CEPC SCRF System Design and RD Jiyuan Zhai

CEPC SCRF System Design and R&D Jiyuan Zhai (IHEP) On behalf of CEPC SRF Team 2020 International Workshop on the High Energy Circular Electron Positron Collider Shanghai, October 26, 2020

Outline 1. CEPC SRF system design update 2. CEPC SRF R&D progress 3. SRF infrastructure status 2

CEPC SRF System TDR Status and Plan TDR Phase 1: 2019 -2020 (Components Prototyping) ü ü SRF system TDR design and optimization (RF staging and bypass scheme proposed) High Q, high gradient cavity, high power components and other key technology R&D (close to CEPC spec) 650 MHz high Q short cryomodule prototyping (assembly in Dec 2020) PAPS SRF facility construction (equipment installation) TDR Phase 2: 2021 -2022 (Cryomodule Prototyping) p p p SRF system TDR design re-baseline and optimization 650 MHz high Q short cryomodule operation and improvement (to meet CEPC spec) 1. 3 GHz high Q full cryomodule prototyping (to meet CEPC spec) 650 MHz high current cryomodule design (some tech verification in BEPC 3) PAPS SRF facility commissioning, operation and upgrade Industrialization in synergy with other SRF projects in China Post-TDR: 2023 -2025 (Mass-Production Preparation) • 650 MHz full cryomodule prototyping, engineering design, mass-production technology preparation 3

CEPC TDR SRF System Design • TDR design of CEPC SRF system is aiming to fulfill requirements of improvements over CDR: − higher luminosity at H (5. 2 x 1034 vs 2. 9 x 1034) → RF basically no change − “high-lumi” for Z (102 x 1034 vs 32 x 1034) → RF staging and bypass scheme − assuring compatibility for top-pair production → reserved tunnel space − capability to handle 50 MW SR power → reserved tunnel space, RF staging and bypass scheme • New RF layout and parameters optimization at each energy with the new schemes is ongoing. SRF layout, configuration, parameters, specifications and cost will be upgraded and rebaselined accordingly. 4

CEPC CDR SRF Layout and Parameters H Collider Ring W Z 650 MHz 2 -cell cavity Lumi. / IP (1034 cm-2 s-1) 2. 93 10. 1 16. 6 / 32. 1 RF voltage (GV) 2. 17 0. 47 0. 1 17. 4 x 2 87. 7 460 SR power / beam (MW) 30 30 16. 5 Cavity number 240 108 x 2 60 x 2 Beam current (m. A) Q 0 at max gradient 2 K cavity wall loss (k. W) 4 E 10 @ 22 MV/m (vertical test) 1. 5 E 10 @ 20 MV/m (long term operation) 6. 1 Booster Ring (extraction) 1. 3 0. 1 1. 3 GHz 9 -cell cavity RF voltage (GV) 1. 97 0. 585 0. 287 Beam current (m. A) peak 0. 52 2. 63 6. 91 96 64 32 Cavity number Q 0 at max gradient 3 E 10 @ 24 MV/m (vertical test) 1 E 10 @ 20 MV/m (long term operation) 5

New RF Staging & By-pass Scheme for CEPC Stage 1 (H/W run for 8 years): Keep CDR RF layout for H(HL-H)/W and 50 MW upgrade. Common cavities for H. Separate cavities for W/Z. Z initial operation for energy calibration and could reach CDR luminosity. Minimize first phase construction cost and hold Higgs priority. Stage 2 (HL-Z upgrade): Move Higgs cavities to center and add high current Z cavities. By-pass low current H cavities. International sharing (modules and RF sources): Collider + 130 MV 650 MHz high current cryomodules. Stage 3 (ttbar upgrade): add ttbar cavities (international sharing): Collider + 7 GV 650 MHz 5 -cell cavity, Booster + 6 GV 1. 3 GHz 9 -cell cavity. Both low current, high gradient, high Q. Nb 3 Sn@4. 2 K or others. • • • Unleash full potential of CEPC with operational flexibility. Seamless mode switching with unrestricted RF performance at each energy until AC power limit. Stepwise construction cost, technology risk and international involvement. 6

CEPC SRF Parameter Comparison BEPCII 500 MHz 4. 2 K BEPC 3 500 MHz 4. 2 K CEPC CDR H 30 MW 3 E 34 CEPC CDR Z 16. 5 MW 32 E 34 CEPC 1 -cell H 30 MW 3 E 34 CEPC TDR Z 30 MW 100 E 34 CEPC TDR H 30 MW 3 E 34 CEPC TDR W 30 MW 10 E 34 CEPC Ultimate Z 50 MW 167 E 34 400 (600) 900 2 x 17. 4 460 2 x 17. 4 838 2 x 17. 4 2 x 87. 7 1400 Cell number 1 1 2/1 1 Cavity number / ring 1 2 2 x 120 60 2 x(90+60) 60 6 (1. 5 MV) 10 (2. 5 MV) 19. 7 3. 6 40 9. 4 19. 7 4. 2 9. 4 1 E 9 1. 5 E 10 3 E 10 1. 5 E 10 6. 1 0. 1 6. 1 0. 35 6. 1 0. 27 0. 35 250 275 250 500 250/125 835 2/1 1 800 800 1200 Beam current (m. A) Eacc (MV/m) Q 0 @ 4. 2 K / 2 K Total wall loss (k. W) 2 Input power (k. W) 110 150 Cavity# / klystron 1 1 SSA 250 150 SSA 2 4 120 60+120 90+120 120 Absorber Hook+ Absorber 8* 20 0. 23 2. 4 0. 46 / 0. 23 1. 5 / 0. 75 4 Klystron power (k. W) Total KLY number HOM damper HOM power (k. W) 2 800 0. 6 800 1. 9 * Bunch length 15 mm, cavity cell HOM loss factor 0. 1 V/p. C, tapers 0. 06 V/p. C, absorbers 0. 26 V/p. C. 7

HL-Z High Current Cryomodule (130 MV) • Three KEKB / BEPCII or BEPC 3 type modules (12 m) in between two quadrupoles (16 m). 60 modules for each ring. Cavity and klystron number: balance gradient & input power vs. beam loading & cost. Parasitic loss power not trivial. • “One cavity, One module, One klystron” advantages: LLRF control for heavy beam loading, RF trip compensation (2 x 60 klystrons), maintenance. • Up to 1. 4 A beam current, 835 k. W ~ 1 MW input power / CAV (two couplers? ), 10~20 MV/m, 1. 5 E 10@2 K (contamination and magnetic field), 4 k. W HOM / CAV, 4~8 frev detuning (Reserves Phase Operation vs. total AC power) 8

Outline 1. CEPC SRF system design update 2. CEPC SRF R&D progress 3. SRF infrastructure status 9

CEPC SRF Hardware Specifications Hardware Qualification Normal Operation Max. Operation 650 MHz 2 -cell Cavity VT 4 E 10 @ 22 MV/m HT 2 E 10 @ 20 MV/m 1. 5 E 10 @ 20 MV/m 2 E 10 @ 20 MV/m 1. 3 GHz 9 -cell Cavity VT 3 E 10 @ 24 MV/m 1 E 10 @ 20 MV/m 2 E 10 @ 23 MV/m 650 MHz Input Coupler (variable) HPT 300 k. W sw < 280 k. W 300 k. W 1. 3 GHz Input Coupler (variable) HPT 20 k. W peak, 4 k. W avr. < 20 k. W peak 650 MHz HOM Coupler High power test at RT and 2 K: 1 k. W < 1 k. W 650 MHz HOM Absorber High power test at RT: 5 k. W < 5 k. W 650 MHz Cryomodule (six 2 -cell cavities) static loss 5 W @ 2 K static loss 8 W @ 2 K static loss 10 W @ 2 K Tuner (Collider & Booster) tuning range and resolution 400 k. Hz / 1 Hz 300 k. Hz / 1 Hz 400 k. Hz / 1 Hz 10

High Gradient 1. 3 GHz 1 -cell Cavity (EP) • Gradient increased rapidly (from BCP to EP, from initial EP to optimized EP). • All 12 single-cell cavities > 40 MV/m. Max 46 MV/m. 11

High Gradient 1. 3 GHz 9 -cell Cavity (EP) • • • High gradient is the basis for high Q because of large gradient degradation With new EP tool, only 5 months to achieve 36 MV/m on 9 -cell cavity All of the five 9 -cell cavities > 30 MV/m, cell gradient near or above 40 MV/m Two main FE sources: EP sulfur contamination, HPR contamination Further push the gradient to 35~40 MV/m with 90% yield 12

High Q 1. 3 GHz 1 -cell Cavity (Mid-T Annealing) S. Posen (FNAL) 2019: Mid-T bake (after HPR). Cavities tested without/with exposing to air. K. Umemori (KEK) 2020: Mid-T furnace bake (before HPR). Cavities tested after exposing to air. LCLS-II & SHINE spec. • High-T annealing + (light EP) + (baseline VT) + expose to air + HPR + 250 -400 C Mid-T annealing (3 hours, time not optimized yet) + HPR • All 13 single-cell cavities > 2. 7 E 10 @ 16 MV/m. 300 C has the highest Q: 4. 9 E 10@16 MV/m. S 6: 3. 5 E 10@30 MV/m. Most of the quench gradient > 30 MV/m. • Will solve the high-field Q-slope problem and reach high gradient with high Q. • Outside BCP sometimes helps to increase Q. Lower magnetic sensitivity than N-doped cavities. 13

High Q 9 -cell Cavity by Mid-T Annealing • World’s first mid-T high Q 9 -cell cavity. 4 E 10@16 MV/m. 3. 9 E 10@22. 7 MV/m quench. • Exceed LCLS-II and SHINE spec. Close to LCLS-II-HE (2. 7 E 10@23 MV/m) and CEPC (3 E 10@24 MV/m) spec. • Further push the gradient and yield by light cold EP between 900 C and 300 C. 14

High Q Mechanism & Mean Free Path Measurement Mid-T, N-dope: Surface Impurity Engineering in 40 nm 15

High Q N-infusion BCP 650 MHz 2 -cell Cavity • Nitrogen-infusion of the BCP cavity: 6 E 10@22 MV/m, exceeds CEPC spec (4 E 10@Eacc=22 MV/m) • World record in BCP cavity. EP to further improve the gradient and Q • 650 MHz cavity EP tooling in construction Courtesy of Jiankui Hao (PKU) 16

650 MHz 2 -cell Cavity Vertical Test with HOM Couplers BCP • BCP 650 MHz 2 -cell cavities: 2. 8 E 10@22 MV/m (CEPC VT spec: 4 E 10@22 MV/m) • Performance no change after install HOM coupler • EP/Mid-T to further improve gradient and Q 17

HOM Coupler RF Performance wrong rotation angle of copper part installation NPT: normal pressure & temperature Spec. 1 W at 20 MV/m Fundamental mode rejecting Qe Near or larger than 1× 1012 at 2 K. Test results before and after VT are repeatable if installed correctly. HOM damping Qe Fulfill Higgs CBI requirement. Need beam feedback for Z. Further damping optimization is under way. 18

650 MHz 2 -cell Cavity Dressing Cavities attached with flux gates and thermal sensors Helium vessel components Cavity local magnetic shield Vertical test 2 -cell cavities with helium vessel and HOM couplers soon. 19

Module Magnetic Shielding and Compensating North Because of beam direction and larger beam pipe than 1. 3 GHz, only two shieldings can reach the magnetic field requirement of high Q 650 MHz cavity: cavity (2 K local) shield and module (RT global) shield. 1. Flux trapping ratio: grain size, high-T annealing, fast cold down 2. Magnetic sensitivity: mean free path and other (thin film) 3. Remnant magnetic field: demagnetization, magnetic shield, magnetic compensation, thermocurrent Magnetic compensation with coils

1. 3 GHz Input Coupler • 1. 3 GHz variable input couplers high power tested to CW 14 k. W TW, 7 k. W SW. • Another six couplers under fabrication. • Ceramic window Ti. N sputtering and coupler copper plating (especially bellow) meet the specifications. 21

CEPC 650 MHz High Power Variable Coupler • 650 MHz variable couplers tested to CW TW 150 k. W (SSA power limit), SW 100 k. W (corresponding to 400 k. W TW power at the window, exceeds CEPC spec 300 k. W). One of the world highest variable couplers. • High power test with CEPC 800 k. W klystron soon (need high power circulator) Bellow on inner conductor. Inner conductor water cooling. Outer conductor He gas cooling. TW high power test to 150 k. W Window SW field and power Window SW field and temperature rise 22

CEPC 650 MHz Test Cryomodule • Cavity string and module assembly soon • Horizontal and beam test at PAPS in 2021 • Demonstrate 650 MHz cavity high Q operation 23

Outline 1. CEPC SRF system design update 2. CEPC SRF R&D progress 3. SRF infrastructure status 24

PAPS SRF Facility Status Clean room construction, equipment installation and commissioning now. Full operation in mid 2021 (CEPC, CSNS-upgrade, HEPS prototype modules, then 1. 3 GHz 8 x 9 -cell modules). 25

PAPS SRF Facility Status Vacuum furnace (doping & annealing) Temperature & X-ray mapping system Nb 3 Sn furnace Second sound cavity quench detection system Nb/Cu sputtering device Helmholtz coil for cavity vertical test Cavity inspection camera and grinder Vertical test dewars 9 -cell cavity pre-tuning machine Horizontal test cryostat 26

PAPS SRF Facility Status 500 MHz/100 k. W SSA Other equipment and devices LLRF for cavity & coupler testing HLRF design for cavity, coupler and module testing 27

CEPC SRF Collaboration • CEPC SRF domestic collaboration • Organization: CEPC SRF Collaboration kick-off soon. Members: IHEP, PKU and 5 companies. • Activities: Already have collaborations on CEPC SRF technology and various SRF projects. • Future collaborations: SHINE and other CW SRF FEL projects in China. • CEPC SRF international collaboration • Organization: TBD • Funding: apply for international team program to support personal exchanges. • Activities: ILC/Super. KEKB (KEK), INFN-LASA, JLAB: SRF system design, technology and infrastructure, annual SRF collaboration meeting with KEK, personal exchanges. • Future collaborations: FCC-ee (CERN, Univ. Rostock), EIC (JLAB, BNL), PIP-II and LCLS-IIHE (FNAL), DESY, CEA-Saclay etc. 28

Summary: CEPC SRF Design and R&D Xiangshan Conference CEPC Pre-CDR 100 km Double Ring CEPC TDR starts CEPC named (6. 14) Partial double ring Beam-cavity interaction EP commissioning, N-doping LL LG 9 -cell 20 MV/m 1. 3 GHz short CM PAPS SRF facility starts 1. 3 G 1 -cell 45 MV/m 2013 2014 2015 2016 2017 2018 2019 2020 50 km Single Ring CEPC MOST fund CEPC CDR RF staging and by-pass scheme CEP SRF system design PDR beam loading issue 650 M 2 -cell&5 -cell PAPS SRF facility near completion 1. 3 G short CM integration TESLA 9 -cell 24 MV/m 650 M test CM design EP 1. 3 G 9 -cell 36 MV/m Mid-T 1. 3 G 9 -cell 4 E 10@23 MV/m N-infusion 650 M 2 -cell 6 E 10@22 MV/m Input and HOM coupler prototypes 650 MHz test module to assemble 1. 3 GHz 8 x 9 -cell module to launch 29

Backup 30

CEPC CDR RF Section with ttbar Extension 31

N-doping of 1. 3 GHz 1 -cell cavity • The best result of N-doping reached 2. 6 E 10@30 MV/m for 1. 3 GHz 1 -cell cavity. • Two kinds of N-doping recipes is adopted: 800 C 3 h + 3/60@800 C, 3/60@800 C 2. 6 E 10@30 MV/m CEPC VT SPEC. LCLS II SPEC. 32