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

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. 10/12/14 HF2014: WG5 Report

Talk 1: FCC-ee SC RF system by Frank Zimmermann/Erk Jensen (CERN) 10/12/14 HF2014: WG5 Report

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

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

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

Talk 2: CEPC SRF system by Jiyuan Zhai (IHEP) 10/12/14 HF2014: WG5 Report

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 (2E10@2K) at 15-20 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 10/12/14 HF2014: WG5 Report

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 10/12/14 HF2014: WG5 Report

Talk 3: 704 MHz BNL3 cavity as an option for CEPC/FCC by Wencan Xu (BNL) 10/12/14 HF2014: WG5 Report

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… 10/12/14 HF2014: WG5 Report

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. 10/12/14 HF2014: WG5 Report

Talk 4: High power couplers for Higgs Factory by Eiji Kako (KEK) 10/12/14 HF2014: WG5 Report

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., 260 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., 125 kW, CW 10/12/14 HF2014: WG5 Report

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 10/12/14 HF2014: WG5 Report

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. 10/12/14 HF2014: WG5 Report

Talk 5: HOM Damper Hardware Considerations for Future Energy Frontier Circular Colliders by Sergey Belomestnykh (BNL) 10/12/14 HF2014: WG5 Report

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. 10/12/14 HF2014: WG5 Report

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

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. 10/12/14 HF2014: WG5 Report

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

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 10/12/14 HF2014: WG5 Report

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

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