ERL RF Systems A. Nassiri November 15, 2006 Presented to the Machine Advisory Committee for the Technical Review of APS Accelerator Upgrade Options – November 15-16, 2006

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Outline Goals Cavity design criterion oCavity parameters oFrequency oCell Shape oNumber of cells oQ vs. Gradient oCavity and wall-plug Power oFundamental RF Coupler oHOM Coupler Cooling Requirement RF power sources and rf distribution LLRF system A summary of cavity design parameters Cavity R&D Conclusion

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Goals To accelerate beam with up to 100 mA and 2-4 ps bunch length to 7 GeV CW operation Achieve and maintain rf amplitude and phase required for stability Preserve nm-type beam emittance in the linac Acceptable machine reliability and beam availability

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Criteria for Cavity Design Cavity parameters ParameterUnitValue FrequencyMHz1300/1408/704 Structure typeStanding Wave Accelerating mode TM 010  mode GradientMV/m20 Quality factor Q 0 1  Active lengthMeter1.04 Number of cells9/7 R/Q  900 Q ext for input coupler 1  10 7 Cavity bandwidth at Q ext Hz400 Fill time ss 500

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity Frequency Frequency scaling oThe losses in a microwave cavity are proportional to oFor a given length of a multi-cell structure oIt becomes independent of oAt, the BCS term dominates above 3 GHz and the losses grow linearly with frequency o Below 300 MHz, the residual resistance dominates and the losses are proportional to

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity Frequency Choice of frequency o1300 MHz ( Single-pass)  Pros Design exists ( TESLA Cavity, DESY) 1 Has been benchmarked Significant working experiences  Cons Will need some modifications to be suitable for High power CW operation BBU threshold and HOM effects Smaller cavity aperture. Beam loss and scraping issues Wakefields issues o1408 MHz (Single-pass)  Pros 4 th harmonic of the APS storage ring frequency Makes it possible to “synchronize” ERL and SR ( hybrid mode operation)  Cons Cavity has to be designed Requires R&D for development, optimization, and rf characterizations for CW operation BBU threshold and HOM effects Smaller cavity aperture. Beam loss and scraping issues Wakefields issues 1 J. Sekutowicz, DESY

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity Frequency Choice of frequency (Two-pass) o704 MHz  Pros 2 nd harmonic of the APS storage ring frequency Makes it possible to “synchronize” ERL and SR ( hybrid mode operation) Higher BBU threshold limit Injector design is simplified. Input power couplers will have higher power handling capability High peak current effects are reduced In principle can accelerate higher than 100 mA  Cons Requires higher bunch charge ( ~150 pc for 100 mA average current), compared to ~77 pc at 1300 MHz This potentially affects emittance and source brightness Cavity fabrication and processing due to larger surface area Lower field gradient

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Number of Cells Effective beam loading in ERL cavities is minimal, close to zero The required RF power per cell is small Large number of cells could be excited via one input coupler (TESLA SS ) Multi-cell cavities with a larger number of cells would also improve linac packing factor, i.e., ratio of active length to total length This will reduce the cost of the ERL linac, BUT Strong HOM damping with higher beam current favors smaller number of cells Extensive SCRF cavity cells optimizations have been done at TESLA and JLab oTESLA – 9-cell cavity oJLab- 7-cell cavity ( CEBAF 12 GeV Energy Upgrade, Renascence) oBNL 703 MHz for eRHIC TESLA 9-cell cavity, 1300 MHz 3 CEBAF 7-cell cavity, 1497 MHz 1 BNL 5-cell cavity, 703 MHz 2 1 Courtesy R. Rimmer, JLab 2 Courtesy R. Calaga, BNL 3 Courtesy J. Sekutowicz, DESY

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Q vs. Gradient CEBAF 7-cell cavity We expect cavity gradient for CW operation to improve in the next five years 1 Courtesy C. Reece, JLab 1

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity and Wall-Plug Power Provide reasonable cavity coupling factor Provide a cavity bandwidth sufficiently large to allow cavity frequency tuning We use TESLA structure as a baseline for the following The strongly over-coupled cavities reflect most of the incident drive power This results in a large requirement for rf power to produce the specified accelerating voltage

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity and Wall-Plug Power The power dissipated in the cavity wall, P cav, is given by Total losses in 350 m of cavities are ~16 kW Since the beam loading of the accelerated and decelerated beam cancel, ideally there is no effective beam loading for the accelerating mode The required RF power to maintain a given accelerating voltage under cavity detuning due to microphonics is given by Allow 20% overhead for control margin and waveguide loss and phase shift ~16 kW klystron is needed.

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity and Wall-Plug Power The required rf power will increase to ~17 kW if one operate the TESLA cavity at the design gradient of 23 MV/m APS-ERL requires ~350 cavities ( ~350m effective accelerating length) Assuming an IOT efficiency of 65% The wall-plug power for the 7 GeV ERL is 7.0 MW Injector beam power ( 100 MeV) is 1MW. 1MW of CW RF power is required Assuming a klystron efficiency of 50% Injector wall-plug power is 2 MW Losses due to synchrotron radiation ( ~15 MeV) ~1.5 MW Total wall-plug power is 11.5 MW Multi-pass recirculation reduces wall-plug power

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Fundamental Power Couplers Waveguide oPros  Simpler in design  Easier to cool  Better power handling  Large size  Bigger heat leak  More difficult to make it variable Coaxial oPros  More compact  Easier to make it variable  Smaller heat leak oCons  More complicated design  Require active cooling  Not so good power handling Cornell 50 kW CW Coupler Design 2 CEBAF 12GeV Upgrade Renascence cryomodule 1 1 Courtesy R. Rimmer, JLab 2 Courtesy M. Liepe, Cornell

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, HOM Losses Large HOM power contributes to large loss factor High Q HOMs contributes to MBI MBI give rise to beam breakup oHigh Q dipole modes oFeedback loop between beam and cavities oIt is worse for high current, high bunch charge HOM losses are This is not an acceptable loss at liquid helium temperature It has to be properly extracted ( with a carefully chosen Q ext ) to outside so only a small fraction of the power is dissipated in the cavity walls (per cavity for two beams)

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, HOM Damping HOM power must be dumped out of the liquid helium HOM couplers must be able to handle large average power Higher order modes must be well coupled to the beam pipe TESLA HOM coupler is not suitable for CW operation Ferrite absorbers Broadband K) Loop couplers Resonant circuit K) (Cornell type) (TESLA type)

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cooling Requirements for 7 GeV APS-ERL will require a cryogenic plant equivalent to 3.5 x CEBAF Electrical power utilities requirement: 16.0 MW ( operating at 2.08 ºK) Multi-pass recirculation reduces power requirement Cavity frequency (MHz) Gradient15 MV/m20 MV/m15 MV/m20 MV/m V cav (MV) Number of cavities (W) Dynamic heat (kW)

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, RF Power Sources We plan to adapt one-source-per-cavity concept Because high loaded Q L of the cavities prohibits vector sum control of many cavities In addition, microphonics would cause unacceptable fluctuations of the individual fields in case of vector sum control RF power source requirements: oReasonably high efficiency oReliability and long life time oAvailability oReasonable price oTechnical support and good customer service Types of RF sources: oKlystrons  High gain – requires low drive power  High efficiency when operated close to saturation  A good choice for ERL injector linac ( constant beam loading)  CW sources below 1 GHz are available from industry  CW sources above 1 GHz under development

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, RF Power Sources Types of RF sources: oInductive Output Tubes (IOTs)  High efficiency  High linearity and smaller pushing factors  No saturation. Can operate up to their maximum output power  Less expensive than klystrons BUT  It has lower gain than klystron and needs higher power drive amplifier  Grid geometry does not allow operation at high frequencies like klystrons CPI 1.3 GHz IOT Prototype 1 Courtesy S. Lenci, CPI MPP, Palo, Alto, CA 1

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, RF Power Sources DeviceKlystronIOT Manufacturere2vThalesCPIe2v Frequency [MHz]1300 Beam Voltage [kV] Beam Current [A] Output Power (CW) [kW] Gain [dB] Efficiency [%]

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, RF Distribution 7- or 9-cell structure One cryomodule will consist of 4 or 8 structures 20 MV/m accelerating field gradient with 60% filling factor 350 cavities ( 3150 or 2450 cells) Each structure is powered by its own rf source 350 power sources with feedback control HOM Damper (4) RF Amplifier Water load Circulator One cryomodule ~600 m

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, LLRF Control Requirements Maintain constant phase and amplitude of the cavity fields within given tolerances oRF phase: RMS oRF amplitude: 1x10 -4 RMS Minimize power needed for control by actively controlling cavity tuners to ensure operation on resonance Build-in diagnostics for calibration of gradient and phase, cavity detuning Fast interlock system for faults during a cavity trip Feedback loops and control to deal with: oBeam current fluctuations oMicrophonics oLorentz force detuning Possible types of control systems: oSelf-excited loop oGenerator driven system and monitor separate amplitude and phase oUse I/Q detector and controller oFPGA/DSP Use JLab, SNS, CORNELL LLRF systems as baseline design

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Cavity R&D Active R&D is needed to address critical SCRF cavity design for CW operation oInvestigate the need for the development of a new cavity that meets APS ERL requirements  Higher fill factor  Strong HOM damping  Low microphonics  TESLA, JLab, Cornell, and BNL experiences are essential oOptimize the shape of the Cornell 7-Cell cavity to further increase HOM damping and to lower cryogenic losses  Collaborate with Cornell SCRF group oDesign and build a prototype multi-cell copper cavity  Measure fundamental rf parameters Q’s of fundamental and HOM modes  Bead-pull measurements to check field flatness  Identify the HOM modes from bead pull field profile oAnalysis and simulation of HOM and damping oDesign of a high power input coupler (FPC)  Use JLab WG coupler and TESLA Coaxial coupler as baseline oDesign and build a multi-cell Nb first prototype cavity oDesign and build a prototype cryomodule oPerform vertical dewar test

ERL RF Systems ( Exploring Options)A. NassiriNovember 15, Conclusion SCRF technology for ERLs and CW machines is advancing at a fast pace We expect cavities development to make possible to operate at energy gains in excess of 20 MV/m A wide range of expertise and experience already exists Our challenge is to: oHow to deal with a 16 kW cryogenic plant ( big footprint, capital+operation)  Note: CEBAF CHL system is K oDesign a CW-specific cavity to meet ERL design parameters  Tesla 9-cell and JLab 7-cell structures are good candidates o Develop a robust HOM damping system oBetter understand and reduce field emission for higher gradient in CW mode oImprove cavity quality factor ( 1  !)  For CW operation highest fields are not important. Highest possible Q values at about 20 MV/m are very critical. This is in contrast with pulsed ILC requirement. oDevelop a robust LLRF control system for CW operation We intend to actively seek collaboration with other laboratories and institutions on the development of SCRF for ERL ( JLab, Cornell, BNL, Daresbury,….)