Dwersteg, B

Matheisen, A

Möller, W D

Proch, D

Renken, D

Sekutowicz, J

CERN Document Server

Particle Accelerators, 1996, Vol. 54, pp. [301-314] /1-14Reprints available directly from the publisherPhotocopying permitted by license only© 1996 OPA (Overseas Publishers Association)Amsterdam B.V. Published in The Netherlands underlicense by Gordon and Breach Science Publishers SAPrinted in MalaysiaOPERATING EXPERIENCE WITHSUPERCONDUCTING CAVITIESIN THE HERA E-RINGB. DWERSTEG, A. MATHEISEN, W.-D. MOLLER, D. PROCH,D. RENKEN and J. SEKUTOWICZDeutsches Elektronen Synchrotron DESY,Notkestrasse 85, 22603 Hamburg, FRG(Received 22 January 1996; infinalform 22 January 1996)In this paper a short description ofSRF part ofthe HERA electron ring is given. Statistics ofbeamloss due to the failures of the superconducting cavities and discharging in the fundamental modecouplers is discussed. Computed and measured effects of wrong phasing on cavity performanceis presented.Keywords: Superconducting cavities1 INTRODUCTIONSuperconducting RF installation in the HERA e-ring has been completedin April 1992. The installation itself and the operation experience tillmid of 1994, have been presented already in the Proceedings of formerSRF Workshops1 and Proceedings of the European Particle AcceleratorConferences.2 The installation is made of:• sixteen sc cavities,• two klystrons and distribution system of the microwave power,• interlock and control electronics,• cryogenic and vacuum systems.A briefdescription of the RF part ofthe installation, interlocks and controlscircuits is given below.[301]/1[302]/2 B. DWERSTEG et al.1.1 Cavities and CryostatsSuperconducting, 4-cell, 500 MHz cavities of the HERA e-ring are made ofbulk niobium. Each cavity is equipped with one fundamental mode couplerand three HOM couplers. All couplers are made in coaxial line technique.Cavity and HOM couplers are dipped in 4.2 K LHe bath. Two cavities areassembled in one cryostat, symmetrically with respect to the mid plane ofthe cryostat. These cavities are connected by 3/2 A long beam tube, to whichboth fundamental mode couplers are attached. Since the nominal Q ext ofall FM couplers has been fixed to 2.5 x 105, frequency adjustment of eachcavity can be performed by means of slow mechanical tuner, which correctsthe length of the cavity.1.2 RF Source and the Distribution SystemMicrowave power is generated by two klystrons and guided down to thetunnel by WR1800 type waveguide. Both klystrons deliver maximum powerof Pk = 1.1 MW under cw operation (0.55 MW each). The nominal value ofPk =1.5 MW (0.75 MW per klystron) was not achievable due to the limitationin the power supply. Recently, problem has been solved and in the future wewill try to operate cavities with more power. The maximum input power percavity was, up to now, 64 kW (waveguide losses are 7%). Equal distributionof the power is done in the tunnel, by 9 directional couplers and 6 magicT's. In front of each cavity, 3-stub waveguide transformer is placed, to makepossible:• change of the Q ext in the range from 5 x 104 to 1.25 x 106,• phase adjustment of the cavity in the range ±50°.1.3 Interlocks and Control ElectronicsTo avoid damage of the system, several signals from each cavity and eachcryostat are monitored during the operation. Interlocks signals are listed inTable I. In addition to the interlock electronics, the SRF system is equippedwith voltage and phase control loops to provide proper operation ofthe systemfor various beam loading conditions. The three most important control loopsare as follows:OPERATING EXPERIENCE WITH SC IN THE HERA [303]13• Cavity Phase Lock Loop which compensates for the beam induced voltageby changing the resonant frequency of the cavity,• Voltage Control Loop (VCL), which adjusts the power of both klystrons,to keep a constant sum of all 16 voltages, Vb = b Vn , for different beamloading and thus makes the efficiency of the system higher for the broaderIb range,• RF Phase Control Loop which keeps constant the phase between Vb andthe reference phase.The profit of using the VCL can be seen in the computed example shown inFigures 1 and 2. The total voltage Vb = 34 MV, as required, stays constant forthe different beam current Ib and the fixed synchrotron phase CPs = 45°. Theklystron power delivered to the cavities is adjusted and the reflected poweris minimized. Without the VCL we would observe a drop of the Vb for Ib 2::25 rnA or a change of the synchrotron phase and increase of the Vb and ofthe reflected power, for Ib ::::: 25 rnA. The second situation would happenat the end of every luminosity run, when Ib is reduced due to the finite lifetime of the beam. The VCL makes the efficiency 1] = Pbeam / Pklystron vs.Ib higher than 75% already for Ib > 22 rnA, although the system has beenmatched for the beam current of Ib =45 rnA.TABLE I InterlocksKind of interlock Measured value Measured byFundamentalMode Couplertemperature of the ceramic windowmultipacting currentlight intensity at the vacuum sidelight intensity in the doorknob regionpressure of air cooling ceramic windowtemperature of He gas cooling the inner conductorinfrared detectorpickup probelight detectorlight detectorpressure sensortemperature sensorHigher OrderMode CouplersCryogenicInterlocktemperature of all 3 feedthroughsplaced between the PTIOOiso-vacuum and airquench detector LHe level in each cryostatVacuumInterlockRF distributionsystempressure in the beam lineforward and reflected power at the input of allabsorberscurrent of ion pumpand vacuum gauge[304]14 B. DWERSTEG et al.1.1 _ L.._L 1. __.1 _._t _L-__ l _.~}=:-j'~.._ _.1 ~ powe~ de1ive~ed by I 1 .,...,.,,.-t--- l ~;~ :=~-:~:=t~t=:E~==I::~3~~~~J::;~f;=:o ············""I"············"1"·············i.. r············ + + ·············T·············oFIGURE 1 Forward and reflected power vs. Ib with VCL.l--···I···o /: : 1: ::oFIGURE 2 Efficiency rJ == P bearn / Pklystron vs. I b with VCL.50502 LUMINOSITY RUNS, HIGH CURRENT EXPERIMENTS ANDSTATISTICSThe operating time of the SRF system since June 1991 till October 1995 isin total about 23000 hours. The system was cooled down and warmed up22 times without mechanical ,or any other kind of damage. Table II containsa brief history of the system. Figure 3 shows the increase of the averagestored current (1991) and the average stored current at the beginning ofluminosity runs (1992 -7- 1995) vs. years of operation. Maximum klystronpower delivered to the sc cavities vs. years of operation is given in Figure 4.DateOPERATING EXPERIENCE WITH SC IN THE HERATABLE IIComment[305]/5June 1991April 1992Oct. 1992Nov. 1992June 4,1995June 6,199512 sc and 84 nc cavities installed in the HERA e-ring.Upgrade of energy from 28 GeV to 30.4 GeVStarting of the operation of all 16 cavitiesbegin of the luminosity runsmax. accelerating voltage V:E = 76 MV achieved aftersystem has been assembled in the tunnelmax. stored h =47 rnA at 27 GeVmax. stored h = 52 rnA at 12 GeVpolarizationmeasurementslimited by globalheating of two cavitiesmachine studymachine study30I 20b[mA.]10o1991 1992 1993 1994 1995FIGURE 3 I b vs. years of operation for the luminosity conditions.1000800P 600[kW]400200o1992 1993 1994 1995FIGURE 4 Maximum klystron power delivered to sc cavities vs. years of operation for theluminosity conditions.[306]/6 B. DWERSTEG et al.SRF klystrons& absorbers. 170/0FIGURE 5 Beam loss statistics July-September, 1995.2.1 Reliability of the SRF SystemThe recent statistics for the period from July to September 1995 says that40% of all losses of the e+ beam are caused by the SRF system (Figure 5).There are two dominating problems:• loss of the beam caused by the failure of klystrons or absorbers,• loss of the beam caused by multipacting or plasma discharging in PMcouplers.One more phenomenon has been observed during the high currentexperiments in November 1994:• loss of the beam caused by strong detuning of cavities with the wrongphasing.3 SRF SYSTEM FAILURES3.1 Multipacting and Plasma Discharging in FundamentalMode CouplersTo investigate these phenomena several signals are continuously recordedby the fast data logging program. If the beam is lost, all signals from 75 msOPERATING EXPERIENCE WITH SC IN THE HERA [307]/7before to 25 ms after the loss of the beam are stored on the computer harddisk. Monitored signals are listed below:Interlock signals:• multipacting current Ie of each cavity* ,• input power P abs of each absorber in the tunnel*.Additional signals:• voltage Vn of each cavity* ,• phase lock signal of each cavity V;h '• total klystron power Pkly ,• beam current Ib,• amplitude of Vb'Signals marked with * are measured by the multiplexer. Sixteen values ofthe Ie, Vn , P abs and V ph , are sent to the computer and re-measured every2ms.The typical event of the multipacting and plasma discharging in the FMcouplers is presented in Figures 6a-c. The electron current Ie (Figure 6a),measured by the electron probe, has a high amplitude and triggers theinterlock loop. Since the delay time of all interlock loops has been fixedto 50 ms, the SRF system continues operation and the beam stays still in thering for this time. At t = 0, klystrons are switched off (Figure 6b) and thebeam is lost (Figure 6c), but discharging in the coupler remains until the inputpower decreases to a quarter of the initial value. The observed hysteresis ischaracteristic for the plasma discharging. It is very probably started by themultipacting, since its initialization happens only at some levels of the inputpower. Two periods of multiplexed Ie and Vn signals, at the beginning of thedischarging, are shown in Figures 7a,b. The first period on both diagramsshows 16 Ie signals and 16 Vn signals, just before discharging occurs. Inthe second period we can see discharging in the coupler of cavity 3 and 4.The voltage in both cavities drops very fast. It gives an indication that theinput power is reflected by the "short" produced by the discharging whilethe still circulating beam takes out the energy from the cavities. The plasmadischarging decouples cavities from the waveguide and causes increase ofthe external Q. It was observed very often that during the plasma dischargingthe cavity was firstly emptied and then filled again by the beam, to thehigher energy, such that the induced voltage reaches a value much biggerthan Ib x (Rj Q) x Q ext (Figure 8). In addition, in many cases we observed[308]/8 B. DWERSTEG et al.16ot[ms]0.L- --.;...... --=-575Ie[J-lA]1dy[MVV]FIGURE 6 Multipacting and plasma discharging in FM couplers.that discharging spreads to the coupler of the neighboring cavity, housed inthe same cryostat. This can be explained by the rather short, 60 cm, distancebetween two couplers. To prevent PM couplers from multipacting and plasmadischarging phenomena, pulse conditioning of the system has been applied.The processing is performed with a pulse length of 5 ms and a klystron powerPkly = 1.2 MW. To enlarge part of the surface in the PM couplers, cleanedduring processing, all cavities are simultaneously detuned by ±45°, to movestanding wave pattern in the couplers. This way of conditioning turned out tobe extremely effective in suppressing ofmultipacting and plasma discharging.OPERATING EXPERIENCE WITH SC IN THE HERA [309]/9Ie[~]'r-:,.....L.-~~+-+-~~~-'"o h--, i I ~ j ~ 1 ~ ~ ~ ~\-55 t [ms]linV.•.••,U:!i.:: I.•,~~~f\.~ ~ ~ ~ j ~ ~ ~ ~ ~ ~ l~-50-.50.61t [ms]~1hl~~ LLlu~~o -A--A 11213 I! i i-.5.5[MV]FIGURE 7 Beginning of the discharging in the FM coupler.t [ms]FIGURE 8 The empty cavity No.3 is re-filled by the circulating beam and 2 ms later, voltageof this cavity reaches 5 MV.3.2 Cavity Trips Caused by Phasing ErrorsThe strongest detuning of the cavities by the beam takes place duringinjection when the average synchrotron phase (¢) is near to 90°. Themeasured phasing errors of the individual cavities were in the range of ±20° .There are two main contributions to them: limited accuracy in the phaseadjustment and thermal effects in the power distribution system, occurringat the operation. The phasing errors and the strong beam loading cause big[310]/10 B. DWERSTEG et al.: : :.o~.,-"",,:,,,-,o.,,o·'··'-·':-··"'··-'·····-r--'-'o--. ",..so403S~ ~5I b [mA]t510'"~ ~:#1\\: ;_~_.o_o.o_..._.._..._!_.. -_..._-.._..._-+_..._..._.. o_.. _..._>_..._o.._..._........!o_..._..._.,._.-,_":,_..._.,,_,,._-.._.. ~__.. ,_..._..._.._... __,~._.._..._..._..........\_<.~._\__\ ..._.........: _.._..._.. '_"'~ IoFIGURE 9a Detuning ofn vs. lb. Phase of cavity No.1 <PsI = 70° and <Ps2 -7- <Ps16 = 85°.c#3I-c#16 . Q-2.5E.5.so45.0352015...~....::::::::::::~:::~ ......_._-........_._...._~~.: .. ;-;;--;;;;--,---10FIGURE 9b Voltage V c vs. h. Phase of cavity No.1 <PsI = 70° and <Ps2 -7- <Ps16 = 85°.differences in the detuning of individual cavities. This has been observedduring the high current experiments, in November 94.3 As the VeL keepsV~ constant and does not control Vn of the individual cavities, voltagesof cavities with smaller ¢s drop while voltages of cavities with bigger ¢sincrease. The first group ofcavities is detuned by many kHz towards the lowerfrequency. Spectral lines of the HERA e-ring are separated only by 47 kHz,OPERATING EXPERIENCE WITH SC IN THE HERA [311]/11TABLE III Measured and re-calibrated values of VnCavity I b = 0 [rnA] h = 47 [rnA]NO. Q ext [105] Vn [MV] Vn [MV]1 2.39 1.7 2.22 0.81 0.9 0.73 2.20 1.6 2.34 2.45 1.7 1.55 2.30 1.7 1.56 2.13 1.6 1.27 2.50 1.7 0.88 2.41 1.7 2.09 2.37 1.7 3.210 2.47 1.7 1.111 2.57 1.8 2.712 2.76 1.9 2.713 2.46 1.7 1.514 2.52 1.7 0.615 2.44 1.7 0.916 2.01 1.6 0.5Vmax / Vrnin 1.19 6.40thus strong detuning may cause the induced voltage by the lower spectral lineto be comparable with the voltage of the fundamental mode. Figures 9a,bshow computed detuning and voltage for individual cavities vs. Ib, when thewhole SRF system is operated at VI: =25 MV. In this computer simulation,cavity No.1 has a synchrotron phase <PsI = 70°. Other cavities have phases<Ps2 -7- <Ps 16 = 85°. We can see here, that even for the smaller phase errorsof ±7.5°, strong beam loading causes that the voltage of cavity No.1, forIb = 50 rnA, drops to 20% of its value at the beginning of the injection.The measured results are presented in Table III. Voltages without beam arelisted in column 3. Column 4 contains Vn measured with 47 rnA beam, at aninjection energy of 12 GeV. In both cases, the system was operated at a totalvoltage VI: =25 MV. The ratio of the maximum voltage Vmax to the minimumvoltage Vrnin (bottom row)Q changes here from 1.2 without the beam to 6.4with the beam. To keep the system less sensitive to phasing errors, Q ext'Sof all cavities, except cavity No.2, have been changed to half of the valueslisted in Table III. In 1995 cavities were mostly operated with the total voltageVI: higher than 30 MV. Under these new operation conditions cavity trips[312]/12c/J[deg] 9)90B. DWERSTEG et al.J 3 4 .5 6 7 8 9 10 11 12 13 14 15 16Cavity NoFIGURE 10 Phases of 16 sc cavities measured at the injection.have not been observed in this year, even for the maximum stored current of52 rnA, although the phasing errors, after the last correction, are still in therange of ±14° (Figure 10).4 CLOSING REMARKSThe last two years of operation have shown that the SRF system is able to:• transfer nearly 1 MW power to the beam,• produce total voltage of 35 -;- 40 MV for luminosity runs,• store and accelerate up to 39 rnA beam for the luminosity runs and storeup to 52 rnA beam for the machine studies (high current experiments).In addition, high current experiments performed during the machinestudies showed that HOM power induced by the beam does not exceed theexpected value of 120 W/cavity and can be transferred outside cryostats tothe extemalloads by the coaxial cables and feedthroughs. Two times (once1993 and once in 1994) the beam was dumped by the feedthrough temperatureinterlock. In both cases, the feedthrough has been replaced and no temperatureaRatio is computed without cavity No.2, because Q ext of this cavity is low for quenchreason.OPERATING EXPERIENCE WITH SC IN THE HERA [313]/13increase has been measured after the exchange. No quenching and no changein PM rejection filter performance have been observed for all 48 couplersduring the five years of operation.4.1 Klystron PowerThe electron beam current, stored for the luminosity runs, still does not reachthe design value of 58 rnA but increases in average by 6.6 mAlyear. To enableoperation at 35 -;- 40 MV for the d~sign current, one should make use of thenominal klystron power of 1.5 MW. As it was mentioned before, very recently,limitations in the power supply have been solved and we will operate bothklystrons in the next year at the nominal power of 1.5 MW. The first attemptto increase the power has been carried out right after this Workshop. Theoperation with the klystron power 1.35 MW and 39 rnA beam at 27.5 GeVwas successfully performed.4.2 Muitipacting and Conditioning of the SystemLast two months of operation showed that 2 hours of pulse conditioning oncea day (during the injection of the proton beam, no additional time needed)suppresses very efficiently the multipacting and the plasma discharging in thecouplers. The only one beam loss, caused by this phenomena, has happenedin that time.We have to proceed this way of conditioning for a longer time to be surethat no other means like the bias voltage or the baking will be necessary toincrease the reliability of the system.4.3 Phasing of the CavitiesPhase adjustment for the high current operation plays an important part. Withthe present values of Q ext, the SRF system is less sensitive to phasing errors.Nevertheless, the injection of 58 rnA beam and/or operation at 1.5 MW,will require higher accuracy in the phasing and better compensation forthe dynamic errors, coming from the heating of the power distributionsystem. For this reason, we plan to build new waveguide transformers,with the stubs driven by stepping motors, to enable phase correction duringoperation.[314]/14ReferencesB. DWERSTEG et al.[1] D. Proch, et al., DESY Laboratory Report (Proceed. of the 6th Workshop on RFSuperconductivity, October 4-8, 1993, CEBAF).[2] B. Dwersteg, et al., Operating Experience with Superconducting Cavities in HERA (Proceed.of EPAC94, 27 June-1 July, 94, London).[3] J. Sekutowicz, et al., Beam Loading for 16 SC HERA Cavities with Voltage Control Loop(HERA Seminar '95, March 1995, Bad Lauterberg).

Operating Experience with Superconducting Cavities in the HERA E-ring

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