ERL RF Systems

1. 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

2. Outline • Goals • Cavity design criterion • Cavity parameters • Frequency • Cell Shape • Number of cells • Q vs. Gradient • Cavity and wall-plug Power • Fundamental RF Coupler • HOM Coupler • Cooling Requirement • RF power sources and rf distribution • LLRF system • A summary of cavity design parameters • Cavity R&D • Conclusion

3. 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

4. Criteria for Cavity Design • Cavity parameters

5. Cavity Frequency • Frequency scaling • The losses in a microwave cavity are proportional to • For a given length of a multi-cell structure • It becomes independent of • At , the BCS term dominates above 3 GHz and the losses grow linearly with frequency • Below 300 MHz, the residual resistance dominates and the losses are proportional to

6. Cavity Frequency • Choice of frequency • 1300 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 • 1408 MHz (Single-pass) • Pros • 4th 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

7. Cavity Frequency • Choice of frequency (Two-pass) • 704 MHz • Pros • 2nd 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

8. 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 • TESLA – 9-cell cavity • JLab- 7-cell cavity ( CEBAF 12 GeV Energy Upgrade, Renascence) • BNL 703 MHz for eRHIC TESLA 9-cell cavity, 1300 MHz3 1 Courtesy R. Rimmer, JLab 2 Courtesy R. Calaga, BNL 3 Courtesy J. Sekutowicz, DESY BNL 5-cell cavity, 703 MHz2 CEBAF 7-cell cavity, 1497 MHz1

9. Q vs. Gradient • CEBAF 7-cell cavity • We expect cavity gradient for CW operation to improve in the next five years 1 1 Courtesy C. Reece, JLab

10. 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

11. Cavity and Wall-Plug Power • The power dissipated in the cavity wall, Pcav , 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.

12. 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 mA @10 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

13. Fundamental Power Couplers • Waveguide • Pros • Simpler in design • Easier to cool • Better power handling • Large size • Bigger heat leak • More difficult to make it variable • Coaxial • Pros • More compact • Easier to make it variable • Smaller heat leak • Cons • More complicated design • Require active cooling • Not so good power handling CEBAF 12GeV Upgrade Renascence cryomodule1 1 Courtesy R. Rimmer, JLab 2 Courtesy M. Liepe, Cornell Cornell 50 kW CW Coupler Design2

14. HOM Losses • Large HOM power contributes to large loss factor • High Q HOMs contributes to MBI • MBI give rise to beam breakup • High Q dipole modes • Feedback loop between beam and cavities • It 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 Qext) to outside so only a small fraction of the power is dissipated in the cavity walls (per cavity for two beams)

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 Loop couplers Resonant circuit (@2 K) Ferrite absorbers Broadband (@300 K) (Cornell type) (TESLA type)

16. 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

17. RF Power Sources • We plan to adapt one-source-per-cavity concept • Because high loaded QL 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: • Reasonably high efficiency • Reliability and long life time • Availability • Reasonable price • Technical support and good customer service • Types of RF sources: • Klystrons • 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

18. RF Power Sources • Types of RF sources: • Inductive 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 1 CPI 1.3 GHz IOT Prototype 1 Courtesy S. Lenci, CPI MPP, Palo, Alto, CA

19. RF Power Sources

20. 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) One cryomodule Water load Circulator HOM Damper (4) RF Amplifier ~600 m

21. LLRF Control Requirements • Maintain constant phase and amplitude of the cavity fields within given tolerances • RF phase: 0.050 RMS • RF 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: • Beam current fluctuations • Microphonics • Lorentz force detuning • Possible types of control systems: • Self-excited loop • Generator driven system and monitor separate amplitude and phase • Use I/Q detector and controller • FPGA/DSP • Use JLab, SNS, CORNELL LLRF systems as baseline design

22. Cavity R&D • Active R&D is needed to address critical SCRF cavity design for CW operation • Investigate 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 • Optimize the shape of the Cornell 7-Cell cavity to further increase HOM damping and to lower cryogenic losses • Collaborate with Cornell SCRF group • Design 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 • Analysis and simulation of HOM and damping • Design of a high power input coupler (FPC) • Use JLab WG coupler and TESLA Coaxial coupler as baseline • Design and build a multi-cell Nb first prototype cavity • Design and build a prototype cryomodule • Perform vertical dewar test

23. 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: • How to deal with a 16 kW cryogenic plant ( big footprint, capital+operation) • Note: CEBAF CHL system is 4.6 kW @ 2.1 K • Design a CW-specific cavity to meet ERL design parameters • Tesla 9-cell and JLab 7-cell structures are good candidates • Develop a robust HOM damping system • Better understand and reduce field emission for higher gradient in CW mode • Improve cavity quality factor ( 11011 !) • 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. • Develop 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,….)