Royer, J P

Royer, Jean-Pierre

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High current with high precision flat-top capacitor discharge power converters for pulsed septum magnets

 HIGH CURRENT WITH HIGH PRECISION FLAT-TOPCAPACITOR DISCHARGE POWER CONVERTERSFOR PULSED SEPTUM MAGNETSJ.P. ROYERABSTRACT In the field of the accelerator, capacitordischarge power converters are widely used.The requirements of the capacitor dischargepower converters imposed by the operation ofthe accelerators, especially for the pulsedseptum magnets, demand particular technicalsolutions.The operational aspects around the protonsynchrotron -PS- at CERN are reviewed.The main features of the capacitordischarge power converters for the pulsedseptum magnets are described with emphasison: -the high current 30kA; -the duration of the flat-top, with high precision10E-4; -thermal and magnetic problems owing to theirregular pulses, current and timing modulation;-the addition of third harmonic components forthe discharge pulse; -the insertion of an active filter; -the general regulation and electronic crate.Recently, to control these converters, a newelectronic (with the G64 bus) has beendevelopped. The continuous evolution allows toplan the manufacture of the power convertersfor the new pulsed septum magnets in thetransfer line Booster (PSB) - PS, taking intoconsideration the future requirements for theLHC, with better standardization and reliability.1- GENERAL  DESCRIPTIONThe accelerators use a large quantity ofcapacitor discharge power converters,particularly in the injection and transfer line toextract and to guide the beam. The principle ofthese power converters is based on discharge ofthe capacitors in synchronization with the beam.Most of the capacitors discharge powerconverters are designed from a resonantdischarge circuit, with the possibility of energyrecuperation (through the magnet or an auxiliaryinductance) or energy destruction (throughauxiliary inductances and resistances). Theenergy destruction circuit is used when pulse topulse peak magnet current modulation isrequired (the voltage after energy recuperationcould be higher than the next pulse expected).Depending on the magnet and the operation,the capacitor discharge power converters forpulsed septum magnet are designed to obtain  aprecision of 10E-4 during 200µsec to 600µsecwith a current of 30 kA maximum.2- PULSED SEPTUM MAGNETS AROUNDTHE PSThe septum magnets [1] require a highcurrent, during the ejection for the extraction ofthe beam. The table 1 shows the current, theresistance and the inductance of the magnets.Table 1MAGNET I max (A) L(uH) R(mOhm)PR.SMH16 30000 5.53 0.50PR.SMH26 8000 1.03 0.80PR.SMH58 24000 2.56 0.24PR.SMH74 14000 2.00 0.34PR.SMH92 14000 2.00 0.34HR.SMH00 20000 2.00 0.34BI1.SMV 20000 0.30 0.03BI2.SMV 20000 0.30 0.03BI4.SMV 20000 0.30 0.03BI.SMH 3500 6.00 1.50BTSMV30 2000 5.90 0.20BT1.SMV10* 30000 2.14 0.11BT4.SMV10* 30000 2.14 0.11BT.SMV20* 30000 2.14 0.11BE.SMH* 6000 20.00 3.32PI.SMH42* 35000 1.23 0.07∗ future manufactureThe deflection of the beam is given by theformula :α =⋅ ⋅300 B lpα  : deflection (milliradians)l   : equivalent length (meters)B  : magnetic field (Tesla)p  : Beam momentum (Gev/c)For example, the deflection of the beamwith the septum 16 is 30 milliradians with 1Tesla of magnetic field and equivalent magneticmagnetic lenght of 2 meters.The situation of the pulsed  septum magnetsaround the PS and the PSB is shown Fig. 1.Fig. 1 : the pulsed septum magnets around the PS and the PSB.3- OPERATIONAL ASPECTS [2]Particle beams of different kinds andcharacteristics are exchanged between the PSand the other accelators or external beamtransport lines as shown table 2 .                           Table 2ACCELERATORS OR BEAM LINES      PARTICULESLinac 2 pLinac 3 ions PbPSB p , ions PbAAC, LEAR p , p barsLIL, EPA e+ , e-PS p,p bars,e+,e-,ions PbTT70 eject. SPS-LEP e-,p  barsTT10eject. AAC et SPS-LEP p ,p bars,e+ ,ions PbThe PS supercycle is made of the basiccycles A,B,C,D,E,F,G,H dependent on the useof the different beams.The Fig. 2 shows thebasic PS cycles.S P SA14 Gev/cE A S TH A L LB24 Gev/cA A CC26 Gev/cL E A R  o r  T S Tf o r  A A C3.5/0.6 Gev/cDS P S / L E P3.5Gev/cES P SF20 Gev/cG(ions)1 Gev/c MD ORE C O N O M YMD (LHC)H26 Gev/cFig. 2 : Basic PS cycles.During operation with leptons at 3.5 Gev/cthe double batch mode requires a stable currentflat-top and double pulses at 30 msec interval.Because of the irregular pulse repetition andthe pulse to pulse modulation, the power supplyof the septum 16 is the most demanding [3]. Anexample of PS supercycle is shown in Fig. 3.The working instants of the septum 16power converters are symbolised by themarkers below.        SPS p        14 Gev/c            A       AAC p  3.5/26 Gev/c          C      LEAR p     3.5 Gev/c           D       AAC p  3.5/26 Gev/c          C      SPS e+    3.5 Gev/c        E      SPS e-    3.5 Gev/c       E septum 16 pulsesFig. 3 : example of PS supercycleThe pulsed capacitor discharge powerconverters for the septum 16 must work duringthe ejection at the values indicated in table 3.                           Table 3Line PLS Beam I septumFE16S-FE16A protons 26 Gev/c 28700 AFE 16I ions 20 Gev/c 21800 ACT protons 14 Gev/c 14400 AFE 16L positrons 3,5 Gev/c   3400 AFI 16A p  bars  3,5 Gev/c   3790 A4- PRINCIPLE OF THE PULSE CAPACITORSDISCHARGE POWER CONVERTERS.The principle is based on the charge anddischarge capacitors through a resonant circuitbetween capacitors and the load.The charging current of the capacitors iscontrolled via thyristors on the primary side of ahigh voltage transformer. The d.c. voltagerequired at the capacitor terminals  comes fromthe rectifier on the secondary side of thetransformer.The d.c. voltage and current are measuredby voltage dividers and a shunt.Once charged, the capacitors aredischarged via a power thyristor through a thirdharmonic circuit and a choke for the active filter.The discharge is adapted with a matchingtransformer to obtain high current. A powerdiode is connected in series with a choke and aresistor to destroy the energy in order to obtainthe pulse to pulse modulation facilities.The scheme of the power converter isshown Fig. 4. 5- THE THIRD HARMONIC CIRCUITThe classical equation of the dischargecurrent in a resonant circuit (with p Laplacian )is:[ ]I p UL p p( ) A= ⋅+ ⋅ ⋅ ⋅ +12 2 20 0ξ ω ω with  ξ << 1 the time domain equation is:i t ULe tt( ) sin=⋅⋅ ⋅− ⋅⋅ωωξ ω0ω 01=⋅L C        ω =⋅−⋅1422L CRL    ξ = R CL2In order to obtain a better flat-top currentthan the basic sinusoidal discharge current, athird harmonic with extra parallel or series L, Ccircuit is added.The  two  possibilities  of  the third harmonicFig. 4 : scheme of the power converterare shown Fig. 5.  With the ideal lossless case,the basic formula is :           i t k t k t( ) sin sin= ⋅ + ⋅1 2 3ω ωa) third harmonic circuit with serial capacitorsCC1 L1LU0U L1U CU Lt*(T/2)II11II/ILMAXUUU1U/U0t*(T/2)1b) third harmonic with parallel capacitorsU0C2C3L2U L2U C3LU LC = C2 + C3U/U0I/ILMAXII1I1t*(T/2)UUU1t*(T/2)1Fig. 5 : electrical circuit and waveforms of the twopossibilities of the third harmonic.To optimize the third harmonic, it isnecessary to adjust the L, C circuit. An exampleof simulation for the design of  the powerconverter SMH42  is also shown Fig. 6.0m 2m 4m 6m 8m 10m-1.00K0.00K1.00K2.00K3.00K4.00KI magnetSTEP : C2FROM : 1186 uFTO : 1386 uFSTEP VA LUE : 50 uF    0m 2m 4m 6m 8m 10m-1.00K0.00K1.00K2.00K3.00K4.00KI magnetSTEP : C3FROM : 1614 uFTO : 1814 uFSTEP VA LUE : 50 uFFig. 6 : waweforms with third harmonic components(ex. : SMH42) and simulation to adjust the flat top.Fig. 7 shows the difference between the topof the magnet current with third harmonic andwithout third harmonic. In Fig. 7, the flat topadjustment possibilities with L variations areshown.I magnet1.88m 2.28m 2.68m 3.08m 3.48m 3.88m2.50K2.60K2.70K2.80K2.90K3.00K1.88m 2.28m 2.68m 3.08m 3.48m 3.88m2.50K2.60K2.70K2.80K2.90K3.00KSTEP : L 2FROM : 0.259 mHTO : 0.299 mHSTEP VA LUE : 0.01 mHI magnetFig. 7 : comparaison and adjustement of the thirdharmonic with the vertical injection booster pulsedseptum power supply.These examples are from the power supplyof the vertical injection booster pulsed septummagnets.6- MAGNETIC AND THERMALPROBLEMS IN RELATION TO TIMING ANDAMPLITUDE MODULATION.The current reproducibility and precision∆I I  for the septum magnet is 100 ppm duringthe flat -top  ( 200 to 600 µsec).Any minor variations lead to the non-respectof this stability.The variation of the temperature andmagnetic problems caused by irregularrepetition rate must be compensated to obtainthe right stability. 6-1 The thermal problem : At high current,with the same repetition rate and the samevoltage on the capacitors, the variations areshown on the Fig. 8.All the variations caused by the temperatureare seen by the converter with the square of theratio of the matching transformer. R R m R= + ⋅122         L L m L= + ⋅122           R L,    resonant circuit ;   R L1 1,   primary circuit  ;   R L2 2,  secondary circuit ;    m  ratio of the matching (4< m <50) ;Tr = 1,2 s   Im = 27 kA   m = 12Im : 30 A/d ; 100 uS/d              Im : 30 A/d ; 5 s/dTr = 2,4 s  Im = 27 kA   m = 12Im : 15 A/d ; 100 uS/d            Im : 15 A/d ; 5 s/dFig. 8 : current variations with the same voltage onthe capacitors and repetition rate.The studies of the variation, with regular andirregular repetition rate are indicated in Figs. 9and 10.a) Regular repetition rate with the samevalue of current:    Two curves of instantaneous temperature forrepetition times of 1.2 second and 2.4 secondsare shown schematically in Fig. 9.1 2 3 4 5 6 7 8 9 10 11K I 2 1.2 t (sec)*0f at 2.4 sec. 0f at 1.2 sec. 0 10 20 30 4O C0Fig. 9 : curve of increase of temperature with 1.2 sec.and 2.4 sec of repetition time.   The mathematical expressions of thesecurves are:θ 1 2= kIθ τ2 2 2= ⋅ +−kI e kIt /θ τ τ3 2 2 2= ⋅ + +− −( )/ /kI e kI e kIt tθ τ τ τ4 2 2 2 2= ⋅ + + +− − −(( ) )/ / /kI e kI e kI e kIt t t-----------------------------------------------------θ τ τ τn t t ntkI e e e kI= + + +− − −2 2 2( ..... )/ / /θ τf t nnnkI e= ⋅ −==∞∑21( )/b)Irregular repetition rate with amplitudemodulation of the current:      An example of an increasing temperaturecurve is shown schematically in Fig. 10.1 2 3 4 5 6 7 8 9 10 11 1.2 t (sec)*O C0supercycle0f average withthis supercycle 0 10 20 30 4Fig. 10 : curve of increase of temperature during asupercycle.The mathematical expressions of thesecurves are:θ 1 12= kIθ τ2 12 1 22= ⋅ +−kI e kIt /θ τ τ3 12 1 22 2 32= ⋅ + +− −( )/ /kI e kI e kIt tθ τ τ τ4 12 1 22 2 32 3 42= ⋅ + + +− − −(( ) )/ / /kI e kI e kI e kIt t t----------------------------------------------------------------θ τ τnt t t nnt nnkI e kI e kI= ⋅ + + ⋅ +−+ + −−−−121 2 11212.. ( ) ( )....θτf nn tnk I e nn= ⋅∑− −∑−2111/We can demonstrate that ∆I kIf f≈ ⋅θ θ .So, with a regular rate (t = regular rate ) andthe same current:                 θ τf tnnnkI e= −==∞∑21( )/                 ∆I k I e t nnn= ′−==∞∑31( )/τWith irregular rates and amplitude varyingcurrent :              ∆I k I I en ntnnnn= ′′ ⋅∑−− −∑ 2111/τ ∆I  is the current correction to obtain acurrent stability independent of the temperature.To correct this variation, the regulation mustincrease or decrease the voltage on thecapacitors, before the discharge.6-2 Magnetic variations: The matchingtransformer is a special manufacture with a verylow stray inductance (0.3µH referred to thesecondary).To reduce the stress on the septum magnetand to obtain pulse-to-pulse modulation, thenegative pulse is supressed. However in thiscase, with amplitude modulation, the remanentinduction is not stable, the level depends on thecurrent value and the precision of the dischargecurrent is influenced. In order to minimize thevariations, an air gap is used in the magneticcircuit.  7- THE ACTIVE FILTER FOR FLAT-TOPCURRENT.To obtain the flat-top current with a stabilityof  10 4−  and a time of 300 µsec  (600 µsec. forthe ions ) an active filter is necessary.    The principle of the active filter is indicatedin Fig. 12. The voltage at the main capacitors isregulated to give a current slightly higher thanthe reference current. As  Z Z1 2〉〉  the current ofthe active filter is ≈ 200A for  a flat top of 300µsec depending on the circuit values.Ref.IseptumU command+-I septum60 MOS INPARALLEL220V DCACTIVE FILTER Iaf I2I1Iaf = I1 + I2 with I1>>I2Iseptum = Idischarge - I2Z1Z2Fig. 12 : principle of the active filterWith Z Z1 2〉〉  we can pull the excess currentthrough the inductance of the active filter. Thesystem acts in closed loop through the matchingtransformer, the current transformer and theactive filter composed of 60 MOS ( type BUZ 45) in parallel. Fig. 13 shows the Bode plot of the gain ofthe ratio I septum / I active filter and the magnetcurrent step response for U command = 1 V inopen loop.10 100 1K 10K-100.00-80.00-60.00-40.00-20.000.00fG=I2/Iaf0m 0.40m 0.80m 1.20m 1.60m 2m-1.000.001.002.003.004.00I2(step answer)Fig. 13 : Bode plot between I2 and I active filter andtransient response of the system in open loop withoutcorrection.We can consider the active filter as acurrent source in opposite direction to thecurrent from the discharge of the maincapacitors. The block diagram of the controlsystem is shown in Fig. 14.kA ( s ) B ( s ) C(s)+ _+_Im ref. ImFig. 14 : block diagram of active filter regulationThus the system is composed of:- a controller : PD or PID as below (PR.SMH16values) :A s k e s e se s s( ) ( ) ( . )( )=⋅ + − ⋅ ⋅ + − ⋅+ − ⋅ ⋅1 11 6 1 1 31 1 6- a block representing the power MOS transferfunction :B s e se s e s( ) ( . ). .=⋅ + − ⋅− ⋅ + − ⋅ +30 1 23 54 75 9 669 6 12- a block representing the power circuit :C s IIafA s B s C s D sa s b s c s d s( ) = = ⋅ + ⋅ + ⋅ + ⋅⋅ + ⋅ + ⋅ + ⋅ +214 3 24 3 2- a parameter K that represents the maincapacitor discharge.This model allows the study of the accuracyand stability of the system. Fig. 15 indicates aresult from simulation with PID controller at15000A and Fig.16 the real curves with differentcurrent values. The simulations are realised withMC4.I septum81m 81.30m 81.60m 81.90m 82.20m 82.50m14.90K14.95K15.00K15.04K15.09K15.14K81m 81.30m 81.60m 81.90m 82.20m 82.50m0.0040.0080.00120.00160.00200.00I active filterFig. 15 : results of a simulation with PID controllerand PR.SMH16 system values.           Im=24000 A                             Im=3500 AIm : 30 A/div. 100uS/div          Im : 30 A/div. Iaf : 50 A/div.Fig. 16 : septum current (PR.SMH16)8- GENERAL REGULATION ANDELECTRONIC CRATE    The general diagram of the regulation circuitsis shown Fig. 17.Fig. 17 : general diagram of the regulation systems.To optimize the results, the completeregulation requires  the following circuits:-regulation of charge capacitors: voltageregulation with charging current loop.-correction for amplitude and timingmodulation: simulation of temperature.-circuit regulation for active filter control-active filter power.To compense the different time constantsand the long term effects, the active filtercurrent is verified at each discharge and acorrection is given if the current is too low or toohigh, so we obtain permanently the right valuefor the the compensation of the temperaturecurve.Electronic crate:The electronic crate uses a G64 bus with a6809 Motorola microprocessor.The electronic cards have been developedto respect different criteria : standardization,maintenance, protocol, timing with sequenceforewarning, warning , start, measure pulses.The cards used inside a crate for these pulsedpower supplies are given in the table 4.Table 4 2 × 32 Europe G64 busGate control "Argentine" Micro term interfaceCapacitor discharge control PIA optoActive filter control 2 PIAPIA interface Fast ADC 16 bitsAnalog default 6809 CPU RAM/ROMThermal correction Surveyor simulation timingTiming unit Single transceiver or 1553Relay unit 1 and 2 DAC 16 or 14 bitsHOLEC DCCTAll the electronic is inside this crate as wellas the interface for the control, with MIL1553 orEurope Single Transceiver.This crate is controlled locally by a touch-panel and a display.9-CONCLUSIONThe enormous work realized on this type ofpower converters have permitted to achieve theextremely demanding performances required forthe ejection of different particles at differentenergies.In spite of the complexity of these powerconverters - different characteristics of themagnets, special operation - we have treatedall the power converters with the sameapproach which leads to a betterstandardization and reliability.The experience in this field allows us to lookat the future with optimism about thereplacement of the d.c. septa of the transfer linePSB-PS by pulsed septa. Consequently,  weforesee the manufacture of capacitordischarge power converters, well able totake into account the future requirements(PSB to 1,4 Gev) for the Large HadronCollider (LHC).ACKNOWLEDGEMENTSI would like to thank the numerous peoplewho have worked on the capacitor dischargepulsed power converters, as well as those whowere in charge of studies and the tests.REFERENCES.[1] M. Thivent ,The septum magnets, privatecommunication,  1994.[2] J. Boileau ,Operation of the complex PS,CERN PS 94-07.[3] C. Ducastel, J.P. Royer, F. Volker,  Thenovel capacitor discharge power converters forthe pulsed septum magnets in the PS straightsections 16 and 58, CERN PS 88-26.[4] Siemens product information, Powersemiconductors SIPMOS and IGBT, 1991.[5] J.M. Cravero, Model studies for the activefilter in the power converters for pulsed septummagnets, to be published.   

High current with high precision flat-top capacitor discharge power converters for pulsed septum magnets

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