion production  target  i s one of the pieces of equipment b u i l t f o r muon s t u d i e s . The muon physics experiment i s a large and complex the RF separator i s only a small part.  The separator  d r i v e n at 23 MHz, the c y c l o t r o n operating phase locked to the primary proton beam. operate;  an on-off  switch  installation  frequency, I t must  and a knob  o f which cavity i s and i t  is  be simple t o  to select  different  p a r t i c l e s , n, u, or e. The RF requirements f o r the separator can be l i s t e d i n three s e c t i o n s .  1. RF Generator,  Cavity,  and Deflection  Plates  • a frequency of 23MHz. • a power output of 120Kw • a peak p l a t e - t o - p l a t e c a v i t y voltage of 360Kv  2. RF Regulator  • phase locked t o the proton beam extracted from the c y c l o t r o n • phase s t a b i l i t y b e t t e r than 1° 3  • amplitude s t a b i l i t y b e t t e r than 1 part i n 10  3. RF Controller  • one button a u t o s t a r t • automatic c a v i t y tuning  6  • automatic spark recovery and f a u l t handling • a s e l f - e x c i t e d , i d l e s t a t e when the c y c l o t r o n RF i s o f f • l o c a l d i s p l a y of the system state • manual c o n t r o l of a l l loop parameters and system s t a t e s  This report describes the a n a l y s i s and design the c o n t r o l l e r , and the RF s i g n a l i n s t a l l e d on the RF separator.  modules  of  that  A c o n t r o l model  the were  regulator, built  i s developed f o r  r e g u l a t i o n and tuning RF systems operated near resonance.  7  and  Chapter 2 The RF C a v i t y and Power A m p l i f i e r  The power a m p l i f i e r , transmission l i n e , and c a v i t y  were  designed  and commissioned by Bob Worsham and V o j t a Pacak who are  principal  members o f the TRIUMF RF group. Their work provides the b a s i s f o r the lumped model which i s used  to develop  a  transfer  function  d e s c r i b i n g the system response to c o n t r o l modulations.  transmission line  .11 1  1 X /A. privity beam high voltage gap  A/4 cavity  fig.4 Schematic of the separator cavity structure.  The RF separator  consists  of two A/4  transmission l i n e c a v i t i e s that are t i g h t l y t i p c a p a c i t y at the high result  voltage  from  short  coupled  electrodes.  resonant  modes  cavities.  These are a push-pull resonance, which  mode, and a push-push resonance  the close  which  8  circuit  occurs  through the  Two  coupling  coaxial  fundamental  between  the  i s the d e s i r e d at the frequency  defined by the A/4  cavities.  At the push-push  resonance, the  c a v i t y Q and the shunt r e s i s t a n c e measured at the high voltage gap are: Q = 4800 Rsh  cavity parameters  = 141k«  A 2X long, 50Q transmission l i n e connects the upper c a v i t y t o the high Q p l a t e c i r c u i t of a c l a s s "C" grounded g r i d power a m p l i f i e r . This introduces an 87ns delay between the p l a t e  circuit  and the  c a v i t y . The Q and shunt r e s i s t a n c e of the p l a t e c i r c u i t are:  Q = 3000  plate  circuit  parameters  R = 12000 P  The response of the separator RF system i s dominated elements i n the p l a t e c i r c u i t of the power a m p l i f i e r cavity.  A  lumped  model  of t h i s  system  also  by high Q and by the  includes  the  t r a n s m i s s i o n l i n e and the power tube represented by an RF current source. S t a r t i n g from the push-push model, one can a r r i v e simple equivalent c i r c u i t f o r the dominant RF elements.  fig.  5  Push-Push model of the cavity  9  and plate  circuit.  at a  The push-push r e s i s t a n c e measured  at the gap i s the p a r a l l e l  combination of the shunt r e s i s t a n c e from each c a v i t y . The push-pull  mode  operates at a  frequency below  resonance defined by the push-push mode. At t h i s c a v i t y appears i n d u c t i v e  and resonance  the  desired parallel  frequency,  i s formed  each  by the gap  capacitance i n s e r i e s with one side. A c a p a c i t i v e voltage probe at one s i d e o f the high voltage gap sees a lumped  model  similar  to  that i n the f i g u r e below.  IP©  fig.  6  Push-Pull model of the cavity  and plate  circuit.  The amplitude and phase of the gap voltage i s regulated using the s i g n a l measured by a c a p a c i t i v e voltage probe on one side  o f the  gap. To develop a t r a n s f e r f u n c t i o n f o r the system i t i s u s e f u l t o transform the p l a t e c i r c u i t  to the high  voltage  gap. In the Zz  c i r c u i t below, Z i i s the transformed p l a t e impedance,  i s the  - s T  c a v i t y impedance seen from the d e f l e c t i o n p l a t e , and e t o t a l delay i n the transmission l i n e s , from the p l a t e the measurement point; a distance of 4A.  10  i s the circuit  to  <- V(s)  -sT  Ks)  fig.  O 7  Zl  Equivalent  Z2  circuit  of high Q elements.  A c o n t r o l model f o r the RF c a v i t y and the a m p l i f i e r p l a t e is  developed  from  the admittance  of  these  r e l a t i o n s h i p between the transformed p l a t e  circuit  elements.  current  The  and the  gap  voltage i s given by: Ks)  = V(s) [ J . + g l ) e  When the p l a t e c i r c u i t i s matched to  s T  = V(s) Y(s)  the c a v i t y ,  p l a t e r e s i s t a n c e seen by the c a v i t y w i l l be equal  the e f f e c t i v e t o the c a v i t y  shunt r e s i s t a n c e , Rsh. The t o t a l shunt resistance seen by the current generator w i l l be 1/2 Rsh.  11  RF  Chapter 3 T r a n s f e r F u n c t i o n s f o r RF R e g u l a t i o n  3.1  An RLC Model  To d e v e l o p a t r a n s f e r f u n c t i o n f o r c a v i t y r e g u l a t i o n , to c o n s i d e r a simple  lumped r e s o n a t o r  driven  by  an  it  is  RF  useful current  source.  fig. The a d m i t t a n c e ,  8  Lumped RLC resonator.  Y =  where:  wo R  Q = woL let  w = wo + Aw 1  Z  R [  wo  12  wo+Aw JJ  1 LC woRC  This approximation, with less than 5% e r r o r f o r Aw ^ very u s e f u l f o r c a l c u l a t i o n s  within  the c o n t r o l  represents the c a v i t y as a f i r s t order frequency, wo. The RLC c i r c u i t  pole  responds  wo/10,  is  bandwidth.  It  about  the  to c o n t r o l  resonant  modulations  l i k e a simple RC low pass f i l t e r with a time constant:  2Q  T =  RF cavity  Wo  3.2  time  constant  A Simple Transfer Function f o r the Separator  A s i m i l a r t r a n s f e r f u n c t i o n f o r the separator  cavity  and  a m p l i f i e r can be evaluated by expanding the equivalent expression about resonance.  power  admittance  For maximum power t r a n s f e r , the  shunt  r e s i s t a n c e of the transformed p l a t e c i r c u i t i s equal to the  shunt  r e s i s t a n c e of the c a v i t y so that the admittance of  the  separator  power a m p l i f i e r (PA), transmission l i n e , and c a v i t y becomes:  Zi sT  ^ e , where:  T2 =  r  9600 Wo  Z2  (1+STl)  (1+ST2)  Rsh  Rsh  _„ = 66us ,  T * 200ns «  xi =  T1.T2  6000 Wo  = 42us  •••=» e * 1+sT s T  Approximating the delay by a f i r s t order expansion gives:  1 ••<!•«»> )(  y , ( s  2  r 1 ^ s(xi+T2)  Rsh"^-  2  13  ^  J  i . s i] . .  .  .  .  over p r a c t i c a l c o n t r o l bandwidths  The r e l a t i v e l y s m a l l d e l a y i n the t r a n s m i s s i o n l i n e a l l o w s resonant  s t r u c t u r e s t o be modeled as a  single  the  two  RLC circuit.  The  t r a n s m i s s i o n l i n e adds a second p o l e at 1MHz.  Z(s)  «  R  where:  1+ST  R =  Rsh  T = ^ (T1+T2)  The c a l c u l a t e d time c o n s t a n t the  two time c o n s t a n t s .  o f the model i s 54us,  Open  loop  pulsed  the average  power  measured v a l u e o f 55us f o r the time c o n s t a n t  tests  o f the RF  of  gave  a  amplifiers  and c a v i t y .  fig.  RF source.  9  p u l se width modulator  Pulsed power tests. power  cavlty  amp1 I f l e r  amp  111 u d e  detector  The photo shows two f a l l i n g edges o f the d e t e c t e d c a p t u r e d on a d i g i t a l o s c i l l o s c o p e . time c o n s t a n t s p a r k which  o f the R F system. effectively  short  One t r a c e  -> t o  cavity  shows  14  the  cavity  voltage  the  The o t h e r t r a c e r e c o r d s circuits  scope  *  50us  a  cavity  and  power  a m p l i f i e r . This avalanche c o n d i t i o n remains i n e f f e c t as the R F d r i v e i s present. The 20Hz, 5% pulse  modulator  long  as  turns o f f  the d r i v e and the short c i r c u i t mechanism i s extinguished  i n the  p e r i o d between pulses.  Other tuned c i r c u i t s i n PA cathode c i r c u i t and the d r i v e r  stage  use d i s c r e t e elements w i t h Q's l e s s than 100. This i s c o n s i d e r a b l y l e s s than the Q's achieved with d i s t r i b u t e d resonant s t r u c t u r e s In the c a v i t y and PA p l a t e c i r c u i t . The e f f e c t o f the  energy  stored  stage  i s to  s l i g h t l y increase the time constant o f the system measured  during  i n the d i s c r e t e coupling  circuits  o f the d r i v e r  pulsed power t e s t s . These t e s t s show that the delays storage  i n the d r i v e r  stages  and transmission  and energy line  can be  described as a transconductance of the form:  Ga  j|  a  (1+STI)  Ga  1+sJ^Ti  Ga  Vln  10  One dimensional  1+STa  R >  1+STa Ta - 2flS  fig.  Ga  a  V 7  1 + ST T - 54US  transfer  function  for the RF system.  Open loop pulsed power measurements i n d i c a t e that a model i s s u f f i c i e n t  second  t o describe the dominant response  order  o f the R F  system. The approximate pole l o c a t i o n s determine adjustment ranges i n the r e g u l a t o r compensation networks. An estimate of Ga i s not needed i f the RF r e g u l a t o r design  15  allows  some  gain  adjustment.  This i s discussed l a t e r i n chapter 5.  3.3  M a t r i x Transfer Functions f o r Amplitude and Phase Regulation  Regulation o f both  amplitude  and phase  of the separator  voltage i s a two dimensional problem i n which  a  current  gap  vector  from the power tube i s used to c o n t r o l the output voltage  vector.  The current vector can be described by a steady s t a t e term  and a  small time varying, modulation term. I  +  d/(t) = I e  l e  j 9 l +  j  9  l  [ ^ l  +  Jd6i(t)]  The gap voltage i s the product of the generator  current  and the  load impedance. The impedance can be w r i t t e n i n p o l a r form as  Zz<p  where: |Z| =  tan# £ - T A W  ^  / 1 +  (TAW)  2  At resonance, Aw=0 and the tube  works  into  a  resistive  V i b r a t i o n and other sudden changes can move the resonant and cause large impedance changes i n high Q systems f i x e d frequency.  When  the gap voltage  modulations of the c a v i t y current, cavity i s  t o cross  couple  the e f f e c t  frequency  driven  i s adjusted  load.  by  at a small  of the detuned  the v a r i a t i o n s i n the phase and  amplitude.  To c a l c u l a t e t h i s e f f e c t i n high Q systems,  i t i s desirable to  express the detuned c i r c u i t impedance at the modulation S t a r t i n g from the previous impedance approximation, can be w r i t t e n  i n terms  of a  displacement  16  from  sidebands.  the admittance the c a r r i e r  frequency rather than a displacement from the resonant frequency, let: Aw = t o t a l frequency displacement from resonance wrf = displacement of the c a r r i e r frequency from resonance w = modulation sideband frequencies  1  ' ~  where:  R  "  R  R ~  Zrf  R  Aw = wrf+w Zrf = c i r c u i t impedance at the c a r r i e r frequency  The c o n t r o l modulations w i l l appear as RF sidebands. variable  for the RF control  is defined  as s=jw,  The  where  Laplace  w=Aw-wrf.  The admittance of the c i r c u i t at these c o n t r o l frequencies i s :  Y(s) = Y l S ;  »  —  Zrf  + —  fi Zrf I-  a Z- L f rf  R  L  1  1  +  S  ST T —  1  R J  ST -v „ _ 1 _ |"(1+ST) - j t a n B z l l-jtan9z J Zrf [ 1 - jtanBz J  1  +  where: tan9z = -TWrf wrf ^ constant  The impedance at the modulation sidebands f o r a c a v i t y detuned  to  an a r b i t r a r y angle, 0z, from resonance i s :  Z(s) s Zrf  l-jtan9z 1 [(l+ST)-jtan9zJ  ^ (1+ST) + tan 9z r f  2  l+ST+tan 9z) -JsTtan0z] 2  2  This expression describes the impedance  17  of an " o f f c e n t e r "  Q  curve. I f there are no c o n t r o l modulations then c i r c u i t impedance i s Z(0)  s  = Zrf. The steady s t a t e  = and  0  and  the  modulation  terms i n the gap voltage are given simply by Ohms law; the product of the current vector and the c i r c u i t impedance.  V  dl^t) = Ve  +  , where  - Ve  j e v  j 6 v  [ *™  + JdBv(t)]  ,, J6v „ J9l Ve = Zrf Ie T  In the Laplace domain (s=jw), the v a r i a t i o n i n the gap voltage can be expressed i n terms of the current modulation. are s i m p l i f i e d i f the amplitude  variations  of  The the  expressions current  and  voltage v e c t o r s are expressed as f r a c t i o n a l changes.  ^dWU  + J d 0 v ( t )  j  = v(s) + Jev(s)  =  [sx+(l+tan 8z) - JsTtanGz][i(s)+jei(s)) 2  (l+sx) +tan 9z 2  2  Separating the r e a l and imaginary parts, t h i s r e w r i t t e n i n matrix  form  to  show  the  expression  can  transmission of  phase  modulations and f r a c t i o n a l amplitude modulations through the Q section.  v(s)  sx+(l+tan9z) (l+sx) +tan 9z 2  9v(s)  Cross Coupling  srtanGz  i(s)  2  -srtan9z  sx+d+tan 9z) 9 i ( s )  Matrix for Amplitude and Phase  18  be  Modulations  high  D r i v i n g the c i r c u i t o f f resonance causes the cross  terms  i n the  matrix t o increase by a f a c t o r tanGz. This detuning can come  from  s e v e r a l sources: • random v i b r a t i o n s and dimensional changes i n the p l a t e and cavity circuits. • changes i n the c y c l o t r o n operating  frequency.  • The separator i s locked t o the beam phase which changes due to a 5 Hz mechanical v i b r a t i o n i n the c y c l o t r o n resonators.  The e f f e c t i v e impedance of the separator RF system i s the p a r a l l e l combination of the p l a t e c i r c u i t means i s p r e s e n t l y  and the c a v i t y  a v a i l a b l e to a u t o m a t i c a l l y  s t r u c t u r e . No  tune  the p l a t e  c i r c u i t . The c a v i t y , which e x h i b i t s considerable frequency i s independently adjusted by an automatic described  tuning  loop  drift, that i s  later.  I t i s l i k e l y that the p l a t e c i r c u i t and the c a v i t y tuned t o d i f f e r e n t  frequencies.  In t h i s  case,  will  become  the e f f e c t i v e  admittance of the high Q c i r c u i t can be r e w r i t t e n as:  Y(s) K  J  =  — f ZrfL  1+  where  s(xi+x ) 1 2-j(tan9i+tan82) •* 2  - L= ^  +  ^  Independent tuning e f f e c t s of the p l a t e c i r c u i t and the c a v i t y are e a s i l y described by a d j u s t i n g the terms i n the coupling matrix. tanBz  -> }r (tan6i+tan02)  19  and  x -» i  (xi+X2)  When automatic tuning i s r e s t i c t e d to the must be able to handle A.C.  cavity,  the  regulator  disturbances i n both the p l a t e c i r c u i t  tune, 9 i , and the c a v i t y tune, 92, and D.C.  errors  in  the  plate  c i r c u i t tune, 9 i . The c y c l o t r o n u s u a l l y operates between 23.05 MHz and 23.07 MHz,  a 20 KHz range. With a p l a t e c i r c u i t Q of 3000  a c a v i t y tuner that works p e r f e c t l y , detuned as much as  |9i| s  detuning angle of |9z| s  70  the  which  plate  circuit  translates  to  and  can a  be  system  54°.  Before the separator i s run f o r an extended period  of  time,  the  p l a t e c i r c u i t i s tuned to match the e x i s t i n g c y c l o t r o n  frequency.  I f the c y c l o t r o n magnet i s s t a b l e and does not  any  suffer  power  bumps, the c y c l o t r o n frequency can u s u a l l y be held w i t h i n 5KHz the i n i t i a l tune which  limits  the  p o s s i b l e separator  of  detuning  range to |9z|s34° when the c a v i t y tuning loop i s a c t i v e .  After  the  separator  system  has  been  manually  adjusted,  expected detuning angle, 9z w i l l be zero. I f the impedance 2  the  matrix  3  i s expanded about tan 9z=0 and terms of  tan 9z  and  greater  are  neglected then the t r a n s f e r f u n c t i o n matrix can be s i m p l i f i e d to: .  v(s)  tan Gz 2  sTtan9z  1 + ST  9v(s)  1+ST  sxtanGz  1+  1+ST  This gives a convenient  form  Ks)  1+ST  U+STT ST  tan  8z  9i (s)  (1+ST)'  to  study  p e r t u r b a t i o n of the resonant system.  20  the  tuning  loop  as  a  v(s)  i(s) 1+ST  91 (s)  9v(s)  i nput vec tor  fig.  3.4  tanGz 1+ST  srtanGa 1+ST  11  Linearized  model for  output vector  -1  tan9z 1+ST  high  Q circuit  elements.  P a r t i c l e Beam Loading as a System Disturbance  The separator uses periodic  group  the RF e l e c t r i c  o f charged  field  particles  and r e j e c t  p a r t i c l e s enter the high voltage gap they f i e l d and so  load  the RF system.  to select  distort  a  given  others.  As  the e l e c t r i c  By superposition,  one  can  determine the voltage disturbance induced on the d e f l e c t i o n p l a t e s by a p e r i o d i c group of charges t r a v e l l i n g between them.  A t e s t charge located near a grounded metal surface w i l l induce an image charge  on the metal  surface.  Similarly,  a beam  t r a v e l l i n g across the gap electrode w i l l induce an image on the electrode. The fundamental component of t h i s induces an a l t e r n a t i n g  bunch current  bunched  beam  current on the d e f l e c t i o n p l a t e s which, i n  turn, creates a disturbance voltage on the high impedance resonant c a v i t y . I d e a l l y , the c a v i t y presents  a  low impedance  t o other  frequency components of the bunched beam and, as a r e s u l t ,  21  these  components generate n e g l i g i b l e disturbance  voltage.  induced current that can d i s t u r b the RF system  The  i s equal  maximum t o the  fundamental component of the beam current i n the gap. E v a l u a t i n g the spectrum of a p e r i o d i c s e r i e s of narrow pulses shows that the fundamental component of beam current i s approximately  twice the  average value of beam current.  fig.  12  Approximately produces a  Beam loading seen by a single  voltage probe.  equal currents are induced on each  common  mode  disturbance  voltage  electrode  which  on the d e f l e c t o r  p l a t e s . To a s i n g l e voltage probe, the beam disturbance appears as an asymmetrical  current source. A d i f f e r e n t i a l voltage measurement  would be needed to cancel the common mode interference.  In a system with only one measurement probe, both the beam and the generator currents  affect  the measured  voltage  but only the  generator amplitude and phase are a v a i l a b l e as c o n t r o l v a r i a b l e s . I f the beam loading current i s large then the c o n t r o l become f u r t h e r cross  coupled  due  t o the steady  loops  state  geometry seen by the s i n g l e probe. W r i t i n g down the steady  22  can  vector state  current that determines the measured voltage:  COS*g  [ig  I = J  + IbL C O S * b L )  +[lbL S i n * b L  2  + Ig  sin$g)'  f IbL SJnSbL + Ig SJnflg 1 [ Ig COS*g + IbL COS*bL J  0i= arc tan I  The RF generator  current  provides  the c o n t r o l  input  for  the  system. As with the previous matrix, the beam loading e f f e c t s a r e simplified  i f the v a r i a t i o n s  i n amplitude  are expressed  as  f r a c t i o n a l changes. "dl "  I  _u  cos(0i -$g)  s i n ( 0 i -*g)  dig' Ig  -sin(0i -*g)  cos(0i -*g)_  d$g  I  d8i  Beam loading  effects  are easily  demonstrated  $g=0  with  $bL=-90 . For t h i s worst case, the c o n t r o l o f c a v i t y  and  current by  the RF generator vector can be w r i t t e n as: dl_  Ig  I d8i  Ig+IbL  -IbL  dig' Ig  Ig  d$g  IbL  where: Ig = amplitude of the generator  current  IbL = amplitude of the beam loading current  I t i s apparent that the cross terms i n the matrices  increase as  the beam loading current increases while the o v e r a l l c o n t r o l decreases.  When the beam  generator  current,  loading  the phase  current  i s greater  and amplitude  reversed.  23  gain  than the  controls  become  In the separator, the loading current has three components due t o e l e c t r o n s , pions, and muons as i n d i c a t e d i n f i g u r e 3. I f a l l three p a r t i c l e groups were i n phase, the t o t a l beam loading still  current  l e s s than luA. To produce a c a v i t y voltage of 130KV  into  is a  r e s i s t i v e load of 75KIJ (Rsh/2), the RF generator must supply about g  1.7A of current; more than 10 this  environment,  the s i n g l e  times the beam loading current. voltage  probe  voltage  In  produces  v i r t u a l l y no d i s t o r t i o n of the gap voltage measurement. The only s i g n i f i c a n t cross coupling terms are introduced by high Q elements i n the RF a m p l i f i e r and c a v i t y .  24  Chapter 4 Closed Loop Control  4.1 Phase and Amplitude Loops  The c o n t r o l vector f o r the separator RF system becomes r o t a t e d as the resonance of the output  circuit  d r i f t s with respect to the  d r i v i n g frequency. This i s described by a p e r t u r b a t i o n matrix, A.  v(s)  ac(s)  <pvis)  (1+STa)(1+ST) 2  where:  STtan6z  1 + STtan 0z (1+ST)' STtanGz  built  phase and amplitude  t o operate  over  1+ST tan 8z 2  1+ST  1+ST  Orthogonal  ^c(s)  (1+ST)'  c o n t r o l s , ac(s) and  <j>c(s), can be  a wide range when the inverse t r a n s f e r  matrix i s known. v(s) <pv(s)  Ka (1+STa)(1+ST)  ac(s)  0c( ) S  Very l i t t l e cross coupling i s introduced i n the low Q RF d r i v e r stages.  The t r a n s f e r f u n c t i o n f o r these  relatively  sections can be approximated by the product  wide  band  o f one dimensional  expressions s i m i l a r to the system i n f i g u r e 10. An inverse matrix,  25  A~  provides the c o n t r o l decoupling at low s i g n a l l e v e l s  the RF phase and amplitude can be regulated  so that  by two independent  c o n t r o l loops.  It i s not convenient t o implement a s o l u t i o n i n t h i s form  because  the A matrix r e q u i r e s an updated estimate of tanGz. The system becomes f u r t h e r complicated i f one must estimate cross terms i n the RF d r i v e r stages. I f a l l the cross coupling terms are t r e a t e d as system disturbances, then the t r a n s f e r f u n c t i o n of the RF system can be described by a simple second order expression l i k e that shown i n f i g u r e 10. Even without the decoupling matrix i t i s s t i l l p o s s i b l e to b u i l d a c o n t r o l l e r f o r t h i s system around two independent second order r e g u l a t o r s i f the cross terms are t r e a t e d as system disturbances.  For c o n t r o l purposes, i t i s convenient to represent the e n t i r e  RF  system as a 2x2 matrix that maps the c o n t r o l input vector i n t o the output  space.  Measurement  and c o n t r o l  functions  are  also  represented by 2x2 matrices while the system s i g n a l s are described by two dimensional vectors.  regulator  Gc  Vref  c  RF s y s t e m Vout  measurement  w fig.  13 Block diagram of the RF regulator  26  loop.  The gap voltage vector, Vout, can be w r i t t e n i n terms of the set point vector, Vref as: Vout = [ Q + P G C M J - V G C  = [W+(PGc) ] _1  Vref  Vref  _1  I f the system i s c o n t r o l l e d by two independent r e g u l a t o r s then becomes a diagonal matrix. The c o n t r o l loop contains an  Gc  amplitude  detector and a phase detector, both of which can be constructed t o provide independent measurements thus e l i m i n a t i n g cross the measurement matrix. Only the RF  matrix,  P,  terms i n  contains  cross  terms. Gc =  gu  0  0  M=  g22  mil  0  0  m22  P =  where:  pn=p22  pil  pi2  p21  p22  and  pi2=-p2i  The plant matrix, P describes the r o t a t i o n of orthogonal phase and amplitude  modulation  vectors.  At  ( S T « 1 ) , the r o t a t i o n i s given by tan#  low modulation sxtanSz  and at  frequencies (sx»l) the r o t a t i o n approaches <f> = 0z.  27  frequencies higher  Phase and amplitude changes i n the gap voltage are shown along the v e r t i c a l and h o r i z o n t a l a x i s . The matrix c o e f f i c i e n t p n i s the p r o j e c t i o n o f the amplitude  c o n t r o l vector, v i , on the amplitude  a x i s and p22 i s the p r o j e c t i o n of the phase c o n t r o l , V2 on the phase a x i s . The determinant o f the IP matrix,  pnp22-pi2p2i  i s the  area o f the parallelogram defined by the input vectors.  After  some algebra,  one can write  the closed  loop  transfer  f u n c t i o n f o r the 2x2 system.  [M+(PGc) i -]i - i  bn  0  0  bi2  0  b22  b2i  0  forward terms  cross terms  The forward t r a n s f e r functions are:  bn  =  k22p22+l mi l + g l l p i 1(k22p22 + l)-k22pi2p21  mi 1 +  gll  p22 piIp22-p2ipi2  where k22 = g22m22 and k22p22 » 1  b22 = m22 +  kilpil+l  g22 p 2 2 ( k l i p i l + l ) - k l i p i 2 p 2 1  m22 +  pi 1 g22 p 2 2 p i I ~ p 2 i p i 2  where k n = g u m i i and k n p n  »1  These forward t r a n s f e r functions are of the usual form: Hii  Gi i  =  where Gn = gi  kjjpjj » 1 PJ j = giipii , kjjpjj « 1  1 + mi iGi i  When an amplitude regulators  will  change attempt  i s made i n the reference t o hold  28  the gap phase  vector the  constant.  At  frequencies where the r e g u l a t o r gain i s large, the the y a x i s , p22 can be held constant and the g a i n i n the plant becomes the  effective  p r o j e c t i o n on  projected amplitude gain i s given  by  the  projection  the  area  x  amplitude axis.  of  on  the  This  control  vector parallelogram, |P|, d i v i d e d by p22. ( f i g . 14)  Within the r e g u l a t o r bandwidth, the cross coupling i s the loop gain and a f a c t o r  equal  to  the  area  reduced  spanned  by  by the  c o n t r o l vectors.  b l 2  b21  Two  _  =  g22pi2  PH.  a  ( k l l p i 1 + 1)(k22p22 + l ) - k l I k 2 2 p i 2 p 2 1  kl1m22(pi 1p22-pi2p21)  g"P ( k l l p i 1 + 1 ) (k22p22+l )-kl Ik22pi2p21  Plik22IM 1 ( p i 1 p 2 2 ~ p i 2 p 2 1 )  2 1  independent PID r e g u l a t o r s are used  to  cancel  the  dominant  poles i n the phase and amplitude c o n t r o l loops. Figure 15 shows s i m u l a t i o n of the small s i g n a l step  response  for  a  detuned  a RF  o  system w i t h 0z  ± 2 7 . When the loops are closed,  between phase and amplitude  control  is  Simulations show that two independent amplitude can r e j e c t cross coupling  cross  practically  regulators terms  for  introduced  coupling  eliminated. phase by  and  an  RF  system operated near resonance ( |Sz | 5 0 ° ) . A c a v i t y tuning loop i s included i n the the tuner  does  reduce  cross  coupling  separator between  system. the  amplitude c o n t r o l s i g n a l s , i t s c h i e f task i s to match  29  While  phase the  and  cavity  impedance t o the transmisson l i n e  and minimize  reflected  power  seen by the tube. S»all Signal Step Besponse of Detuned BF Sustea  0 1.1 1  0  .9  closed loop forward tern with PID regulator  .8 .7  legulator Gain = 1BE6 Detuning Angle = 27 deg.  .6 .5 .4  open loop cross tern  .3  note r e j e c t i o n of cross term in closed loop system  .2 .1  v  I e  fig.  4.2  15  closed loop cross t e n with PID regulator  V i  ' v r - ' i  i  i  t  •  i  •  •  i  i  i  .a  i  i—i—  1  Ti«e t»l&-5) Regulator  simulation  showing  cross  term  rejection.  The C a v i t y Tuning Loop  The model developed f o r the RF system  allows  both  the power  a m p l i f i e r and the c a v i t y to be tuned t o resonant frequencies  that  d i f f e r s l i g h t l y from the d r i v i n g frequency. The e f f e c t i v e detuning f o r t h i s system, tanGz, i s the average o f the c a v i t y detuning and the p l a t e c i r c u i t detuning.  tanGz = ^ (tanGi + tan62) where:  tan8i = plate c i r c u i t detuning tanG2 = c a v i t y detuning  A tuning loop i s used to c o n t r o l the c a v i t y steady s t a t e value o f tan62 = 0. The tuner  30  drift  and f o r c e  mechanism  a  produces  small mechanical deformations  of the c a v i t y which i n turn produces  a s h i f t i n the c a v i t y resonance. A l i n e a r i z e d  transfer  function  f o r the tuner mechanism and the tuning loop i s o f the form:  d  KM  motor+gearlng  fig.  KM * 2mm/s/V,  AW2  K2  s(1+STM)  c a v i t y small freq.a3justment  16 Cavity  tan02  T  s  =  TAW2  cavity time c o n s t a n t  tuning mechanism.  K2 - 5KHz/mm - 10 n/s/mm, 4  C a v i t y tune i s measured by a phase  T * 70us,  T M 0. Is  which  compares the  detector  phase o f the transmission l i n e voltage i n j e c t e d and the gap voltage. At resonance,  these  into  the c a v i t y  two voltages  are  in  o  phase. A 90 phase s h i f t i s introduced between the s i g n a l s so that the phase detector w i l l r e g i s t e r zero when the c a v i t y i s d r i v e n at i t s resonant frequency. such that  tan82  62,  Operating near  resonance  (|tan92|s . 5 ) ,  the output of the phase detector w i l l give a  good estimate o f the c a v i t y tune. The tuning loop, shown below, i s designed to operate with the reference, R = 0. D  R  =  0  - ^ E ) -  1.  I  KCM  >  tuner  controller  Kd phase  fig.  detector  17 Cavity  31  tuning loop.  d1st urbance  -> tan02  Output from the phase detector i s a l i n e a r f u n c t i o n of the phase d i f f e r e n c e between the RF i n j e c t e d into the c a v i t y and the gap voltage.  The  mechanical  detector  responds  tuning system  much  (100ms) with  faster  ( l u s ) than  the r e s u l t  that  the  when a  simple proportional c o n t r o l l e r i s used, the loop t r a n s f e r f u n c t i o n is  dominated  by  the motor.  Disturbance inputs  such as  static  tuning e r r o r s and long term d r i f t s are reduced by the closed loop.  tan92 =  H =  1 + H  K t  S(1+STM)  The r a t i o of r e f l e c t e d power to forward power seen by the c a v i t y transmission l i n e i s :  f  2  =  |Z-Ro|  _ (RL-RO) +  IZ+RoI  =  2  (RL+RO) + X  2  2  where  „2  X  2  2  Z = .  ,?° „  = RL + j X  l+jtan02 ° = transformed cavity impedance seen by the transmission line  tan 82+tan 62 — — 4  2  2  (2+tan 92) +tan 92  -  tan 62 „2^ — ^ — for r ^ 2  _  0. 1  To keep the power r e f l e c t e d from the c a v i t y to l e s s than 1%, the tuning control  loop must keep bandwidth  |62|sl0°.  however,  This e a s i l y  achieved over the  the tuner must transmit  mechanical  motion through a vacuum seal and there i s reason to reduce the bandwidth of t h i s movement. Depending on the disturbance spectrum, the r e f l e c t e d power can show s i g n i f i c a n t f l u c t u a t i o n s while the average value i s regulated to less than 1%.  32  The c a v i t y time constant o f about 70us produces a l a g between a phase change i n j e c t e d into the c a v i t y and the appearance change a t the c a v i t y gap. Phase d i f f e r e n c e s cavity  a r e the r e s u l t  o f tuning  measured  disturbances,  propagation delay of phase modulations  through  tuning loop t r e a t s the output of the phase  of  the  across the D, and the  the c a v i t y . The  detector  as system  disturbances and attenuates both s i g n a l s by a f a c t o r (1 + H).  In operation, the loop keeps the c a v i t y tuned near resonance  such  2  that terms o f tan 82 can be neglected and the phase s i g n a l s i n the c a v i t y can be represented as: ,  V i S j  ^ ST2tan82 , ,  1+ST2  c r o s s term  D d i s t urbance  <f>(s) cav1t y input phase  (pout ) cavity output phase  62 to tuner phase measurement  u fig.  82  18  = D+— —  Tuner small  f 0(s) + v(s)  1+ST2 ST2 + 1+ST2 C r  V  1-  S T 2 t a n 6 2  (  S  1+ST2  )  "*  Both the cross modulation terms and tuning attenuated by the tuning  model.  1+ST2 . , ST2tan82 ,,  r  = D  signal  loop.  Phase  (  S  • > )  </>(s) r  )  disturbances,  modulations  are t r e a t e d  d i f f e r e n t l y . The c a v i t y output phase i s 0out = 82 + #(s). D and v ( s ) t o zero such that only the phase  control  D are  Setting  modulations  i n t e r a c t w i t h the tuning loop, one can w r i t e #out i n terms o f  the  82  #(s) and the closed loop disturbance o f the tuning system, T—rj •  33  =  C  1  - iSi  = T^— 1+ST2  f  1  +  T+H ) *  ( s )  ST2 TTu 1 *ts) 1+H J  Over the range where |H| » 1, the tuning loop e f f e c t i v e l y f l a t t e n s the c a v i t y response to phase modulations such that  = 0(s).  (pout  The bandwidth of the motor d r i v e n tuning loop i s of the order of a few h e r t z , much less than the c a v i t y bandwidth  of »  2.5KHz  and  much l e s s than the r e g u l a t o r bandwidth. As a r e s u l t , the separator tuning loop i s not s t r o n g l y coupled with the amplitude  and  phase  r e g u l a t i o n . Since cross coupling introduced by a detuned RF system i s t r e a t e d as a disturbance by the r e g u l a t o r  system,  the  tuning  loop acts l i k e a slow moving, feedforward term that i s w e l l w i t h i n the bandwidth of the r e g u l a t o r s .  From the c o n t r o l point of view, the c a v i t y should be tuned the complex conjugate of the transformed p l a t e to give  8i=-62  and  tan8z=i/2(tan8i+tan82)=0.  circuit  In i t s present  t o be  impedance form,  the tuning loop eliminates phase and amplitude cross c o u p l i n g  due  to c a v i t y d r i f t s but i t has l i t t l e influence on e f f e c t s introduced by a detuned p l a t e c i r c u i t .  I t i s not d i f f i c u l t  e x i s t i n g resonance tuner to a conjugate tuning t h i s may change the power a m p l i f i e r design.  34  to change the  c o n f i g u r a t i o n but  Chapter 5  Hardware Implementation of the C o n t r o l l e r  5.1  Overview  The  RF c o n t r o l l e r  handles  routine  system  faults,  provides  automatic s t a r t up, and keeps the c a v i t y tuned t o the c y c l o t r o n frequency. I t also regulates  the phase and amplitude o f the gap  voltage, providing wideband c o n t r o l of the p a r t i c l e f l u x d e l i v e r e d to the experiment. The system i s b u i l t around analog c o n t r o l loops which were designed t o have u n i t y gain bandwidths lOOKHz. These loops are supervised  greater  than  by a computer which c o n t r o l s  the setpoints and the regulator parameters. Computer  ^  RF  RF Regulator ^  }  Ana l o g  fig.  5.2  19  System  -> V R F  Loop  Basic controller  concept.  System D e s c r i p t i o n  The RF c o n t r o l l e r was developed as a modular system. system functions were i d e n t i f i e d and then hardware  35  Individual  modules were  b u i l t to perform that s i n g l e function. Such modules can be e a s i l y changed  or upgraded and complex  from these basic elements  systems can be pieced together  i n much the same way that  they are  assembled on paper using block diagrams.  Beam Si gnal  Phase  \  )  Cyclotron  Lock  RF  d r i v tsn RF  s e 1 f - e x c it e d  s oru r c e swi t c h  Cavlty Vo 11 a g e Feedbac k  amp 1 i t u d e  amp1i t ude  amp 1 1t u d e  detector  regulator  mo d u  ator  s i g na1 splitter phase  phase  detector  phase  regulator  modulator  Phase Reference  RF  On/Off switch  , Fig.  20  Control modules.  RF  t  L  —>  RF Out t o PA  interlock trip  duty  eye 1 e  1  Regulator I/O Bus -> t o  optical  couplers  A dedicated computer provides access to the c o n t r o l l e r v a r i a b l e s . It  also handles graphic d i s p l a y of the system  information and  sequencing f o r automatic s t a r t - u p and spark recovery. The computer permits the modular design to be extended to the user i n t e r f a c e , sequencing operations, and upgrading to provide new system states. The  regulator  I/O l i n e s  connect  to the same backplane as the  computer but are o p t i c a l l y i s o l a t e d from the bus.  The software f o r automatic sequencing and system I/O was developed  36  on an MS DOS computer using compiled BASIC. This system had a l l o f the graphics and timer support  needed to produce the bar graphs,  system p i c t o r i a l s , and sequencing operations. The c o n t r o l l e r was b u i l t using a 7MHz PC on STD bus with Prolog's System 2 operating system. This c o n f i g u r a t i o n worked w e l l and could e a s i l y handle the necessary tasks using a simple p o l l i n g loop.  A gas plasma d i s p l a y i s used as the f r o n t panel d i s p l a y . I t i s compatible  with IBM's EGA graphics but u n l i k e a CRT i t i s not  s e n s i t i v e to magnetic f i e l d s o r phosphor burn. The amplitude and phase c o n t r o l can operate  i n open loop o r closed loop with t h e  s t a t e o f each loop shown g r a p h i c a l l y on the f r o n t panel d i s p l a y . Set point and readback values are displayed numerically and with horizontal  bar graphs.  adjustable  drive  limit  The modulator value  drive  are displayed  signals  and an  i n a v e r t i c a l bar  graph. System status i s shown i n the r i g h t side of the d i s p l a y . AMPLITUDE LOOP  setpoint 2843 e  2  4  18 6  6  4 2  4  1 1 1  6  8  readkack 2843  18  MANUAL  8  18  S  8  control —i  C D  C a i n : 58 T a u l i 32 32 l a u D : (9  axpl i tilde mdulator  interlocks OK  2. 8. source DRIVEN  PHASE LOOP  setpoint -1848 1 1 IB  I  I  I  I  8  I  I  I  I I  I  I  I  I I  • Mode  18  drive I I I I I — I  -18  8  RF OH  18  rea<U>aclc 14  Fig.  21  Front panel  37  display.  During  start  up,  the RF  voltage  i s pulsed  to  overcome  m u l t i p a c t o r i n g i n the c a v i t y . A p e r i p h e r a l card f o r the STD bus was b u i l t t o handle t h i s operation. The computer w r i t e s the pulser frequency operated  and pulse  width  to t h i s  card.  When the system i s  i n pulsed mode, the pulse width can be adjusted by a  f r o n t panel  knob from 0% to 100%. In CW operation, the RF i s  turned On or Off by w r i t i n g a pulse width of 100% or 0%.  Push buttons  on the f r o n t panel  are used to toggle the system  s t a t e s such as RF On/Off, RF Pulsed/CW, etc. Continuous parameters can be adjusted using f i v e o p t i c a l shaft encoders on the f r o n t panel. Each loop has one shaft encoder that can be assigned t o adjust e i t h e r the regulator gain, the two t r a n s f e r f u n c t i o n zeros, or  the d r i v e  limit  f o r the loop.  The remaining  three  shaft  encoders are not assignable but remain dedicated to the amplitude and phase set points and the RF pulser duty cycle. The assignable knobs are used mainly during setup and commissioning while the dedicated knobs are used during manual operation o f the RF system. Keeping m u l t i p l e assignments to a minimum reduces the complexity of the f r o n t panel c o n t r o l s .  5.3  S e l f - E x c i t e d Operation  To s t a r t the RF system, the c a v i t y and power a m p l i f i e r u s u a l l y need to be tuned before f u l l power i s applied. Experience with the c y c l o t r o n c a v i t y has shown that i t i s best s t a r t e d i n s e l f - e x c i t e d mode where the s i g n a l from the c a v i t y pickup i s f e d back t o the low  level  RF d r i v e r  amplifier.  38  The s e l f - e x c i t e d  frequency i s  determined by the c a v i t y resonance and the phase s h i f t around the loop. SIGNAL  FEEDBACK RF SAMPLE FROM CAVITY  POWER AMPLIFIER  AMPLIFIER  DC BLOCK -> BANDPASS FILTER  fig.  22 Self-excited  configuration.  S e l f - e x c i t e d operation provides an i d l e mode where the RF system operates independent of a reference phase or frequency and can be tuned by a phase s h i f t e r i n the feedback loop instead of the usual mechanical tuning loop. This arrangement has proved to be u s e f u l for  commissioning and debugging the various RF systems on s i t e and  has become a standard requirement f o r TRIUMF RF systems.  interlocks  ) Rf  off  Ok  drive  1  Freq.  >——  self  i  >  >  <—  (  If  23  Principal  system  beam production i s i n t e r r u p t e d ,  reference  signal  *  driven  excited  fig.  Ok  and the system  states.  the separator looses i t s RF waits  at f u l l  power i n  s e l f - e x c i t e d mode u n t i l beam i s again d e l i v e r e d to the experiment. S i m i l a r l y , from a c o l d s t a r t , the system waits at f u l l s e l f - e x c i t e d mode u n t i l  the c a v i t y frequency i s matched  reference frequency.  39  power i n t o the  L i m i t e r s and a bandpass f i l t e r are included i n the s e l f - e x c i t e d s i g n a l path. The l i m i t e r s keep the s i g n a l amplitude the same as i t would be i n the driven mode while the bandpass f i l t e r ensures that the c a v i t y i s s e l f - e x c i t e d by the desired push-pull mode.  The  s e l f - e x c i t e d system behaves  like a classical  oscillator i n  which the t o t a l phase s h i f t around the loop i s 2nir. When the loop i s i n i t i a l l y closed, system noise generates a current i n the power tube which e x c i t e s the high Q plate c i r c u i t and c a v i t y .  I f the  delay around the loop at the c a v i t y resonant frequency, wo, i s 2rm-<p then a steady s t a t e frequency i s reached when the c a v i t y i s e x c i t e d at a frequency above resonance, w=wo+Sw where the c a v i t y provides the e x t r a phase l a g to make the loop delay equal t o 2n7r. The impedance of the c a v i t y at t h i s frequency i s :  z  h  =  l  z  l ^  — l 20 At steady s t a t e , <t> = t a n f — Sw") and the s e l f - e x c i t e d frequency v. Wo  i s given by:  -'  /-, tan®-> w = wo + 5w = Wo[1+ n J 9  5.4 Drift  S e l f - E x c i t e d and Driven Tuning Systems i n the c a v i t y resonance or i n the phase delay around the  loop w i l l  cause the system to operate o f f resonance. In terms of  r e f l e c t e d power and load matching, the s e l f - e x c i t e d mode, l i k e the d r i v e n system,  needs an automatic tuning  40  loop.  When i n d r i v e n  mode,  the  resonance  mechanical and  tuning  minimizes  loop  keeps  reflected  the  cavity  power.  In  tuned  to  self-excited  operation, the tuning mechanism drives the c a v i t y to the c y c l o t r o n frequency  while the  phase r e g u l a t i o n  loop minimizes  reflected  power.  RF RF phase regulator  phase modulator  source swltch  feedback  loop  RF amp 1 i f l e r  cavity  TX 1 i ne phase detector AF  tuner  d> \  frequency c o mpar e  manual O—  auto  Ok  < d r i v e <present  fig.  24  Self-excited  RF detect  reference frequency  tuning loop.  In the s e l f - e x c i t e d mode the frequency comparator allows the tuner to adjust the c a v i t y resonance RF.  This  module  produces  a  to w i t h i n 1 KHz of the c y c l o t r o n signal  that  i s positive  i f the  s e l f - e x c i t e d frequency i s greater than the c y c l o t r o n frequency, negative i f the frequency i s less than the reference and zero i f the  self-excited  frequency  i s within  1 KHz  of  the  cyclotron  frequency.  The  phase  r e g u l a t o r maintains  a  delay  of  2nrc  around  the  s e l f - e x c i t e d loop. The bandwidth  of the phase r e g u l a t o r i s very  much greater than the mechanical  tuning system and no  reflected  power f l u c t u a t i o n s are detected when the s e l f - e x c i t e d tuning loop  41  i s functioning.  Under automatic c o n t r o l , the system can be taken  into the driven s t a t e when (AF_Ok AND Drive_Present) i s true.  reference RF  phase regulator  1  frequency  source swi t c h  phase nodulator  1 i TXne RF amp 1 i f i e r  phase detector  auto  q/o-  cavity  tuner  reference frequency  phase detector d r i v e <present  fig.  25  manual  RF detect  Driven Tuning Loop  In driven mode, the phase regulator locks the c a v i t y s i g n a l to the RF reference s i g n a l . When Drive_Present i s False,  the c o n t r o l l e r  w i l l r e t u r n to the RF Off state. Under automatic c o n t r o l , i t w i l l b r i n g the system back to the s e l f - e x c i t e d state and wait f o r the RF drive.  To change from s e l f - e x c i t e d to driven, the RF source i s switched from  the c a v i t y  configures  to the reference  signal.  A  DPDT switch  box  the c o n t r o l s i g n a l s f o r the tuner and f o r the phase  regulator.  42  Ref  erence-)-  switch  X  control  dr i v e n  phase detector  -O  -> t o P h a s e Regulator  Cavity-»phase detector TX 1 lne4Cavity-*-  -> to T u n e r Dr 1 ve  frequency c o mpar e  self Tuner  Switch  Module  Ref e r e n c e - ) -  fig.  5.5 The  26 Signal switch for tuner and phase  regulator.  The Frequency Comparator automatic  c o n t r o l l e r must be able t o tune the s e l f - e x c i t e d  c a v i t y t o the reference frequency  before  the system i s driven.  Necessary information f o r t h i s task i s provided by the frequency o  comparator which uses 2 mixers,  a 0  o  power s p l i t t e r  and a 90  power s p l i t t e r to derive two s i g n a l s , sin(wi-w2)t and cos(wi-w2)t. With respect t o the cosine term, the sine term i s i n v e r t e d when W2>W1.  COs(wi-W2)t = COs(w2-Wl)t sin(wi-w2)t = -sin(w2-wi)t The  quadrature  s i g n a l s are converted  decoded by simple l o g i c c i r c u i t s .  to digital  Experience  waveforms and  has shown that when  the two frequencies are matched to w i t h i n 1 KHz, the system can be driven.  A r e t r i g g e r a b l e one shot  condition.  The time  constant  and a  latch  of the one shot  c l o s e l y the frequencies are matched.  43  detects  this  determines how  90  W2 splitter  D  Q  -> <J1>U2  C  0  -> W2>W1  splitter low p a s s f1 Iter  mixer  fig.  5.6  Sin(a>l-6)2)t  low p a s s f1 Iter  ml x e r  27  T one shot  COs((Jl-W2)t  -> Aw>T  Frequency comparator.  Spark Detection  Sparks i n the RF c a v i t y e f f e c t i v e l y short c i r c u i t power  amplifier.  initiates,  The avalanche  i s extinguished  condition,  which  when the RF d r i v e  prevent damage t o the RF system, the d r i v e  the c a v i t y and the spark  i s removed. To  i s turned o f f f o r a  minimum o f 1 second when a spark i s detected. A photograph o f the d i f f e r e n t waveforms f o r normal RF Off and a spark are shown i n f i g u r e 9. The spark detector responds t o amplitude s i g n a l s fall  that  70% i n l e s s than 5us. Normal RF Off s i g n a l s decay t o t h i s  l e v e l i n about 50us and do not t r i g g e r the c i r c u i t .  F ron Amplitude Detector  •ne Shot Conpo.ro.tor  fig.  28  Spark Detector  44  5.7  System Hardware C o n f i g u r a t i o n  Long cable runs (60m) between the c a v i t y and the c o n t r o l l e r can introduce diagram  ground  loop  f o r hardware  noise modules  into  the system.  includes  DC  The  blocks  connection  and  optical  couplers that i s o l a t e the c o n t r o l l e r from 60Hz ground loops. The DC blocks are coupling capacitors that pass only the RF s i g n a l s on the cable s h i e l d and center conductor.  Connections  to the tuning  motor and to the computer I/O bus are o p t i c a l l y i s o l a t e d .  o  Phase detectors operate over a r e s t r i c t e d range, u s u a l l y ±90 . The c o n t r o l l e r contains 4 manual phase s h i f t e r s which are adjusted when the system i s i n s t a l l e d . One of the phase s h i f t e r s adjusts the phase delay i n the s e l f - e x c i t e d loop. The other three set the operating range of the phase detectors. Bandpass f i l t e r s  are included on the reference  input  and the  c a v i t y feedback. They have a pass band of ±1 MHz about the 23 MHz o  center frequency and introduce less than ±1 separator's  10 KHz operating range.  phase e r r o r over the  The f i l t e r  i n the c a v i t y  feedback path attenuates out of band noise before the s i g n a l i s presented  t o the phase and amplitude  detectors. A f i l t e r on the  reference input i s needed to c o n d i t i o n the various s i g n a l s that can provide the reference frequency.  45  tuner odjuat TRANSMISSION LINE  ECBLOCK  —>  0  MANUAL TUNING  sample  23 MHZ BANDPASS FILTER  I/O PHASE DET. sample,  TUNER SIGNAL SWITCH  0 AF DET.  I/O  FEEDBACK RF SAMPLE FROM CAVITY  23 MHZ BANDPASS FILTER  <—  RF SOURCE  6 WAY POWER SPLITTER  0 self-excited adjust  z>  m  m  SPARK DETECT RF PRESENT  CYCLOTRON REFERENCE FREQUENCE  WAY POWER SPLITTER 4  PS  PHASE MOD  0 H LAexternal  O o  I*  RF DET.  •H C  I/O -> I/O  LOCK TD BEAM  BEAM LINE 1A CAP PROBE SIGNAL  MOD.  o  DC  BLDCK V  23 MHZ BANDPASS FILTER  (/) r- AMP.  AMP DET.  MOTOR)  detector zero  PHASE DET.  DC BLOCK  MOTOR DRIVE  OPTO COUPLER  TUNER CONTROL  I/O PVM 8. RF SWITCH  fig.  23 MHZ BANDPASS FILTER  DC BLOCK  29  Separator RF Control Modules  RF OUT  5.8  Regulator E l e c t r o n i c s  A s i m p l i f i e d diagram of the regulator loop and i t s connection to the RF system i s shown i n f i g u r e 30. I t i s a PID c o n f i g u r a t i o n i n which 8 b i t m u l t i p l y i n g DACs are used to change the loop gain and the r e g u l a t o r zeros. A 12 b i t DAC i s used to c o n t r o l the set point and an 8 b i t ADC i s multiplexed to monitor the d r i v e l e v e l and the detector output.  Wide band phase and amplitude  modulators and detectors add high  frequency poles to the RF system. These e f f e c t s and the high order poles i n the regulator are not included i n the s i m p l i f i e d diagram because the c i r c u i t i s designed  to reach u n i t y gain before  these  high order terms a f f e c t the loop s t a b i l i t y .  Control s i g n a l s to the RF modulators are generated by operational a m p l i f i e r s and vary between ± 10V. Signals from the amplitude and phase detectors are processed  by s i m i l a r devices and are a l s o i n  t h i s ±10V range. Within an order of magnitude, one can w r i t e the product o f the plant gain and the measurement gain as KaKm « 1. If the  regulator  design cancels  order plant and is unity be adjusted  to be closed  the two dominant poles in the second  gain stable loop stable  then the regulator  gain can  in all TRIUMF RF systems  where KaKm ~ 1. An 8 b i t m u l t i p l y i n g DAC i s used as a v a r i a b l e r e s i s t o r to adjust the c o n t r o l l e r gain, Kc, over 2 decades (48db) between 10 t o 2.5xl0 . T i s adjustable from 0 t o 330us i n 255 5  7  steps. This range i s able to compensate f o r Q's u s u a l l y a t t a i n e d i n copper c a v i t i e s .  47  R-  XD\  1  ;  >  loop switch  open  Pl  o  (1+ST)(1+STa)  closed  Kc , . A . —(1+ST) s  V  Ka  1 (1+STf) open loop switch  lead  measurement  c 1 osed  Km  (1+STa)  (1+STm)  Regulator ffig.  30  -» Plant  PID Reguiator and RF System Model  When the RF system i s operated  i n open  loop,  the PI term i s  strapped f o r u n i t y gain and f o l l o w s the set point. the  i n t e g r a t o r from  drifting  and permits  t r a n s f e r between open loop and c l o s e d loop  This  prevents  a nearly  bumpless  control.  xe  Ideally,  should be equal t o T and, at the cost o f increased complexity, i t i s p o s s i b l e t o make T f adjustable so that i t t r a c k s T .  A simpler  c i r c u i t c o n f i g u r a t i o n with a f i x e d value of T f was used  t o make  the r e g u l a t o r u n i t y gain s t a b l e under a l l s e t t i n g s of Kc and T .  The d e r i v a t i v e term i s r e s t r i c t e d  t o the feedback  c o n f i g u r a t i o n provides less DAC noise on the output  path.  This  as the  set  point changes. I t i s adjustable from 0 to lOOus i n 255 steps. For most systems, the dominant pole can be canceled i n e i t h e r path.  48  The  simplified  circuit at  high  discrete series input  i n the  schematic feedback  frequencies. components with  a  of path.  as  regulator  This c i r c u i t  A better shown  common base  and o u t p u t .  the  stage  with  i n the  which g i v e s an input r e s i s t a n c e o f the  emitter  values  (8f2  should f a l l  follower and t o 45  feeding  O.Oluf) o  the  32.  It  emitter  can is  that  the  is  stage  about  simulation.  I 2.71  49  of  phase  2.21  •4-120  Component  4.5mA  differentiator  4—!••  Discrete  is  the  These  02  32  on  in  2.5fl.  1.21  fig.  from  resistance  8.1UF  8. IUT  built  followers  around 2MHz. T h i s agrees w i t h the  12U-L  lead  perform w e l l  be  The output  capacitor  op-amp  a capacitor  common base  5.5fl.  indicate  an  does not  differentiator  in figure  The c u r r e n t  shows  Differentiator  Discrete Differentiator Response  100  IK  10K  100K  1M  Frequency in Hz. fig.  Several  companies  bandwidths  33  Differentiator  now  i n excess  provide  Simulation  operational  of 20 MHz. Most  of these  transimpedance a m p l i f i e r s which have a high input and a low impedance negative typically  a common base  stage  impedance over a wide frequency  amplifiers  devices are  impedance p o s i t i v e  input. The negative which  with  maintains  a  input i s low input  range. Bench t e s t s show that a  useful d i f f e r e n t i a t o r can be b u i l t using one of these devices.  The  transimpedance op-amp i s also well s u i t e d t o the m u l t i p l y i n g  DAC c i r c u i t s i n the PID regulator. The DACs have a large output capacitance  (120 p f ) which can reduce the s t a b i l i t y of voltage  op-amp c i r c u i t s unless the high frequency  gain i s reduced. Bench  t e s t s i n d i c a t e that the bandwidth and gain of the r e g u l a t o r can be considerably improved i f current input op-amps are used with the m u l t i p l y i n g DACs.  50  Chapter 6 Software  6.1  The Main Program Loop  A program was w r i t t e n t o monitor system t r a n s i t i o n o f the system provides  an  operator  from  operation  one s t a t e  interface  and  supervise  t o another.  and g r a p h i c  display  c o n t r o l l e r v a r i a b l e s . Manual o r a u t o m a t i c c o n t r o l can be f r o m the f r o n t panel.  The  of  the  selected  system  under  i s constrained  to  rules.  c o n t r o l l e r follows a polling  Do  also  The s o f t w a r e p e r m i t s more f l e x i b i l i t y  manual c o n t r o l . In automatic mode the follow rigid  It  loop o f the  form:  While Control=True Scan Knobs Scan Buttons  state  transition  requests  Read_Inputs Fault  Control  Control  Devices  Display_Data End  To  fault  { apply  control  { update  rules rules  display  While  achieve  vectors;  { apply  a new s t a t e , the machine needs t o c o n s i d e r  the present  s t a t e and e x t e r n a l  two  i n p u t . The p r e s e n t  input state  i s a v a i l a b l e i n the machine memory while external input  can come  from the operator or from the RF system as a  spark  detected  or  l o s s of d r i v e , etc. External input acts as a request to change the r e s i d e n t image of the system s t a t e and the appropriate  hardware.  Safe  arbitrary  operation  of the RF  system  does  not permit  t r a n s i t i o n from a given s t a t e t o any other s t a t e . In manual  mode  and i n automatic mode, the requests f o r s t a t e change are f i l t e r e d by c o n t r o l r u l e s . As experience i s gained  with  the system, the  c o n t r o l r u l e s are changed to accommodate new f u n c t i o n s .  6.2  Task Communication  System information i s stored i n g l o b a l v a r i a b l e s , a v a i l a b l e t o a l l tasks. Flags are used  to communicate  between  tasks.  p r i n c i p a l tasks are d i s p l a y and c o n t r o l and the f l a g s  The two associated  w i t h these tasks are:  Amplitude_Display_Mail  Amplitude_Control_Mai1  Phase_D i sp1ay_Ma i1  Phase_Control_Mai1  Sys t em_D i sp1ay_Ma i1  Sys t em_Co nt ro1_Ma i1  Pulser_Display_Mai1  Pulser Control Mail  Each mail f l a g i s two bytes long and has an  internal  structure  i n d i c a t i n g the i n d i v i d u a l requests. I f the f l a g i s zero, the task i s not invoked.  I f the f l a g i s non-zero, b i t s  f l a g as each request i s processed.  52  are reset  i n the  6.3  Control Tasks  A task such as Control_Devices is of the form:  SUB Control_Devices Check_Control_Rules IF System_Control_Mail THEN System_Control IF Amplitude_Control_Mail THEN Amplitude_Control IF Phase_Control_Mail THEN Phase_Control IF Pulser_Control_Mail THEN Pulser_Control END SUB  Check_Control_Rules  filters  the requests  flags to make them compatible  in individual  with the present  control  system state.  Changes are made to the system i f the validated control flag i s non-zero.  Program constants have been declared as integer masks that test individual bits in the appropriate display and control flags. If an individual bit tests TRUE then that specific action is taken and then the bit is reset. This process will clear a l l the bits in the control  flag.  Amplitude_Control  i s typical  of the control  modules.  Sub Amplitude_Control IF Amplitude_Control_Mail AND Setpoint_Flag THEN OUT Amplitude_Setpoint_Port, Amplitude_Setpoint Amplitude_Control_Mail = Amplitude_Control_Mai1 END IF  53  XOR Setpoint_Flag  IF Amplitude_Control_Mail AND Tau_D_Flag THEN OUT Amplitude_Tau_D  Port, Amplitude_Tau_D  Amplitude_Control_Mail = Amplitude_Control_Mai1 XOR Tau_D_Flag END IF END  SUB  Phase_Control and Amplitude_Control w r i t e new values to the phase and amplitude r e g u l a t o r s . The loop v a r i a b l e s that are changed by these r o u t i n e s are: • Setpoint  6.4  Front  • Limit  - hardware l i m i t f o r the modulator d r i v e  • Gain  - loop gain  • Tau_I  - r e g u l a t o r zero  • Tau_D  - r e g u l a t o r zero  Panel  Input  A p e r i p h e r a l card was b u i l t  to l a t c h the front panel knobs and  push buttons. When a button i s pushed or a knob turned, the event sets  a single  b i t i n one  of two  8  b i t registers.  These  two  r e g i s t e r s are read during the p o l l i n g loop and ANDed with masks that  enable  Action  inputs compatible  i s taken  only  with the present  i f the r e s u l t  machine  i s non-zero. The  state. hardware  a u t o m a t i c a l l y c l e a r s the r e g i s t e r s at the end of the read c y c l e , minimizing latency i n scanning the front panel.  54  Push buttons are assigned to toggle boolean system v a r i a b l e s . Both the d i s p l a y and c o n t r o l f l a g s are set by front panel input that i s enabled by the appropriate masks. No problems have  occurred  with  t h i s procedure, however, i t i s probably b e t t e r to s e t the d i s p l a y f l a g when the a c t u a l c o n t r o l i s accomplished.  SUB Scan_Buttons Push_Buttons = INPUT(Push_Button_Port) AND Button_Mask IF Push_Buttons THEN IF Push_Buttons AND On_Off_Flag THEN System_Display_Mail = System_Display_Mai1 OR On_Off_Flag System_Control_Mail = System_Control_Mail OR On_Off_Flag END IF o o o o o END IF END SUB  Input from the f r o n t panel shaft encoders i s more complicated than the boolean i n f o r m a t i o n received from the push  buttons.  I f the  Knob_Register i s non-zero then the D i r e c t i o n _ R e g i s t e r i s read. I f a b i t i s s e t i n the Knob_Register then the D i r e c t i o n _ R e g i s t e r b i t i s t e s t e d t o see i f the associated v a r i a b l e should be or decremented.  incremented  The changed v a r i a b l e i s v a l i d a t e d t o ensure  55  that  (0 £ value £ max_value) and then the appropriate b i t s are s e t the c o n t r o l and d i s p l a y f l a g s .  The system  only  responds  in  to a  person turning one knob at a time.  SUB Scan_Knobs Knob_Register = INPUT(Knob_Port) AND Knob_Mask IF Knob_Register THEN D i r e c t i o n = INPUT(Direction_Port) Increment setpolnt  step s i z e  ( i n i t i a l value = 0)  IF Setpoint_Step < Max_Step THEN Setpoint_Step = Setpoint_Step +1  D  IF Knob_Register AND Amplitude_Setpoint_Knob  THEN  IF D i r e c t i o n AND Amplitude_Setpoint_Knob THEN Amplitude_Setpoint = Amplitude_Setpoint + Setpoint_Step ELSE Amplitude_Setpoint = Amplitude_Setpoint - Setpoint_Step END •  set  2)  ELSE IF o  3)  ELSE IF o  4)  IF • validate 0 S setpolnt c o n t r o l , display  S  max_setpoint  flags  Phase set point  amplitude  loop parameters  ELSE IF o  5)  phase loop parameters  ELSE IF o  pulse width  END IF no knob turned, relax setpolnt  step size  IF Setpoint_Step > 0 THEN Setpoint_Step = Setpoint_Step - 1 END SUB  56  One advantage of scanning the f r o n t panel knobs i n t h i s f a s h i o n i s that i t produces no large step functions outside the response time of the c o n t r o l and d i s p l a y tasks i n the p o l l i n g loop. The loop can be slow (>10ms) and ramping the voltage with a 12 or becomes a tedious j o b with a counts. The apparent  system  response  that  looses  i s changed  s e t p o i n t by an amount that depends on how  by  16  b i t DAC  shaft  encoder  incrementing  fast  the  the f r o n t  panel  the p o l l i n g  loop,  knob i s turned.  I f the program detects that each time through  the s e t p o i n t i s always flagged, then the setpoint turned f a s t e r than  the system  can respond.  knob  The  s e t p o i n t step i s increased u n t i l i t reaches  a  u n t i l the program detects that the setpoint  i s not  size  maximum  i s being of the value  flagged  or and  then the setpoint step i s decreased. In t h i s way the " f e e l " o f the system i s t a i l o r e d to s u i t manual operation.  6.5  Auto  Start  The i n i t i a l s t a r t - u p c o n d i t i o n i s : RF Off Phase and Amplitude loops open Amplitude setpoint = 0 mode = s e l f - e x c i t e d  When a spark i s detected, hardware immediately  turns  d r i v e and s e t s an I/O b i t that i s scanned  the computer  57  by  o f f the RF each  time through the p o l l i n g  loop.  The computer  will  r e s t a r t the  system i f i t i s i n automatic mode.  IF Spark_Detected THEN Spark_Count = SparkjCount + 1 IF Spark_Count = 1 THEN Auto_End_Amplitude  = Amplitude_Setpoint  Initialize Display_Message("SPARK: Pause (8)  w a i t i n g f o r vacuum")  wait 8 seconds  System_Control_Mail = Auto_Start_Flag Fault = True END IF  Sparks can occur during the auto s t a r t process. I f sparking occurs too many times the system w i l l abandon i t s attempts t o s t a r t and wait f o r an operator. Auto_Start can be halted by a system or operator intervention. A s i m p l i f i e d auto s t a r t sequence the form:  Initialize =* E x i t  • Max_Sparks? set Button Mask enable RF Fault or button pushed?  Exit  Pulse at 5% • set amplitude to 60% of target Exit  RF not detected? • wait 2 seconds  58  fault is  of  Fault?  => E x i t  • go CW Fault?  =» E x i t  • wait 1 second Fault or button pushed?  => E x i t  • Close Amplitude Loop Fault?  =* E x i t  • wait 1 second • Close Phase Loop • ramp to target voltage Fault or button pushed?  =* E x i t  • wait f o r beam Fault or button pushed?  => E x i t  • match c a v i t y freq. Fault or button pushed?  =» E x i t  • go d r i v e n Fault or button pushed?  => E x i t  • s e t Button_Mask • Spark_Count =0 Exit  The auto s t a r t routine provides: - a means f o r the operator t o t u r n on the RF system without s p e c i a l knowledge of the system. - a means to a u t o m a t i c a l l y recover from known f a u l t s and t o r e - e s t a b l i s h operation of the separator.  The present auto s t a r t r o u t i n e performs s a t i s f a c t o r i l y but  59  i t is  not well structured. During the wait sequences, the computer scans f o r f a u l t s and can i n i t i a l i z e the system i f a f a u l t  i s detected.  The code associated with t h i s routine needs work.  In general, the perceived performance of the RF w i t h the software and the operator example, i f there i s low voltage  interface from  controller  rests  i t provides. For  the screen  grid  power  supply, i t i s reported as a c o n t r o l l e r f a u l t ; the system does not come up t o voltage when the on button operator  interface  requires  i s pushed.  more d i a g n o s t i c s  A successful and  as  much  development as the does the r e g u l a t o r hardware.  The TRIUMF Controls Group i s working to develop  workstations f o r  s u p e r v i s i n g s i t e processes. Such a system i s used at Los Alamos t o supervise RF r e g u l a t o r loops and provide the user i n t e r f a c e . c o n f i g u r a t i o n w i l l improve the RF c o n t r o l l e r .  60  This  Conclusions  A c o n t r o l system f o r the TRIUMF M9  Separator  designed, and b u i l t . Experience w i t h t h i s  has been  and other  modeled,  RF  systems  i n d i c a t e s that a second order model i s s u f f i c i e n t to c o n t r o l most, i f not a l l , of the TRIUMF RF systems. I t i s not necessary t o know the exact pole l o c a t i o n s or the system gain. The can be adjusted over a range s u f f i c i e n t  regulator  to cancel  zeros  system  poles  introduced by the RF a m p l i f i e r s and by copper c a v i t i e s . Within order of magnitude,  the product  of the plant  measurement gain i s  1 f o r the i n s t a l l a t i o n s  gain  and the  at TRIUMF.  r e g u l a t o r design i s u n i t y gain s t a b l e and the plant  an  loop  The  can be  made s t a b l e given the 48db of gain adjustment i n the r e g u l a t o r .  Significant  cross  coupling  between  the open  amplitude c o n t r o l s i s introduced when the p l a t e  loop  phase  circuit  power a m p l i f i e r i s detuned. This can be almost eliminated  i n the i f the  c a v i t y i s tuned t o the complex conjugate of the p l a t e c i r c u i t . systems where the transmission wavelengths,  line  i s an  integral  tuning  c a v i t y tuning w i t h i n 5° of resonance.  system  keeps  resistive  the average  The disturbance spectrum can  sometimes exceed the bandwidth of t h i s loop, causing  fluctuations  i n r e f l e c t e d power greater than 1% of the forward power.  61  In  number o f  a conjugate tuning scheme should present a  load t o the tube. The present  and  References  1)  P. Sigg, A General RF Control System Concept TRIUMF Design Note TRI-DN-85-27  2)  J . Cherix, RF Phase Detectors TRIUMF Design Note TRI-DN-86-17  3)  J . Cherix, RF Control System Summary TRIUMF Design Note TRI-DN-86-18  4)  L. Durieu, p r i v a t e  5)  F. Pedersen, Beam Loading E f f e c t s i n the Cera Booster IEEE NS-22, June 1975, pl906  6)  R. Burge, Control Options f o r Kaon Factory Beam Loading Los Alamos AHF Accelerator Workshop Proceedings February 1988, p298-307  7)  R. Haussler, Modelluntersuchungen f u r das Regelsystem der 150 MHz-Flattop-Anlage Swiss Nuclear I n s t i t u t e (SIN) Design Note TM-04-33, November 1974  8)  S. K o s c i e l n i a k , A General Theory of Beam Loading TRIUMF Design Note TRI-DN-89-K25  9)  E. Blackmore et a l , An RF Separator f o r Cloud Muons at TRIUMF Nuclear Instruments and Methods, A235(1985) p235-243  communication  10) T. Enegren, R. Burge, D. Dohan A Modular RF Control System at TRIUMF IEEE 1987 P a r t i c l e A c c e l e r a t o r Conference Proceedings, p532 11) T. Enegren, L. Durieu, D. Michelson, R. Worsham Development of a Flat-Topped Voltage f o r TRIUMF IEEE Transactions on Nuclear Science 1985, NS-32  62  12) D. Boussard, Control of C a v i t i e s with High Beam Loading IEEE Transactions on Nuclear Science 1985, NS-32 13) F. Pedersen, A Novel RF C a v i t y Feedback Tuning Scheme f o r Heavy Beam Loading IEEE Transactions on Nuclear Science 1985, NS-32  63  

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