1. Report of WG 5: Superconducting RF
The 55th ICFA Advanced Beam Dynamics Workshop on
High Luminosity Circular e+e- Colliders – Higgs Factory (HF2014)
Report of WG 5: Superconducting RF
Yoshiyuki Morita (KEK) & Sergey Belomestnykh (BNL)
10/12/14
HF2014: WG5 Report

2. WG 5 Introduction
At WG 5, we reviewed SRF systems of FCC-ee and CEPC and considered SRF structures, peripheral components and other issues relevant to the future colliders.
In particular, we discussed the validity of cavity parameters and cavity design (frequency, voltage, input RF power, coupling, detuning and HOM damping scheme), high power couplers, HOM dampers, frequency tuners, operating experience and other issues.
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HF2014: WG5 Report

3. Talk 1: FCC-ee SC RF system by Frank Zimmermann/Erk Jensen (CERN)
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HF2014: WG5 Report

4. FCC-ee RF system Main RF parameters
Synchrotron radiation power: 50 MW per beam
Energy loss per turn: 7.5 GeV (at 175 GeV, t)
Beam current up to 1.4 A (at 45 GeV, Z)
Up to 7500 bunches of up to 4 x 1011 e per ring.
CW operation with top-up operation, injectors and top-up booster pulsed
Basic choices for RF system and RF system size:
Frequency range (200 … 800) MHz with 400 MHz as starting point, Harmonics of 40 MHz required, harmonics of 200 MHz preferred
Preferred technology: Thin films on Cu substrate (allows scaling to very large overall size)
System dimension compared to LHC:
LHC 400 MHz  2 MV and ~250 kW per cavity, (8 cavities per beam)
Lepton collider ~600 cavities 20 MV / 180 kW RF  12 GV / 100 MW
HF2014 Beijing, 9-12 Oct 2014
Frank Zimmermann, Erk Jensen
FCC-ee RF System
10/12/14
HF2014: WG5 Report 4

5. FCC RF R&D Subjects Superconducting RF Large RF Systems
Cavity technology
Power couplers
Cavity optimization
Cryomodules
Large RF Systems
Availability
Reliability
Maintainability
Operational aspects
Energy Efficiency
Efficient power sources
Lowering cryogenic load
Energy recovery?
HF2014 Beijing, 9-12 Oct 2014
Frank Zimmermann, Erk Jensen
FCC-ee RF System
10/12/14
HF2014: WG5 Report 5

6. Summary
The areas of R&D identified to prepare technology for the Future Circular Collider are
Superconducting RF R&D
focus on Nb on Cu, but explore alternatives!
High Efficiency RF power generation
Design of complex systems for high availability
In all these areas, the R&D has significant synergies with ongoing studies and projects, with which the R&D should be coordinated.
HF2014 Beijing, 9-12 Oct 2014
Frank Zimmermann, Erk Jensen
FCC-ee RF System
10/12/14
HF2014: WG5 Report 6

7. Talk 2: CEPC SRF system by Jiyuan Zhai (IHEP)
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HF2014: WG5 Report

8. General Approach to CEPC SRF System (Jiyuan Zhai, IHEP)
CEPC SRF system and cryomodule layout, design criteria, and key parameter choices
frequency, cavity number, cell number, gradient, quality factor.
cavity with very high Q0 at MV/m (use N-doping and flux expulsion). Plan to demonstrate in-house and push technology frontier.
Preliminary analysis of key HOM issues of both main ring and booster. Possible solutions or concepts.
HOM power, spectrum, heat load, QL limit, frequency spread
trap mode, better heat load model, feasible HOM damping scheme to be studied
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HF2014: WG5 Report

9. General Approach to CEPC SRF System (Jiyuan Zhai, IHEP)
Preliminary cavity RF design; power coupler and tuner requirement and baseline (or candidate) type.
Preliminary cost and power consumption (efficiency) model of main ring and booster SRF system (cryomodules, cryogenics and RF sources) .
More precise and comprehensive model (and scaling) needed for further optimization.
Cooperation or synergy with other projects (R&D and industrialization): CADS & PIP-II, Euro-XFEL, LCLS-II, ERLs & ILC, FCC-ee
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HF2014: WG5 Report

10. Talk 3: 704 MHz BNL3 cavity as an option for CEPC/FCC by Wencan Xu (BNL)
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HF2014: WG5 Report

11. 704 MHz BNL3 cavity design BNL 3 cavity Administrative limit : 20 MV/m
Enlarged beam tube: to attenuate the fundamental mode but propagate all the HOMs. The cutoff frequency is below the first dipole mode.
Comparing with BNL1 cavity, BNL3 improves the fundamental mode performance, resulting in the R/Q and Geometry factor increase of about 20% and 30% respectively.
Through avoiding the room-temperature ferrite damper on the beam pipe, the real-estate gradient of BNL3 cavity has improved by about 50% as compared with the BNL1 cavity.
To reduce the cross-talk between neighboring cavities and avoid RF heating on the flange seal, tapered sections to a reduced diameter beam pipe are added on both sides of the cavity. The quasi symmetric taper will not affect the loss factor.
3 D bead pull is essential to indentify HOMs and detachable end group allows us to measure the HOMs damping affected the HOM and FPC ports layout.
The BNL3-AES cavity was tested vertically and reached the goal for CEC POP. It is undergoing cryomodule assembly now.
BNL 3 cavity
Administrative limit : 20 MV/m
SRF photo-injector includes SRF cavity, HTS, laser, merger…
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HF2014: WG5 Report

12. HOM damping 6 coaxial HOM damper = Beam pipe damper
Coaxial HOM coupler has similar damping capability with the ideal case;
Two potential HOM dampers are investigated.
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HF2014: WG5 Report

13. Talk 4: High power couplers for Higgs Factory by Eiji Kako (KEK)
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HF2014: WG5 Report

14. Consideration of Input Coupler for Higgs Factory (1)
Facility
Frequency
Window, DC-Bias,
Coupling, Cooling
Maximum RF Power
CepC /IHEP
650 MHz
Under design
Spec., kW, CW
SRF-Gun /BNL
704 MHz
Coax. Disk, no-Bias
Fixed, water & He gas
Test-stand, 125 kW, CW
250 kW, pulse (20% duty)
HIPPI /CEA
Test-stand,
1.2 MW, pulse (10% duty)
SPL /CERN
Disk/Cylind., DC-Bias
Fixed, & He gas
1.0 MW, pulse (10% duty)
ESS /CEA
Coax. Disk, DC-Bias
Specification,
1.2 MW, pulse (4% duty)
SNS /ORNL
805 MHz
Test-stand, 2 MW, pulse
Operation, 350 kW, pulse
FCC /CERN
802 MHz
Spec., kW, CW
10/12/14
HF2014: WG5 Report

15. Consideration of Input Coupler for Higgs Factory (2)
Requirement
FCC (TLEP)
CepC
Frequency
802 MHz
650 MHz
RF power
125 kW, CW
260 kW, CW
Beam current
varied / (fixed)
Operating Eacc
Adjustable coupling
yes / (no)
Range of QL / (Fixed)
x 106 / (5.4 x 106)
- x 106 / (2.4 x 106)
Coupler type
coaxial
RF window
single warm window
Ceramics window
coaxial disk / cylindrical
coaxial disk
Bias voltage
yes
Cooling method
inner conductor : water
outer conductor : He gas
outer conductor : N2 gas
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HF2014: WG5 Report

16. Summary
Coaxial CW high power input couplers with a single warm RF window have been developing in the frequency range of 500 – 1300 MHz at the power level higher than 100 kW in many laboratories of the world.
Design studies of high power couplers with 802 MHz, 125 kW-CW for FCC and 650 MHz, 260 kW-CW for CepC should be started as soon as possible.
Fabrication of the prototype high power couplers and RF conditioning at a test-stand should be carried out at an early stage.
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HF2014: WG5 Report

17. Talk 5: HOM Damper Hardware Considerations for Future Energy Frontier Circular Colliders by Sergey Belomestnykh (BNL)
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HF2014: WG5 Report

18. Introduction
Future high-luminosity energy frontier circular colliders will operate with high average beam currents. Radio-frequency systems in these machines will utilize superconducting RF structures with strong damping of higher order modes.
There are three main design types, which use different transmission lines/coupling circuits: beam pipe HOM absorbers, rectangular waveguide HOM couplers, and loop/antenna coaxial HOM couplers.
There is a wide variety of HOM damper designs for SRF cavities. However, very few of those are designed to handle high average HOM power and even fewer demonstrated this in operation.
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HF2014: WG5 Report

19. Pros & cons of different HOM damper types
Beam pipe absorbers are arguably the most efficient and likely will be required to absorb very high frequency portion of HOM power, which propagates along the beam pipe.
Room temperature HOM loads demonstrated capacity to absorb several kW of HOM power at CESR and KEKB.
Drawbacks of beam pipe absorbers: i) most absorber materials are brittle, can create particulates that contaminate SRF cavities; ii) parasitic beam-absorber interaction is significant and contributes to the overall HOM power; iii) the main disadvantage for large SRF systems is that they occupy real estate along the beam axis and thus reduce the fill factor.
Waveguide couplers can provide very efficient damping in a broad frequency range and don’t compromise the fill factor. However, waveguides significantly complicate the cavity and cryomodule design. This damping scheme is worked on at Jefferson Lab primarily.
Coaxial loop/antenna HOM couplers require means of rejecting the fundamental mode. Rejection filters can be very narrowband and difficult to tune. LHC type couplers were designed for ~1 kW HOM power levels, but operate at lower HOM power levels. High-pass filters are promising and under development at BNL.
10/12/14
HF2014: WG5 Report

20. Summary There are many proven designs of HOM dampers.
However, so far only beam pipe absorbers demonstrated power levels of interest to future circular colliders.
LHC type couplers were designed for ~1 kW HOM power levels, but operates at lower HOM power levels.
Also, there are several new designs under development, which might be suited for future circular colliders.
Strong R&D program must be established to develop suitable (robust and efficient) HOM dampers for future colliders.
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HF2014: WG5 Report

21. Talk 6: Other issues by Yoshiyuki Morita (KEK)
10/12/14
HF2014: WG5 Report

22. Summary 1 Many excellent tuner designs Can select a suitable mechanism
Reliable
Compact
Cost efficient
Easy for maintenance
Need to select cold or warm locations
Lever system and piezoactuator
Familiar mechanism
Long life
TRISTAN/KEKB driver: 28 years old
Will be used for SuperKEKB
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HF2014: WG5 Report

23. Summary 2 Operational experience Performance recovery
0.5 trip/day for 8 cavities at 1.4A operation
Maintenance work to keep low trip rate
Warm up
RF conditioning of cavity
Coupler conditioning with voltage biasing
Adjustable coupling desired
Greasing of tuner once a year
Performance degradation
KEKB cavities can provide 2 MV after 10 years of operation
Unloaded Q factors degraded with field emission
Performance recovery
Needed in the long term operation
Low risk, low cost and short period of time
Horizontal HPR developed
HHPR applied to tow degraded cavities
Those cavities recovered successfully
Performance recovery after HHPR
D11B
Trip rate during KEKB operation
Q degradation at 2 MV
D11C
10/12/14
HF2014: WG5 Report

24. WG 5 Summary
At WG 5, we have considered parameters of SRF systems for FCC-ee and CEPC, requirements and challenges. Also, there we discussed SRF cavity designs, HOM dampers, RF input couplers, frequency tuners and operational experience, performance degradation and recovery.
There are a number of important issues that have to be addressed in future studies:
HOM studies, including: trapped modes; efficient HOM coupler designs; propagation of the very high frequency portion of HOM power, its absorption, and associated parasitic heat load.
SRF cavity design: frequency; number of cells per cavity; optimal operating temperature; N-doping at low frequencies; new materials?
Does FCC-ee at Higgs and Z energies require two different SRF systems? It appears that these are two very different regimes: LEP-like vs. B-factory-like.
General SRF/cryogenic system optimization.
Efficiency of RF power sources.
Ideally, these studies will be executed in collaboration, utilizing synergy with other projects and labs.
10/12/14
HF2014: WG5 Report