HOM Power Challenge for CEPC Jiyuan Zhai On behalf of the CEPC SRF Design Group of IHEP TTC Meeting, December 2-5, 2014, KEK

Content CEPC-SppC Project CEPC SRF Parameters HOM Damping Challenges Summary

CEPC-SppC Project 50-70 km in circumference CEPC is a 240 GeV Circular Electron Positron Collider, proposed to carry out high precision study on Higgs boson, which can be upgraded to a 70 TeV or higher pp collider SppC (construction time: 2035-2042), to study the new physics beyond the Standard Model. BTC IP1 IP3 e+ e- e+ e- Linac LTB CEPC Collider Ring CEPC Booster SppC ME Booster SppC LE Booster IP4 IP2 SppC Collider Ring Proton Linac SppC HE Booster 50-70 km in circumference

Potential Site: Qinhuangdao, Hebei Province, China

CEPC Design Baseline Tunnel circumference: ~ 54 km Tunnel size: ~ 6.5 m (LEP tunnel: 3.76 m) 8 arcs and 8 straight sections: 4 straight for IPs and RF, another 4 for RF, injection and beam dump, etc. A 6 GeV linac on the surface (with the option for an XFEL in the future) A full-energy 120 GeV Booster in the tunnel A 240 GeV e+e- Collider in the same tunnel underneath the Booster A single beam pipe for both e+ and e- beams (similar to LEP, CESR) Synchrotron radiation budget: 50 MW per beam Two SRF systems: Booster: 1.3 GHz 9-cell cavity, similar to the ILC, XFEL, LCLS-II Collider: 650 MHz 5-cell cavity, similar to the China ADS, PIP-II

CEPC Main Technical Challenges IR optics with appropriate dynamic aperture for off-momentum up to 2% Pretzel orbit: Tolerance for orbit offset in the RF Tolerance for orbit offset in the quads and sextupoles Bunch tilt effect due to transverse wakefield Machine-detector interface (MDI) Low energy injection (6 GeV) to the Booster: Earth field effect: bend field ~30 Gs, measured earth field ~0.5  0.03 Gs HOM damper for the Collider RF cavity High average beam current: 2 x 16.6 mA High HOM loss: 3.5 kW per cavity

CEPC SRF System Layout Total SRF 1.4 km, 12 GeV RF voltage Booster magnet field cycle extraction to main ring 120 GeV collider and booster in one tunnel one vacuum chamber for collider injection from linac 6 GeV 4 IPs, ~ 1 km each 4 straights, ~ 850 m each RF section length ~ 175 m Cavity voltage ramping Total SRF 1.4 km, 12 GeV RF voltage 104.5 MW beam power, 2 MW HOM power 124 MW RF power, > 200 MW RF AC power 126 kW (4.2 K equiv.) installed cryogenic power Booster RF and cryogenic duty factor ~ 22 % for continuous injection. Each collider or booster cavity is driven by an individual RF power source.

CEPC Top-level Parameters related to SRF System (1) Unit Main Ring Booster Injection Booster Extraction Beam energy GeV 120 6 Circumference km 54.752 Luminosity / IP cm-2s-1 2.04E+34 - Energy loss / turn 3.11 17.6 keV 2.81 SR power MW 103.42 16.2 W 2.46 Revolution period s 1.83E-04 Revolution frequency kHz 5.4755 Bunch charge nC 60.56 3.35 3.2 Bunch number 100 50 Beam current (total) mA 33.2 0.92 0.87 Bunch spacing μs 1.825 3.65 Beam spectrum spacing MHz 0.55 0.274

CEPC Top-level Parameters related to SRF System (2) Unit Main Ring Booster Injection Booster Extraction Momentum compaction - 3.36E-05 7.69E-05 Energy spread % 0.1629 0.1 (linac) 0.127 Bunch length mm 2.65 1.5 (linac) 2.66 RF voltage GV 6.87 0.213867 5.12 RF frequency GHz 0.65 1.3 Harmonic number 118800 237423 Synchrotron tune 0.180 0.32076 Energy acceptance (RF) 5.99 17.307 2.091 Transverse damping time turns 77.05 682122 (125 s) 85.2 (15.56 ms) Longitudinal damping time 38.52 341219 (62 s) 42.7 (7.789 ms) Lifetime Bhabha min 51.77 Lifetime BS (sim.) 47

CEPC SRF System Parameters CEPC-Collider CEPC-Booster LEP2 Cavity Type 650 MHz 5-cell Nitrogen-doped Nb 1.3 GHz 9-cell 352 MHz 4-cell Nb/Cu sputtered Cavity number 384 256 288 Vcav / VRF 17.9 MV / 6.87 GeV 20 MV / 5.12 GeV 12 MV / 3.46 GeV Eacc (MV/m) 15.5 19.3 6 ~ 7.5 Q0 4E10 @ 2K 2E10 @ 2K 3.2E+9 @ 4.2K Cryo AC power (MW) ~15 ~2.3 (21% DF) 6.1 Qualified test 20 MV/m @ 4E10 23 MV/m @ 2E10 / Cryomodule number 96 (4 cav. / module) 32 (8 cav. / module) 72 (4 cav. / module) CW RF power / cav. (kW) 280 20 125 RF source number 384 (330 kW) 256 (25 kW) 36 (1.2 MW/8 cav) HOM damper (W) 10k ferrite +1k hook 50 (hook+ceramic) 300 (hook)

SRF Program in Possible CEPC Timeline 2015 2020 2027 Pre-CDR Pre-studies (2013-2015) R&D Engineering Design (2016-2020, 5 yrs.) Construction (2021-2027, 7 yrs.) SRF Pre-design SRF Initial Technology R&D (2016-2020, 5 yrs.) HOM damping, high Q0 cavity, coupler window, heat load SRF Pre-Production R&D (2019-2022, 4 yrs.) SRF Production and Installation (2023-2027, 5 yrs.) IHEP site infrastructure Large scale SRF lab (infrastructure) on CEPC site and in industry (2018) CEPC SRF Team (~6) CEPC SRF Core Team (main ring 10 + booster 10 in parallel) (start from 2017) Facility upgrade, additional space SRF ~ 200 FTEs (start from 2020), engineers and technicians Components Prototyping & Performance Demonstration: Cavity (four for each), helium vessel, vertical test Power coupler (four for each), HOM dampers, tuners Short (two-cavity) module for each type, horizontal test Booster SRF in three years (2023-2025): test two 1300 MHz cavities per week assemble and test one cryomodule per month Main ring SRF in four years (2023-2025): test two 650 MHz cavities per week assemble and test two cryomodules per month Demonstrate Robustness of Fabrication and Assembly Processes of Cryomodule and its Components. Establish procedures, quality control steps, test set ups, assembly sequences, etc. for the production run. Build and test two booster cryomodules and three main ring cryomodules.

HOM Power Very small beam spectral line spacing: main ring 0.55 MHz, booster 0.275 MHz. Impossible to detune HOM modes away from beam spectral lines with large HOM frequency scattering from cavity to cavity by fabrication tolerances and RF tuning of the fundamental mode DESY TESLA cavity measured spread: TM011 9 MHz, TE111 5 MHz, TM110 1~6 MHz, TE121 2 MHz HOM power damping of 3.5 kW for each 650 MHz 5-cell cavity and 21 kW for each cryomodule of the CEPC main ring. Possible resonant excitation for low frequency modes below cut-off. For the main ring cavity TM011 power at Qext = 1E4 is 937 W (310 W of the single pass case).

HOM Spectrum 80 % above cut-off frequency , propagate through cavities and finally absorbed by the two HOM absorbers at room temperature outside the cryomodule. Each absorber has to damp about 10 kW HOM power, can’t be in the cryogenic region. LEP2: 300 W / cavity above cut-off : 200 W KEKB and SuperKEKB ferrite HOM absorbers 20 kW. IHEP 4.5 kW tested. XFEL/ILC type HOM coupler and absorber can easily handle the booster HOM power. Ceramic HOM absorber at 70 K in the cryomodule beam line. LEP/LHC-type HOM coupler for the kW level power capacity. BNL also developing kW class coaxial HOM couplers. Waveguide at the cavity beam pipe is also a possible solution for the main ring cavity HOM power extraction, but with large size waveguides, more complicated structure and interfaces of the cryomodule, and large heat load.

HOM Damping Scheme Inter-cavity beam pipe (and bellow) diameter will determine the fraction of HOM power propagating out of the cryomodule. Let most HOMs confine in one cavity and extract them out or let them go through cavities? How much will trap, especially the grey zone? LEP2 type Conservative scheme: two cavities in one module. Another possible solution is hybrid module with water cooled HOM absorber inside (Carlo Pagani). Need further study. Critical issue.

HOM Heat Load Budget   Main Ring Booster HOM power / cavity 3.5 kW 5.3 W HOM power / module 21 kW 56 W HOM 2K heat load / module 13 W 7.2 W HOM 5K heat load / module 39 W 3.2 W HOM 80K heat load / module 390 W 52.8 W Percent of total cryogenic load 22 % 15 % Estimated upper limit, design goal, enough margin Main ring cryomodule: RF shielded bellows (copper plated) and gate valves, flange connection with gap-free gaskets Booster heat load scaled from ILC TDR. Duty factor of booster HOM power is 50 % for continuous injection (much lowered, e.g. 1-5%, for longer beam lifetime => long injection cycle and/or low charge).

Heat Load Estimation Assume 10 kW HOM power propagating through the beam tubes and bellows (thin copper film RRR=30, in abnormal skin effect regime), power dissipation < 2 W/m @ 2K. Even RRR=1 for copper plating (in normal skin effect regime), power dissipation < 10 W/m. Note 300 kW power through input coupler connected to 2 K and 5 K. HOM power loss in cavity at 2 K less than 0.3 W (all modes Qext < 106) Heat load at 5 K and 80 K dominated by HOM cable heating. Assume 0.1 dB/m power dissipation of the LEP/LHC-type rigid coaxial line (copper plated stainless steel) and 1 m length, total heat load is 10 W for the resonant excitation of TM011 mode. Further study: HOM propagating, heat load calculation, statistical approach to study heat load at 2 K (LCLS-II), trap mode … beam test?

Main Ring 650MHz Cavity HOM Q Limit Monopole Mode f (GHz) R/Q (Ω) Qlimit σf = 0 MHz σf = 0.5 MHz σf = 5 MHz TM011 1.173 84.8 5.1E+5 2.9E7 5.8E7 TM020 1.350 5.5 6.8E+6 3.7E7 7.5E7 Dipole Mode R/Q (Ω/m) TE111 0.824 832.2 2.3E+4 1.2E6 2.4E6 TM110 0.930 681.2 2.8E+4 1.5E6 3.0E6 TE112 1.225 36.2 5.2E+5 1.9E6 3.7E6 TM111 1.440 101.5 1.9E+5 1.0E7 2.0E7 Large HOM frequency spread from cavity to cavity relaxes the Q requirement. However, some modes far above cut-off frequency may become trapped among cavities in the cryomodule due to the large frequency spread Further work: enlarge the iris diameter to decrease loss factors while keep relatively high R/Q and low surface field of the fundamental mode, identify trapped modes within the cavity and cryomodule, reduce the cavity cell number or design asymmetry end cells to avoid trapped modes (e.g. TE121)

Booster 1.3GHz Cavity Impedance

Transverse Mode Coupling Instability (TMCI) at Booster Injection Energy (6 GeV) The single bunch threshold current is 27 A (I0th=1.36mA). This is higher than the design bunch current, but doesn’t leave enough safe margin.  Objects number ky [V/pC] Resistive wall (Al) 1 4.4104 RF cavities 256 5.7103 The transverse kick factor is dominated by the resistive wall impedance. More impedance objects will be included when the vacuum components are designed.

Coupled Bunch Instability at Booster Injection Energy (6 GeV) Monopole Mode f (GHz) R/Q (Ω) Q* f ** (MHz) TM011 2.450 156 58600 9 TM012 3.845 44 240000 1 Dipole Mode R/Q (Ω/m) Q TE111 1.739 4283 3400 5 TM110 1.874 2293 50200 TM111 2.577 4336 50000 TE121 3.087 196 43700 * Data of TESLA 9-cell cavity ** J. Sekutowicz. EuCARD Editorial Series on Accelerator Science and Technology, Vol.01 (2007) Most dangerous longitudinal mode growth rate: 472 ms. Most dangerous transverse mode growth rate: 22 ms. Feedback system will be needed and able to mitigate the multi-bunch instabilities. 20 20

~100 kW per cavity if 800 MHz cavity! 50MHz 4MHz Parameter LEP2 FCC-ee Z Z (c.w.) W H t Ebeam [GeV] 104 45 80 120 175 circumference [km] 26.7 100 current [mA] 3.0 1450 1431 152 30 6.6 PSR,tot [MW] 22 no. bunches 4 16700 29791 4490 1360 98 Nb [1011] 4.2 1.8 1.0 0.7 0.46 1.4 ex [nm] 29 0.14 3.3 0.94 2 ey [pm] 250 60 1 b*x [m] 1.2 0.5 b*y [mm] 50 s*y [nm] 3500 32 84 44 sz,SR [mm] 11.5 1.64 2.7 1.01 0.81 1.16 sz,tot [mm] (w beamstr.) 2.56 5.9 1.49 1.17 hourglass factor Fhg 0.99 0.64 0.79 0.80 0.73 L/IP [1034 cm-2s-1] 0.01 28 212 12 6 1.7 tbeam [min] 434 298 39 73 21 F. Zimmermann, HF2014 10% HOM power of CEPC 20 times HOM power of CEPC. ~100 kW per cavity if 800 MHz cavity! 50MHz 4MHz 28.8 nC 7 nC

Thin Film - Nb3Sn for CEPC? S. Posen & M. Liepe, PRSTAB 17.112001 (2014)

Summary and Outlook http://cepc.ihep.ac.cn CEPC SRF system: unique challenge and opportunity for China & world SRF accelerator community. Need strong international collaboration (TTC) to succeed on schedule. Three main challenges, especially the HOM power issue Huge HOM power extraction with low heat load (efficient, low cost) Very high power CW coupler (robust, clean assembly and low heat load) Cavity with very high Q0 at 15-20 MV/m (use state-of-the-art technology) SRF R&D planned for extensive development of key technology, personnel, infrastructure and industrialization. Cooperation or synergy with other projects (R&D and industrialization): CADS & PIP-II, Euro-XFEL, LCLS-II, ERLs & ILC, FCC-ee http://cepc.ihep.ac.cn

秦皇島 Qinghuangdao

Back ups

Yifang Wang, IHEP

Yifang Wang, IHEP

Yifang Wang, IHEP

CEPC SRF Injector Linac Parameters Unit Value RF voltage GV 6 RF frequency GHz 1.3 Bunch charge for CEPC nC 3.2 Bunch repetition rate for CEPC Hz 100 Bunch charge for XFEL pC Bunch repetition rate for XFEL MHz 1 Aver. beam current for XFEL mA 0.1 Cavity CW gradient MV/m 20 Cavity effective length (9-cell) m 1.038 Number of cavities 289 Cavities in one cryomodule 8 Cryomodule length 12.2 Number of cryomodules 36 SRF Linac length 500 Parameter Unit Value R/Q Ω 1036 Geometry factor 270 Iris diameter mm 70 Operating temperature K 2 Cavity quality factor 2×1010 4K equiv cryogenic installed kW 40 Total cryogenic AC power MW 8.5 External Q of input coupler 2E+07 RF power per cavity 6.4 Solid State Amplifier power 8 Number of SSA 289 Total RF source power 2.3 Total RF AC power 4.6 RF&Cryo AC power 13.1 Alternative to normal conducting linac. Simultaneous XFEL user facility?

CEPC Cryomodule Layout Euro-XFEL/ILC/LCLS-II type X Enlarge two- phase pipe and chimney. Omit 5K shield while keep 5K intercept for power coupler. As simple as possible. scaled from 1.3 GHz cryomodule X

Cavity Design Parameters Unit Main Ring Booster Cavity frequency MHz 650 1300 Number of cells - 5 9 Cavity effective length m 1.154 1.038 Cavity iris diameter mm 156 70 Beam tube diameter 170 78 Cell-to-cell coupling 3 % 1.87 % R/Q Ω 514 1036 Geometry factor 268 270 Epeak/Eacc 2.4 2 Bpeak/Eacc mT/(MV/m) 4.23 4.26 Cavity longitudinal loss factor* k∥ HOM V/pC 1.8 3.34 Cavity transverse loss factor* k⊥ V/pC/m 35.3 Qualified gradient MV/m 20 23.5 Qualified Q0 at qualified gradient 4E10 2E10 * main ring bunch length 2.65 mm, booster bunch length 2.66 mm

FNAL 650 MHz Single Cell Cavity Melnychuk, IPAC2014 CEPC new PIP-II (Project X) CEPC old Bp/Eacc=3.75 CEPC cavity with large iris Bp/Eacc = 4.2

Booster Cavity Bandwidth Qopt = 4E7 (BW 33 Hz, LCLS-II), df = 10 Hz, 10 kW input power, SSA 12.5 kW, hard for transient LLRF control. LLRF control difference between booster and SRF linac with low beam loading? Baseline: over coupled (Qext =1E7, df = 50 Hz ), 20 kW input power, SSA 25 kW