Two new high gradient C100 cryomodules with a total of 16 new cavities were installed at the end of the CEBAF south linac during the 2011 summer shutdown as part of the 12-GeV upgrade project at Jefferson Lab. We surveyed the higher order modes (HOMs) of these cavities in the Jefferson Lab cryomodule test facility and CEBAF tunnel. We then studied recirculating beam breakup (BBU) in November 2011 to evaluate CEBAF low energy performance, measure transport optics, and evaluate BBU thresholds due to these HOMs. This paper discusses the experiment setup, cavity measurements, machine setup, optics measurements, and lower bounds on BBU thresholds by new cryomodules

Ilkyoung Shin, Todd Satogata, Shahid Ahmed, Slawomir Bogacz, Mircea Stirbet, Haipeng Wang, Yan Wang, Byung Yunn, Ryan Bodenstein

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RECIRCULATING BEAM BREAKUP STUDYFOR THE 12 GEV UPGRADE AT JEFFERSON LAB ∗I. Shin, University of Connecticut, Storrs, CTT. Satogata, S. Ahmed, A. Bogacz, M. Stirbet, H. Wang, Y. Wang, d B.C. YunnJefferson Lab, Newport News, VAR.M. Bodenstein, University of Virginia, Charlottesville, VAAbstractTwo new high gradient C100 cryomodules with a totalof 16 new cavities were installed at the end of the CE-BAF south linac during the 2011 summer shutdown as partof the 12 GeV upgrade project at Jefferson Lab. We sur-veyed the higher order modes (HOMs) of these cavities inthe Jefferson Lab cryomodule test facility and CEBAF tun-nel. We then studied recirculating beam breakup (BBU)in November 2011 to evaluate CEBAF low energy perfor-mance, measure transport optics, and evaluate BBU thresh-olds due to these HOMs. This paper discusses the exper-iment setup, cavity measurements, machine setup, opticsmeasurements, and lower bounds on BBU thresholds bynew cryomodules.BACKGROUNDThe CEBAF 12 GeV upgrade requires reliable 5-passoperation of upgraded 7-cell 1497 MHz cavities, namedC100. Each cryomodule will contain 8 cavities, designed toprovide CW 108 MV per cryomodule (including overhead)at 5-pass total beam currents up to approximately 400µA,limited by electron gun current to two 1 MW hall dumps.In 2007, CEBAF with an earlier high-gradient cryomoduleprototype experienced recirculating BBU at about 40µA,so great effort was made to improve C100 HOM dampingand performance with DESY-type coaxial HOM couplersand careful control of fabrication methods [1, 2].The first two C100 cryomodules, named C100-1 andC100-2, were surveyed in the Jefferson Lab cryomoduletest facility (CMTF) and installed in the west (high energy)side of the CEBAF south linac (SL) during the 2011 sum-mer shutdown. The availability of these cryomodules withbeam enabled an experimental evaluation of C100 dipoleHOM damping and BBU thresholds in November 2011during CEBAF startup for the 2011-2 run.THEORY AND EXPERIMENTAL SETUPAssumming one cavity, one HOM, and one recirculation,the threshold current for two-pass BBU is given by [3]Ith = −2pcq1(Rd/Q0)Qd km⋆ sin(ωTr), (1)∗Authored by Jefferson Science Associates, LLC under U.S. DOEContract No. DE-AC05-06OR23177. The U.S. Government retainsnon-exclusive, paid-up, irrevocable, world-wide licenseto publish or re-produce this manuscript for U.S. Government purposes.wherepc is the particle energy on the second pass,q = eis the particle charge,(Rd/Q0) is the dipole HOM shuntimpedance,ω is the HOM frequency,Qd is the quality fac-tor of the dipole HOM,k = ω/c is the HOM wave number,Tr is the recirculation time, andm⋆ ≡ m12 cos2 α+(m14+m32) sinα cosα+m34 sin2 α,(2)whereα is the mode polarization angle andmij are com-ponents of the recirculation transport matrix from firstto second cavity crossings. This equation is valid whenm⋆ sin(ωTr) < 0 to make the threshold current positive; ifm⋆ sin(ωTr) > 0, numerical methods are required to de-termine threshold current [3, 4, 5] .The only variables available in the CEBAF BBU ex-periment were particle energypc and opticsm⋆ contribu-tions. Attempts to lowerpc and thusIth by tuning CE-BAF to 150 MeV/linac failed after several attempts. Alow-energy setup of 282 MeV/linac (compared to nominal551 MeV/linac) succeeded, providingpc =1160 MeV af-ter two passes. Unfortunately the Hall C high-current dumpwas not available, so operational beam current was limitedto 80µA. We also acquired C100-1 data at 180µA parasiti-cally during the CEBAF experimental program. These cur-rents are much lower than the lowest BBU threshold cur-rents of about 26 mA predicted from simulation (see nextsection). We were therefore unable to observe BBU di-rectly as was done in previous BBU studies at the JeffersonLab FEL [5, 6].Instead, we surveyed C100 HOMs for both cryomod-ules with three beam currents and three optics configura-tions as shown in Table 1 to compare to offline simulationsand to extrapolate lower bounds on measured BBU thresh-olds. Recirculation optics to calculatem⋆ were measuredusing RayTrace, which transports a synthetic beam ellipsethrough CEBAF [7].Table 1: BBU Experiment ParametersSecond-pass beam energypc 1160 MeVBeam currentI 0, 40, 80, 180µAPhase advance/cell in linacs 90, 105, 120◦BBU SIMULATIONSTDBBU is a multi-pass beam breakup simulation pro-gram developed at Jefferson Lab [8]. The beamline in-formation for TDBBU was obtained from snap shot datafiles from the CEBAF control system for comparison withthe experimental data. The RF focusing effect was imple-mented into TDBBU to improve optics calculation [9].The threshold current predicted by simulation is about26 mA due to TM111π/7 modes in C100-1, which aresharp and narrow peaks near 2893 MHz in Fig. 2. Simu-lations were performed to predict threshold currents in theprimary TM110 and TM111 modes of concern.0 2 4 6 8 10 12x 1060.04690.04690.04690.047TM111 π/7 modeBunch NumberDispalcement [cm]  26 mA0 2 4 6 8 10 12x 1060.04690.04690.04690.047TM111 π/7 modeBunch NumberDispalcement [cm]  27 mAFigure 1: TDBBU simulation results. Thex axis repre-sents bunch numbers, and they axis is transverse positions.Beam is stable at 26 mA, but the transverse position in-creases exponentially at 27 mA, which means beam insta-bility.Table 2: CEBAF C100 HOM Survey ParametersC100 fundamental mode 1497 MHzTE111 survey range 1850-2050 MHzTM110 survey range 2050-2250 MHzTM111 survey range 2850-3050 MHzDATA ACQUISITION AND ANALYSISHOM surveys for frequency ranges listed in Table 2 wereconducted for all cavities in both C100-1 and C100-2 cry-omodules in the CMTF and the CEBAF tunnel through ex-ternal HOM coupler ports using a 4-port vector network an-alyzer (VNA). C100 cavities are installed in the cryomod-ule with their HOM ports oriented away from the cryomod-ule centerline. VNA channels 1 and 2 were connected tohorizontal and vertical HOM ports of the surveyed cavitywhile channels 3 and 4 were connected to correspondingHOM ports of the next cavity towards the center of themodule.2×104 points were sampled on each survey rangewith 1 kHz IF bandwidth. Previous HOM survey proce-dures have required time-consuming, manual Q measure-ments using the VNA at each HOM of interest. We tookadvantage of recent advances in efficient extraction of pole2.890 2.895 2.900 2.905 2.910HOM frequency [GHz]-120-100-80-60-40-200Sxx Power [dB]S31S32S41S42Figure 2: Example Sxx TM111 mode transfer functionsmeasured in the CEBAF tunnel for cryomodule C100-1,cavity 6. The most dangerous modes are at the lower fre-quencies in the TM111π/7 mode; evidence of a narrow,high Q can be seen near 2893 MHz.and Q values to automatically and accurately extract Q val-ues of all HOMs in our sampled frequency ranges by usingan enhanced version of the Mathematica programpolfit,developed at Universität Rostock and Jefferson Lab [10].polfit also compensates for cable phase shifts in its analy-sis. Fitted HOMs were spot-checked with detailed at sam-ple high Q HOMs to confirmpolfit accuracy.An overview of the most important TE111, TM110, andTM111 modes for all cavities in C100-1 and C100-2 cry-omodules is shown in Fig. 3. Dipole mode impedanceswere calculated from measured Q values, and R/Q valuesas modeled by CST/Microwave Studio. Surveyed Q valuesare quite consistent over all cavities, which makes outlierssuch as those in C100-1 cavity 4 TM110 (π) and 5/7 (5π/7)modes quite apparent. The circled impedances are of themost concern for CEBAF BBU performance, though whenextrapolated they are still far below the 5-pass 400µAbaseline impedance threshold of 1010 Ω/m.Although multi-pass BBU is a threshold phenomenon,it is not necessary to exceed the threshold current in orderto measure it. This can be measured by the beam trans-fer function (BTF) measurement. The accessibility to theHOM ports of the cavities enabled us to directly excite thebeam through an HOM port of the cavity. The responsesignal was measured from the other HOM port of the sameor an adjacent cavity.We planned to extract the threshold currentIth by mea-suring the effectiveQeff as a function of average beam cur-rentI and usingQeff =IthIth − IQL, (3)whereQL is the quality factor with no beamUnfortunately we could not detect the difference inQeffas a function of current because the experimental beam cur-r nts (up to 180µA) were much smaller than the expectedthreshold current (≈26 mA). Alternatively, by measuringthe ratio of the stored energy with beam off to the storedFigure 3: Measured Dipole HOM impedances for the first two production C100 cryomodules, C100-1 and C100-2,installed in the CEBAF south linac locations SL24 and SL25. The 5-pass 400µA 12 GeV baseline impedance thresholdis 1010 Ω/m. The highest surveyed impedances are TM111π/7 modes near 2893 MHz.energy with beam on, the threshold current can be obtained[11] fromIth =I1−√P (0)P (I)(4)HereP (0) andP (I) are the HOM powers radiated withbeam currents of zero andI respectively. The data analysisusing this approach is in progress. This requires detailedcharacterization of attenuation of C100 HOM cables usedin the study, which will occur in June 2011. To characterizethe case ofm⋆ sin(ωTr) > 0, the analysis using secondorder perturbative solution will be performed.CONCLUSIONSWe have characterized the HOM and BBU performanceof two CEBAF 12 GeV upgrade C100 cryomodules, eachcontaining eight 7-cell 1497 MHz C100 cavities. FullHOM surveys across TE111, TM110, and TM111 modeswere performed in a test facility and the CEBAF tunnel.These surveys, supported by simulation and experiment,show that the lowest 1-pass BBU thresholds for C100-1/2cryomodules are over 20 mA; 5-pass thresholds are likelywell over the 5-pass 400µA 12 GeV spec. Detailed analy-sis of extrapolated BBU experiment data to lower measuredBBU threshold lower bound uncertainties is ongoing.ACKNOWLEGEMENTSThe authors thank the CEBAF operations staff for theirsupport, and C. Potratz and F. Marhauser for support anduse ofpolfit. I. Shin would like to thank his advisor, K.Joo, for supporting him.REFERENCES[1] F. Marhauser, J. Henry and H.Wang, “Critical Dipole Modesin JLab Upgrade Cavities”, THP009, Linac 2010, TsukubaJapan 2010.[2] H. Wang et al., “C100 Cryomodule HOM Damping Qual-ity Assurance and Control through Production Processes”,WEPPC098, these proceedings.[3] E. Pozdeyev, “Regenerative Multipass Beam Breakup in TwoDimensions”, PRST:AB8, 054401 (2005).[4] N. Sereno, “Experimental Studies of Multipass BeamBreakup and Energy Recovery Using the CEBAF InjectorLinac”, Ph. D. Thesis, Univ. Illinois at Urbana-Champaign,1994.[5] C. Tennant, “Studies of Energy Recovery Linacs at JeffersonLaboratory”, Ph. D. Thesis, William and Mary, 2006.[6] C. Tennant et al., “Experimental Investigation of Multibunch,Multipass Beam Breakup in the Jefferson Laboratory FreeElectron Laser Upgrade Driver”, PRST:AB8, 064403 (2006).[7] R.M. Bodenstein et al., “Further Analysis of Beam Line Op-tics from a Synthetic Beam”, TUPPC046, these proceedings.[8] G.A. Krafft, J.J. Bisognano, “Two Dimensional Simulationsof Multipass Beam Breakup”, PAC 1987, pg. 1356 (1987).[9] I. Shin et al., “Comparison of RF Cavity Transport Modelsfor BBU Simulations”, WEP048, Proc. of the 2011 ParticleAccelerator Conference, New York NY.[10] C. Potratz et al., “Automatic Pole and Q-Value Extractionfor RF Structures”, WEPC098, Proc. of the 2011 Interna-tional Particle Accelerator Conference, San Sebastián, Spain.[11] C.M. Lyneis et al., “Standing Wave Model of RegenerativeBeam Breakup in Recirculating Electron Accelerators”, Nu-clear Instruments and Methods, 204 (1983), 269-284.

Recirculating Beam Breakup Study for the 12 GeV Upgrade at Jefferson Lab

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Recirculating Beam Breakup Study for the 12 GeV Upgrade at Jefferson Lab

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