Belomestnykh, S

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Presented at the 7th European Particle Accelerator Conference, Vienna, Austria, 26 – 30 June, 2000             SRF 000712-08RUNNING CESR AT HIGH LUMINOSITY AND BEAM CURRENT WITHSUPERCONDUCTING RF SYSTEM*S. Belomestnykh, P. Barnes, E. Chojnacki, R. Ehrlich, R. Geng, D. Hartill, S. Henderson,R. Kaplan, J. Knobloch, H. Padamsee, S. Peck, P. Quigley, J. Reilly, D. Rubin, D. Sabol, J. Sears,V. Veshcherevich, Laboratory of Nuclear Studies, Cornell University, Ithaca, NY 14853, USA                                                       * Work supported by the National Science Foundation.AbstractThe transition from normal conducting tosuperconducting RF system has been completed atCESR in 1999.  The new system is one of the changes,which allowed the collider to operate at higher currentsand reach higher peak luminosity.  The commissioningresults and operating experience with the SRF systemare presented alongside with describing its effect onCESR performance.  Future plans are discussed.1  INTRODUCTIONThe Cornell Electron Storage Ring (CESR), a e+e-collider operating on and near the Υ(4S) resonance (E ≈5.3 GeV), is the only single-ring symmetric B-factory inthe world.  CESR has achieved a peak luminosity of8.3×1032 cm-2s-1, a total beam current of 650 mA and abeam-beam tune shift parameter ξv = 0.05.  At presenteach beam consists of 9 bunch trains, each is populatedwith 4 bunches spaced by 14 ns, but the machine opticsallows increase of number of bunches per train to 6.CESR is nearing completion of a Phase III of itsluminosity upgrade [1] which includes upgrading RFsystem from four 5-cell normal conducting cavities tofour single-cell superconducting niobium cavities.  This,together with installation of new interaction region (IR)optics, will lead to the total current increase up to 1ampere and luminosity up to 2.2×1033 cm-2s-1.  The RFsystem upgrade was successfully completed and newsystem is in operation.  The high-gradient capability ofthe SRF cavities will allow reduction of bunch lengthfrom 19 mm to 13 mm.  RF power delivered by eachcavity will be 325 kW at 1 A beam current.2  RF SYSTEM DESCRIPTION The superconducting RF system [2] consists of twoRF stations (East and West).  Each station includes two600 kW YK1300 Phillips klystrons, two single-cellsuperconducting niobium cavities in their individualcryostats (termed cryomodules), and a station cryogendistribution valve box.  WR1800 waveguide can bearranged to feed two cavities (E1 and E2 in the East orW1 and W2 in the West) from one klystron via magic TRF power splitter or to feed each cavity from its ownklystron.  In addition, a processing area (PA) canaccommodate one cryomodule or RF window assemblyfor high power testing and conditioning.  One of theWest klystrons can be connected to the PA test set up. Liquid helium is provided by two 600 W refrigeratorssupplying 2000 liter storage dewar.  Rigid transfer linestransport liquid helium, cold gaseous He, and liquidnitrogen between the refrigerator and storage dewar to acentrally located main distribution valve box.  From themain valve box, rigid transfer lines lead to satellite valveboxes or directly to the superconducting elements. 3  COMMISSIONING RESULTS ANDOPERATIONThe RF system replacement began in September 1997and was performed in stages, with installation of SRFcryomodules one at a time in place of old normalconducting cavities.During commissioning phase of the first (E2) cavityinstallation, CESR beam currents were limited bytravelling wave power (forward power minus reflectedpower) which could be tolerated by the cavity inputcoupler.  We observed vacuum trips at rather repeatablepower levels due to gas evolution brought on bymultipacting in the window and coupler region.  Twoprocessing techniques have proven to be useful in raisingthe travelling wave power limits: high RF power pulseprocessing and beam processing.  Detail description ofthem can be found elsewhere [3].A second variety of vacuum trips arose from RFheating of the waveguide thermal transition section.Such vacuum events showed a measurable temperaturerise on the transition section, and were accompanied bya large release of hydrogen as observed on an RGA.Cryosorbed hydrogen was released as the surface washeated.  After warming up the cavity to roomtemperature and baking the window in situ to 110°CPresented at the 7th European Particle Accelerator Conference, Vienna, Austria, 26 – 30 June, 2000             SRF 000712-08during scheduled CESR shutdown, the beam powerdelivered began to rise steadily and reached 180 kW.After about 2 months in operation, the ability of thecavity to deliver RF power began to deteriorate againdue to the fast vacuum trips.  After another warm up theE2 power delivered to beam eventually reached 212 kW.Experience with the first SRF cavity suggestedmodification to the cryomodule design and handlingprocedures, which were incorporated into subsequentinstallations.When the second (E1) cryomodule was tested at highpower in the processing area, the cavity quenched atgradient around 5 MV/m, below the 6 MV/m acceptancethreshold and significantly below 11 MV/m reached in avertical test.  Since disassembly and re-assembly of thecryomodule to re-etch the cavity is a several month longprocedure, we performed a risky operation of using along stick to wipe the cavity equator bottom in situ [4].Apparently, a stainless sliver from a sheared tongue andgroove seal migrated to the cavity bottom and partiallymelted during high power RF testing.  Fortunately, themelted stainless steel did not wet well to the niobium,and after removal, cavity performance improvedconsiderably, exceeding the 6 MV/m acceptancethreshold.  The second cavity was installed in October1998 and reached 218 kW within one month ofoperation.The third (W1) cavity was installed in March 1999.  Ashort 6-week running period in April-May 1999 wasdevoted to commissioning of the new SRF cavity,exploring beam current limits and running beams forCHESS (Cornell High-Energy Synchrotron Source)experimental users.  At first, the vacuum waveguide arcsdiscussed above limited the power, delivered to thebeam by this cavity.  The cavity was warmed up to 50 Kto remove absorbed hydrogen.  Following this warm upand subsequent high-power pulsed processing the beampower delivered through W1 increased from 140 kW to260 kW.The last cavity (W2) was installed in September 1999.This cavity has thermal breakdown at 7.8 MV/m due toNb material defect.  It was easy to process thiscryomodule, but after about one month of running webegan to get frequent RF trips due to vacuum bursts inthe cavity.  The problem was traced to a vacuum leak ona wire seal.  Partial warm ups to 60 K every month ofoperation allowed us to reduce the frequency of vacuumtrips and run the machine until scheduled down inFebruary 2000.  During down period we installedvacuum dam around the leaky seal.  This temporary cureallows us to keep the cryomodule in CESR until sparecryomodule is ready.SRF cavities have operated in the range of 5 to 7MV/m with average accelerating gradient at present of5.6 MV/m (total voltage of 6.7 MV).  This is not limitedby the performance of cavities, but by operation reasons:with present lattice design it is easier to tune CESR tohigh peak luminosity with lower synchrotron frequency.The maximum power delivered to date by SRF cavitiesis 260 kW (W1), 225 kW (E1), 220 kW (E2), and 212kW (W1).  Figure 1 illustrates CESR peak beam currentincrease with installation of SRF cavities. Since early 1996 CESR beam currents have begun tobe limited by a longitudinal coupled bunch instabilitydue to high Q factor of parasitic modes in normalconducting RF cavities.  A staged replacement of thenormal conducting RF allowed us to study the impact onlongitudinal threshold instability at each step [5,1].  Theincrease of instability threshold with installation of SRFcavities is not as quite dramatic as we might haveexpected.  A detailed calculation and comparison tothreshold data has been performed.  The analysis showedgood agreement between calculations and measurementsfor the case of four normal conducting RF (NRF)cavities and three NRF plus one SRF, but began toworsen for the two NRF plus two SRF case.  Theseresults suggest that as NRF cavities are removed, theimpedance of the rest of the ring becomes moreimportant and dominates the longitudinal dynamicswhen all NRF cavities are removed.  Recent studies ofelectrostatic separators [6] show that their higher-ordermode (HOM) impedance is rather high and can explainlower than expected longitudinal instability threshold.We must note though that if before the SRF systeminstallation longitudinal instability usually caused beamdump, now instability is self-limiting and does not causebeam loss.  A digital multi-bunch longitudinal feedbacksystem has been developed to damp the excitedlongitudinal motion [7].  With the longitudinal feedbacksystem in operation, we have been already able tooperate at beam currents a factor of 2.3 above theinstability threshold, and it will provide enough stabilitymargin to operate with 1 A beam current in CESR. Figure 1: CESR peak current increase with installationof SRF cavities.CESR peak current01002003004005006007001993 1994 1995 1996 1997 1998 1999 2000mAE2 installationE1 installationW1 installationW2 installationPresented at the 7th European Particle Accelerator Conference, Vienna, Austria, 26 – 30 June, 2000             SRF 000712-08 5  FUTURE PLANSInstallation of two superconducting quadrupole lensesin the interaction region (IR) will allow CESR to operatewith small β* [1].  To take advantage of this IR opticsimprovement we have to shorten the bunch length to beabout equal to β*.  With the existing 9 bunch trainoptics, momentum compaction α = 0.0112.  A totalaccelerating voltage of 22 MV yields a bunch length of10.5 mm at 9.3 mA/bunch.  We are planning to installtwo additional SRF cavities in the IR region.  TotalTable 1: CESR parameters for Phase III upgradePhaseII Phase IIIParameter A B CNo. of cavities 4 4 4 6Acc. gradient[MV/m]5 5 10 12Accel. voltage[MV]6 6 12 22Bunch length(zero current)[mm]18.1 18.1 12.8 9.5Bunch length(@9.3 mA)[mm]19.5 19.5 13.8 10.5β* [mm] 18 18 13 10No. ofbunches/train4 6 6 6Peak current[A]0.54 1 1 1Current/bunch[mA]7.5 9.3 9.3 9.3Loss factor[V/pC]6 6 8 13.2Beam power[MW]0.7 1.3 1.35 1.5Transmittedpower/cavity[kW]175 325 337 250Vertical tuneshift param.0.05 0.05 0.05 0.06Luminosity[×1033 cm-2s-1]0.83 1.6 2.2 3.5voltage of 22 MV corresponds to an average acceleratinggradient of 12 MV/m.  This should give us ability to tunemachine to higher peak luminosity (see Table 1).In order to reach high gradient of 12 MV/m we willhave to improve performance of our RF system.  E1cryomodule has to be taken out of the ring, opened andcavity repaired.  W1 cryomodule has to be taken apart toreplace the cavity and fix leaky seal.  Our cryostats cantolerate dynamic heat load up to 200 W per cryostat.This means that at 12 MV/m cavity quality factor shouldbe ≥109.  To reach this value we are going to exploitcavity baking technique.  Experiments at severallaboratories showed that after baking cavity in vacuumat temperatures of 80°C to 150°C, its Q factor increasesby a factor of 2.  We baked our spare cavity at 140°C for48 hours and obtained similar result (Figure 2.)One can see from Table that beam power delivered byeach cavity will drop from 337 to 250 kW when we addtwo more cavities to the system.  This means that withfixed coupling cavity is not matched any more to thewaveguide at 1 A beam current, and there will besignificant amount of reflected power from each RFcoupler.  Thus efficiency of the RF system will not begood.  Therefore we can not easily change cavitycoupler, we are going to use a high RF power waveguidetransformer similar to proposed in [8] and successfullyused at DESY laboratory.  The use of transformer willallow us to reach matching conditions at 1 A.Addition of two cavities will require upgrade ofcryogenic system and RF transmitters.  One more 600 Wrefrigerator will be added to two others, which are inoperation already.  Also, we consider possibility ofrunning cryomodules at temperature lower than 4.5 K.Slight modification of the cryostat design is necessary tofit in very tight space available in the IR region.Figure 2.  Q vs. Eacc plot for spare cavity testedbefore and after 140°C, 48 hr bake.6 CONCLUSIONSCornell design of a superconducting HOM damped RFcavity cryomodule for high-current electron storage ringproved to be successful.  Installation of new SRFcavities in CESR already helped to increase total beamcurrent to 650 mA and peak luminosity to 8.3×1032 cm-2s-1.  In the near future we should be able to reach CESRPhase III design beam current and peak luminosity.Based on our successful operating experience we plan toupgrade our SRF system by adding two morecryomodules and increasing accelerating gradient to 12MV/m.  This will help us to shorten the bunch lengthand as result to reach peak luminosity as high as 3.5×1033cm-2s-1, taking full advantage of focusing strength of newsuperconducting IR quadrupole lenses.1.0E+081.0E+091.0E+101.0E+110 1 2 3 4 5 6 7 8 9 10 11 12 13 14Eacc, MV/mQobefore bakingafter bakingat 2.1 KPresented at the 7th European Particle Accelerator Conference, Vienna, Austria, 26 – 30 June, 2000             SRF 000712-08REFERENCES[1] S.Hensderson, LNS Report CBN 99-28.[2] S.Belomestnykh, et al., Proc. 5th EPAC., p. 2100.[3] S.Belomestnykh, et al., Proc. 1999 PAC., p.980.[4] E.Chojnacki, J.Sears, LNS Report SRF990716-09.[5] M.Billing,S.Belomestnykh, Proc. 1999 PAC,p.1112.[6] A.Temnykh, Cornell LNS Report CBN 00-4.[7] J.Sicora, et al., Proc. 1999 PAC, Vol. 2, pp.1115.[8] B.Dwersteg, Q.Yufang, Report DESY-M-89-08.B.Dwersteg, Proc. 4th SRF  Workshop, KEK Report89-21, p.53

Running CESR at High Luminosity and Beam Current with Superconducting RF System

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