The CEBAF recirculating superconducting electron linac at Jefferson Lab is now in full operation supporting nuclear physics experiments in three target halls at up to 4.4 GeV. The 330 SRF cavities, operating at 2.0 K, continue to perform well above design specifications, and have accumulated over 8,000,000 operating cavity hours. The authors have to date no evidence of degradation of cavity performance. The SRF cavities have demonstrated excellent reliability. The one klystron per cavity design provides CEBAF with flexibility and redundancy for normal operations. Several techniques have been developed for establishing optimum operating conditions for the 330 independent systems. Operation of the cavities and control systems at the full design current of 1 mA has recently been achieved. The principal constraints on usable gradient for low current operations are (1) discharge at the cold ceramic rf window induced by electron field emission in cavities, (2) tuner controls, and (3) stability of the waveguide vacuum in the region between the warm and cold windows. Several cryomodules have been improved by application of rf helium processing while installed on the beamline

Reece, C E

Reece, C.E.

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Operating experience with superconducting cavities at Jefferson Lab

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Particle Accelerators, Vol. 60, pp. 43-52Reprints available directly from the publisherPhotocopying permitted by license only© 1998 OPA (Overseas Publishers Association) N.V.Published by license underthe Gordon and Breach SciencePublishers imprint.Printed in India.OPERATING EXPERIENCE WITHSUPERCONDUCTING CAVITIES ATJEFFERSON LABC.E. REECE*Thomas Je.fferson National Accelerator Facility, MS 58, 12000 Je.ffersonAvenue, Newport News, VA 23606, USA(Received in final fonn 15 January 1998)The CEBAF recirculating superconducting electron linac at Jefferson Lab is now in fulloperation supporting nuclear physics experiments in three target halls at up to 4.4 GeV.The 330 SRF cavities, operating at 2.0 K, continue to perfonn well above design specifi-cations~ and have accumulated over 8,000,000 operating cavity-hours. We have to dateno evidence of degradation of cavity perfornlance. The SRF cavities have denl0nstratedexcellent reliability. The one-klystron-per-cavity design provides CEBAF with flexibilityand redundancy for nonnal operations. Several techniques have been developed forestablishing optilnum operating conditions for the 330 independent systems. Operationof the cavities and control systeins at the full design current of 1InA has recently beenachieved. The principal constraints on usable gradient for low-current operations are(1) discharge at the cold ceramic rf window induced by electron field elnission in cav-ities, (2) tuner controls, and (3) stability of the waveguide vacuum in the region betweenthe warm and cold windows. Several cryollloduies have been improved by applicationof rf-helium processing while installed on the beamline. lKeywords: Superconductivity; Radiofrequency; CavitiesINTRODUCTIONThis paper presents the operating experience with SRF caVItIesen1ployed in the CEBAF recirculating linac at the Tholnas JeffersonN-ational Accelerator Facility (Jefferson Lab). The full design operat-ing envelope for CEBAF called for a 200 f.lA CW electron bealn at* Tel.: (757)-269-7645. Fax: (757)-269-7658. E-nlail: REECE(a),JLAB.ORG.4344 C.E. REECE4.0 GeV with low emittance and energy spread (s ~ 2 X 10-9 mr, aEIE9:!:2.5 x 10-5). This envelope has been demonstrated and exceeded.Productive exploitation for nuclear physics research is nowunderway.2The CEBAF beamline's 330 solid niobium SRF cavities operate at1497 MHz and 2.0 K. 3 Each one has an active length of 0.5 m and isseparately powered by a 5 kW klystron. Eight such cavities are con-tained in each cryomodule. The design accelerating gradient is 5 MV1m,so that by up to five passes through the two linacs, electrons canattain 4 GeV. Beams can also be extracted at less than five passes,enabling multiple simultaneous experiments in three experimentalhalls at differing, but correlated, energies.OPERATIONAL ACHIEVEMENTSIn September 1997, a demonstration test was mounted to confirm thefull beamloading design envelope of CEBAF. With the north andsouth linacs trimmed to 400 MV each, five-pass CW beam was placedon the Hall C dump, and the current was increased to 200 J.lA. Theincreased beamloading exercised the rf control systems in newregimes, and driveline limits were encountered for 12 cavities. Slightredistribution of the gradients removed the obstacles.Stable 200 J.lA, 4.0 GeV beam was maintained on the dump for overan hour. There were no unexpected difficulties. No target was in placefor this run, and no single-user program presently requires this fullcurrent.At the opposite end of CEBAF's dynamic range was the stabledelivery of a 100 pA CW beam for commissioning tests with the LargeAcceptance Spectrometer in Hall B. The first physics runs above4GeV took place during November 1997. One hall received up to115 J.lA at 4.4 GeV, while another hall simultaneously received four-pass beam at 3.5 GeV. The maximum beamloading current in thelinacs was 805 J.lA, constrained by the present administrative limit of180 J.lA on the net output current. This running mode presented nochallenges to the SRF systems.In April 1997, another demonstration test was mounted to beginthe process of extending the energy reach of CEBAF. While the linacSUPERCONDUCTING CAVITIES AT JEFFERSON LAB 45voltages have significant headroom, the power supplies for the arcdipoles were not capable of supporting greater than about 4.4 GeV.As a test of the SRF systems, ~ short high-gradient single-pass runtook place. The linacs were each trimmed to 560 MV, 40% abovenominal design. With the injector providing its nominal 45 MV, a90JlA~ 1.165GeV CW test beam was placed on the beam switchyardtune-up dump.Had the magnetic element systems been available, the rf could havesupported 5.6 GeV, five-pass beam delivery. With only an hour or sofor trimming gradients, cold window discharge arc trips occurredapproximately every 5 min, unacceptably high for most experiments.The test provided opportunity to exercise the cavities at higher thannormal operating gradients and to assess their stability at these levels.Knowledge from this experience is being used to increase stability andreliability for normal operations.OPERATIONAL LIMITATIONSCavity GradientsThe energy reach of CEBAF is fundamentally tied to the sustainablegradients of the linac cavities. Each cavity has its performance char-acteristics, the most important of which are the Q vs. gradient andsusceptibility to arcing at the cold ceramic window. Figure 1 showsthe distribution of maximum sustainable gradients in CEBAF bytype of limitation. The assortment by limitation is also displayed inFigure 2. The principal limitation of the installed cavities - affecting80% of them - is electron field emission and associated phenomena,such as X-ray production, charging and arcing at the cold ceramic rfwindow,4,s and anomalous 2 K heat load. In addition, 12% of thecavities are limited by quench and about 4% are constrained by thestability of the vacuum in the region between the 2 K and 300 K rfwindows.Starting in the fall of 1996, we began developing methods forapplying rf-helium processing to the cavities while still on the beam-line. Initial results were very encouraging, so a total of 12 cryo-modules have thus far received this processing, yielding an additional468070 ~60 -rJ).~ 50-4-J.~:> 40 -rn-U 30 =:tt:: =20 -1002C.E. REECEFE loading &radiation4 6 8 10 12 14Max Gradient for Ops (MV/ m) 7/97 cerFIGURE 1 Distribution of maximum gradient by type of limitation.SRF Cavity Gradient Litnitations in CEBAFOthers (5%) July 1997Waveguide vacuum (4%)ced arcing (48%)FE loading & radiation (32%)cer 7/97FIGURE 2 Type of limitation on gradient in CEBAF SRF cavities.41 MeV/pass capability.6 The indicators of improvement via thisprocessing technique are the raising of onset levels for X-ray produc-tion by field emission, and the reduction of arcing at the cold rfwindow. When adequate time was available to make calorimetric QSUPERCONDUCTING CAVITIES AT JEFFERSON LAB 47measurements on the first module processed, corresponding reductionof losses was observed. See Table I.We intend to exhaust the opportunities for such gains as a firststep toward significant upgrades of the machine into the 6-8 GeVregion.Figure 3 presents the maximum voltage from each CEBAFcryomodule in November 1997, including the gains from rf-heliumprocessIng.TABLE I Relief from arcing and FE loading after rf-helium processing of NL03Cavity MaxG Initial G Qo Qo MaxG Finalbefore limitation (MV/m) (x 109) (x 109) after limitationprocessing Before After processing(MV/m) (MV/m)2 4.2 arcs 7.7 1.5 3.3 8.7 arcs4 5.5 FE 8.7 2.5 4.1 9.0 FEloading loading6 4.9 arcs 6.8 2.1 > 5.3 7.7 FEloading8 5.5 arcs 8.8 2.85 4.7 6.9 arcsNovember 19974GeV6GeVeer 11/24/97low current « 20IlA)o~ 35 -------,---,-----i" -==------ -----J;:j"'ClS30oC25u[20~m>~ 1550.--------1 • Maximum cryomodule voltage without helium processingGain from helium processing45-+-----------l D Potential additional capacity with relaxed limits: FE loading and40+--__-'-__q_ue_n_ch_m_ar_g_in """'--I 8 GeVFIGURE 3 MaxiIlluil1 voltages obtainable frOIll the CEBAF cryoIllodules.48 C.E. REECEOperational StabilityThe SRF cavities in CEBAF have been quite stable and reliable. ByOctober 1997 over 8,000,000 cavity-hours and 3,000,000 cavity-beamhours had been accumulated. To date, there is no evidence for degra-dation of cavity performance.The occurrence of periodic arcing at the cold windows has provento be a manageable phenomenon, as it is a quite reproducible func-tion of gradient for each cavity with the charging field emission behav-ior. Operationally, the maximum useable gradient for a cavity isbounded by that field at which it predictably arcs luore than threetimes per day.The vacuum between the cold and warm waveguide windows issubject to transients which require the rf to be shut off and beamdelivery to be briefly interrupted. This occurs for a subset of cavitiesand only when there are significant changes in the standing wave pat-tern, as when higher powers are applied to allow a detuned cavity tostill achieve its nominal gradient setpoint. Such instabilities have,however, not been observed as the cavities are beamloaded and theincreasing forward power is more closely matched into the beam. Thepressure excursions are interpreted, based on partial pressure analysis,as the redistribution of adsorbed hydrogen and helium in the thermaltransition region of the waveguide. This type of problem has becomemuch less significant as we have gained greater control over maintain-ing the proper tune of the cavities.Of the 330 installed SRF cavities, two are not operational. One ofthese is disabled by a mechanical tuner problem; the other suffersfrom a helium leak into the intermediate waveguide vacuum space.Neither of these is repairable without removing and disassembling itscryomodule.About 25% of the cryomodules have active turbo-pumps on theinsulating vacuum. In one case the leak rate is such that occasionallythe turbo-pump has insufficient capacity to handle the gas desorbedwhen the cryomodule warms above 4 K.Tuning ControlLooking to higher-energy operation, we antIcIpate a new prob-lem with tuning control. The backlash in the mechanical tunersSUPERCONDUCTING CAVITIES AT JEFFERSON LAB 4940 ---.------T----.--~__.__T"--r-_.,..._---,-----...,..---,-~___,._--.--.----.,.----,-----.,..-----r-_.,..._~35 ~~------I----+----1r--t--+---t--~­30-+--+--+--+--J----t--+---+--+--+--+--r./J.~ 25 --+--+-----+-----+---+----+--+--+---+-----+-----+--~.~ 20-I---+----+---l----I-----f--+-+--I---+--U ~4--1o 15 --+--+---+---+--+---+---+--+-----t-----+--::t:l:10~~------I----+----1r--t--+----+--5+-------t-----+----+---t-----+-------t-+-------t--O+--.....-~+-+---4 -3 -2 -1 0Lorentz Force Tuning Coefficient (Hz/(MV/m)2)FIGURE 4 Distribution of Lorentz force tuning coefficients for the CEBAF cavities.complicates the turn-on and tune-up of cavItIes with gradientsgreater than 9 MV/m, in which region the Lorentz force detuningapproaches one bandwidth. The distribution of Lorentz force tuningcoefficients for the CEBAF cavities is provided in Figure 4.The intention is to accommodate this backlash in the future viasoftware.The microphonic environment of the CEBAF cavities has provento be quite good. The rms microphonic phase angle is typically lessthan 2°. Figure 5 presents the distribution for all CEBAF cavitiesafter the addition of mechanical isolation reduced the rms phase anglefor about 20 cavities from the 5-12° range.FlexibilityAt 4 GeV the CEBAF SRF system has generous flexibility, with morethan 25% voltage headroom at full current. If a problem developswith a klystron or other driveline component, that cavity can bedetuned and shut down and beam operations restored in 10 min. Ithas also been demonstrated that 4 GeV beam can be provided withseveral whole cryomodules turned off.50 C.E. REECE82 3 4 5 6 7RMS Microphonic Phase Angle (0)oc20 t-J--t--J--+-1060 f--j-t-J--+-Microphonic Phase Noise70 I---t---;f----I---l--+--l-.-+-+---+-+-+--t-+-_+__+__+_+_ CEBAF SRF CavitiesOctober 1997FIGURE 5 Microphonic phase noise of operating CEBAF SRF cavities.Procedures have also been developed to economize the consump-tion of line power by the rf systems when less than full beamloadingis required by the progralll. This permitted the trimming of 1MWload between the 200 ~A run and the following 2 nA physics runwithout changing any fields seen by the beam.The day-to-day operational setup of each of the rf systems isderived from the following: (1) the maximum power available fromeach klystron, (2) the maximum gradient of each cavity and (3) itsloaded Q-factor, (4) the control headroom needed to counteractmicrophonics, (5) the maximum intended beamloading current, and(6) the total desired linac voltage. The maximum gradients consistentwith the available power and bealnloading for each cavity that ispresently on are sUlnmed for each linac. The setpoint gradient ofeach cavity is reduced proportionally such that the desired sun1 isobtained. Four cavities in each linac are used to provided voltagestabilization against drifts in proper phasing and calibration of thecavities.SUPERCONDUCTING CAVITIES AT JEFFERSON LAB 51DYNAMIC HEAT LOAD WITH BEAM CURRENTOne of the last performance parameters to be measured for theSRF cavity systellls in CEBAF was the dynamic heat load with beam.During the high-current runs of fall 1997, the integrated 2 K loadfor the whole accelerator was monitored as a function of beam cur-rent, (see Figure 6) with all gradients held constant. Potentialcontributions to such a dynamic load are additional dissipative lossesin the cold rf windows, changing thermal profile in the waveguidetransition region, and beam-induced losses via higher-order-mode (HOM) generation. (In CEBAF, the higher-order modes aredissipated at 2 K.)The observed dynamic load follows a quadratic dependence onlinac beam current. Such a functional dependence is not expectedfrom increased losses in the input rf system. The quadratic depen-dence is appropriate, however, for beam-induced HOM effects.The observed amount of heating, while operationally of little con-sequence, is more than an order of magnitude larger than has yet100 200 300 400 500 600 700 800 900 1000I tot (totallinac current - f.1A)I I I I I I /P2K =2.5E-4 * Itot2 - 5.1E-2 * Itot + 1 //1/I~/Vi 1-VYW 1--------~•~'200~180~ 160"-"~ 140o~ 120~N 100~~ 80$:::l.8 60~.1"""""1~ 40~ 20ooFIGURE 6 Dependence of 2 K load on totallinac current.52 C.E. REECEbeen analytically attributed to HOM generation by the CEBAF 2 psbunchlength in the CEBAF cavities.?SUMMARY AND PLANSThe SRF cavities in CEBAF continue to perform quite well withinthe presebt operational requirements. To extend the gradient capa-bility of the cavities, we plan to apply helium processing to thebalance of the machine as opportunity permits. It is expected that thiswill raise the energy bound of CEBAF to close to 6 GeV.Subject to accumulating physics justification, the upgrade path forCEBAF beyond 6 GeV involves gradually replacing cryomodules andpopulating 10 empty slots in the linacs with new cryomodules whichhave improved filling factor and take advantage of improvements inSRF cavity preparation techniques of the past 7 years.AcknowledgmentsThe experience described here is the fruit of many persons' labor,including all of those involved in the construction of CEBAF. Cur-rent activities on this subject fall to menlbers of either the AcceleratorDevelopment Department or the Operations Group in the JeffersonLab Accelerator Division. Particular credit goes to L. Doolittle, whoperformed the measurement of the Lorentz force tuning coefficients.References[1] Work supported by the U.S. Departnlent of Energy, contract DE-ACOS-84ER401S0.[2] H. Grunder, CEBAF conl1nissioning and future plans, PAC 95 Proceedings,pp.I-3.[3] C. Reece et al., Perfornlance experience with the CEBAF SRF cavities, PAC 95Proceedings, pp. 1512-1514.[4] V. Nguyen-Tong, N. Luo, L. Phillips and C. Reece, Electronic activity at CEBAF coldRF window induced by cavity operation, EPA C 94 Proceedings, pp. 2200-2202.[5] T. Powers, P. Kneisel and R. Allen, Arcing Phenonlena at CEBAF RF-windows atcryogenic telnperatures, PAC 95 Proceedings, pp. 1645-1647.[6] C.E. Reece, M.Drury, M.G. Rao and V. Nguyen-Tuong, hnprovelnent of theoperational perfonnance of SRF cavities via in situ heliunl processing and wave-guide vaCUUln processing, Contribution to the 1997 Particle Accelerator C011lerence,Vancouver, B.C., Canada.[7] B. Yunn and J. Bisognano, private COll1111unicatioll.

Operating Experience with Superconducting Cavities at Jefferson Lab

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