1. Peng SHA, 20180525 On behalf of CEPC SRF group
CEPC SRF Cavity
Peng SHA,
On behalf of CEPC SRF group
IHEP, CAS

2. Introduction Outline Development of 650 MHz cavity
N-doping/N-infusion research
Test cryomodule of CEPC 650 MHz SRF
Other work
Next
IHEP, CAS

3. SRF cavities for CEPC 650 MHz 2-cell cavity 240 19.7 1.5E10 22 2E10
Cavity amount
Gradient (MV/m)
Q for long period
Gradient in horizontal test (MV/m)
Q in horizontal test
Gradient in vertical test (MV/m)
Q in vertical test
650 MHz 2-cell cavity
240
19.7
1.5E10 22
2E10
4E10
1.3 GHz 9-cell cavity 96
19.8
1E10 24
3E10
1.3 GHz Cryomodule (8 cavities inside, as E-XFEL, LCLS-II, ILC, SCLF) 3
650 MHz Cryomodule (6 cavities inside, new!)

4. Progress of CEPC SRF cavity study
Vertical test result of 650 MHz 2-cell cavity: which is close to the CEPC target
N-doping research has gained progress. Q0 increased obviously for 650 MHz 1-cell cavities after N-doping.
Design of the test cryomodule is nearly finished, which consist of two 650 MHz 2-cell cavities and other components (main coupler, HOM coupler and absorber, tuner…).
The fabrication of a new N-doping/N-infusion furnace has completed over 50%. Design of Nb3Sn coating equipment has been finished. Main components of electropolishing (EP) finished factory acceptance tests.
IHEP, CAS 4

5. Development of 650MHz cavity
Outline
Introduction
Development of 650MHz cavity
N-doping/N-infusion research
Test cryomodule of CEPC 650MHz SRF
Other work
Next
IHEP, CAS

6. RF design of 650 MHz 2-cell cavity (1)
Cavity type
CEPC-V1
CEPC-V2
Parameters
Mid & end half cell
Mid half cell
End half cell
Riris (mm) 78 75 80
Alpha (deg)
3.22 8
15.3
A (mm)
94.4 93
B (mm)
a (mm)
20.123 20
b (mm)
22.09 30
D (mm)
204.95
412.5
R/Q (Ω)
213
211
G (Ω)
284
279
Ep/Eacc
2.4
Bp/Eacc
4.2
IHEP, CAS 6

7. RF design of 650 MHz 2-cell cavity (2)
Cavity type
CEPC-V1
CEPC-V2
Design goals
Ep/Eacc: 2~2.4
Bp/Eacc: 3.8~4.2
Same half cells design
α：8~10°
Trap modes: TM012 & TE121
Advantages
Four same half cells, save dies, economic
Different designs for mid half cell and end half cell, improve the trap modes
Easy to extend to multi-cell cavity
Large side wall angle is better for surface processing and cavity mechanics.
Disadvantages
Trap modes
Hard to extend to multi-cell cavity
Two different end half cell design is better for trap modes
IHEP, CAS 7

8. Comparison IHEP, CAS f (MHz) R/Q (Ω) Ep/Eacc Bp/Eacc Ri/Rt (mm)
CEPC-V2
(2-cell)
650
211
2.4
4.2
75/80
CEPC-V1
213
78/78
FCC
(4-cell)
435
2.2
4.1
74/80.2
TESLA
(9-cell)
1036
2.0
4.3
70/78
* All geometrical parameters are scaled to 650 MHz.
IHEP, CAS 8

9. Vertical test (11 May, 2018) IHEP, CAS
Vertical test result: is close to the CEPC target
IHEP, CAS 9

10. N-doping/N-infusion research
Outline
Introduction
Development of 650MHz cavity
N-doping/N-infusion research
Test cryomodule of CEPC 650MHz SRF
Other work
Next
IHEP, CAS

11. Surface resistance (Rs) of SRF cavity
Depend on smoothness, cleanliness and flux trapping, etc
If T<TC/2, then Δ(T) ≈ Δ(0)
A is a constant related with London penetration depth, coherent length, Fermi velocity, mean free path…
Δ(T) is the energy gap.
IHEP, CAS 11

12. N-doping and N-infusion: reduce RBCS, increase Q!
High Q (LCLS-II,
SCLF, CEPC)
N-doping
High Q and gradient (ILC)
CEPC Goal
N-doping
N-infusion
N-doping for PIP-II 650 MHz single-cell cavity
A. Grassellino, ICFA seminar 2017
IHEP, CAS 12

13. N-doping at OTIC and KEK
N-doping at KEK
N-doping at Ningxia Orient Tantalum Industry Co., Ltd (OTIC)
OTIC: The furnace heater is made by Tantalum, not molybdenum. N-infusion isn’t allowed, because N2 injection at 120C for 48 hours may harm the heater.
KEK: The furnace can be used for N-doping and N-infusion.
IHEP, CAS 13

14. SIMS Results of N-doping at OTIC and KEK
No N, NbN
N increase!
NbN
NbN
H decrease
H decrease
N increase!
Before N-doping
N-doping at OTIC
(~3 Pa, 800℃ for 2 minutes)
N-doping at KEK
(~3 Pa, 800℃ for 2 minutes)
IHEP, CAS 14

15. Results of SEM, R-T SEM 9.05K: TC of pure Nb 9.86K: TC of Nb N-doped
846nm
9.86K is between Nb (9.23K) and NbN (16.2K), a mixture of Nb and NbN.
After N-doping, clear formation of NbN precipitates in the surface.
IHEP, CAS 15

16. SIMS Results of N-infusion at KEK (1)
Nb sample with Buffered
Chemical Polishing (BCP)
SIMS Results of N-infusion at KEK (1)
Nb sample with
Electropolishing (EP)
H decrease
N, NbN
H decrease
N, NbN
BCP sample
before N-doping
BCP sample after N-infusion （~3 Pa, 125℃ for 48 hours ）
BCP sample after N-infusion （ ~3 Pa, 160℃ for 48 hours ）
IHEP, CAS 16

17. SIMS Results of N-infusion at KEK (2)
H decrease
H decrease
N, NbN
N, NbN
Intensity of N, NbN
EP sample after N-infusion （~3 Pa, 125℃ for 48 hours ）
EP sample after N-infusion （ ~3 Pa, 160℃ for 48 hours ）
EP sample before N-doping
There’re fewer N/NbN of N-infusion than N-doping.
IHEP, CAS 17

18. N-doping process of 650 MHz 1-cell cavities
1st, the baseline vertical test of 650 MHz single-cell cavity before N-doping.
2nd , this cavity received light BCP and was N-doped.
3rd, the cavity received High Pressure Rinsing (HPR) , following with class -10 assembly for vertical test.
4th, the cavity received vertical test again.
N-doping
HPR
Assembly
Vertical test
IHEP, CAS 18

19. Vacuum and temperature curve during N-doping
~3 Pa for 2 minutes
IHEP, CAS 19

20. Vertical test results IHEP, CAS
After N-doping, Q0 increased obviously at low field for both cavities.
650S1: But Q0 decreased quickly at high field (>10 MV/m) because of no BCP/EP after N-doping.
650S2: Quench at
CEPC Target
IHEP, CAS 20

21. Test cryomodule of CEPC 650MHz SRF
Outline
Introduction
Development of 650MHz cavity
N-doping/N-infusion research
Test cryomodule of CEPC 650MHz SRF
Other work
Next
IHEP, CAS

22. Test cryomodule IHEP, CAS
Design of the test cryomodule is underway, which consist of two 650MHz 2-cell cavities and other components (main coupler, HOM coupler and absorber, tuner, Helium Vessel, etc).
The assembly will be finished in the middle of 2019.
IHEP, CAS 22

23. HOM Coupler for 650MHz 2-cell cavity
Result measured is almost the same as simulated. Error is introduced during machining and assembly.
IHEP, CAS 23

24. Helium vessel, main coupler, HOM absorber
Helium vessel design is being optimized.
HOM absorber is under fabrication.
Main coupler finished design and began fabrication.
IHEP, CAS 24

25. Other work Outline Introduction Development of 650MHz cavity
N-doping/N-infusion research
Test cryomodule of CEPC 650MHz SRF
Other work
Next
IHEP, CAS

26. N-doping/N-infusion furnace、EP
The fabrication has completed over 50%. The furnace will be transported to IHEP in middle of 2018.
Factory acceptance test of mechanical part for EP.
IHEP, CAS 26

27. Nb3Sn film
Nb3Sn: high Tc, much higher Q than Nb at 4.2K, higher gradient (120MV/m in theory) than Nb (50 MV/m). Nb cavity coating with Nb3Sn film is promising in future.
Vertical test results of Cornell
Martina Martinello, FCC week 2018.
~30 times!
IHEP, CAS 27

28. Equipment for Nb3Sn coating
Parameters
Value
Min vacuum
5.0E-5 Pa
Max temperature
1400℃
Min
9.0E-4 Pa
Uniform temperature area
Φ300mm×500mm
Nb chamber
Main heater
(molybdenum-lanthanum alloy)
Inner vacuum
Heater (tungsten) Sn
Nb cavity
Outer vacuum
Design has been completed (bid opening on 30th May).
It will be transported to IHEP in the end of 2018.
IHEP, CAS

29. Frontier research of Fe-pnictides
Fe-pnictides: high Tc, much higher gradient (200 MV/m in theory) than Nb (50 MV/m).
Key: coating with curved surface.
RS Measurement Dewar will be completed in June.
Theoretical Model (by Alex Gurevich)
First step (simplified)
Nb（9.2K）+ FeSe（11K)
FeSe
LiF Nb
IHEP, CAS 29

30. Next Outline Introduction Development of 650MHz cavity
N-doping/N-infusion research
Test cryomodule of CEPC 650MHz SRF
Other work
Next
IHEP, CAS

31. Upcoming IHEP, CAS June ~ July，vertical tests of over 10 cavities.
1st week: 650 MHz 2-cell (baseline), 1.3 GHz 1-cell (baseline)
2nd week: 650 MHz 5-cell (baseline)
3rd week: 650 MHz 2-cell (baseline), 1.3 GHz 1-cell (baseline)
4th week: 650 MHz 1-cell (N-doping), 650 MHz 1-cell (N-doping)
5th week: 650 MHz 1-cell (baseline)，650 MHz 1-cell (Japan Nb + baseline)
6th week: 650 MHz 1-cell (Japan Nb + baseline), 650 MHz 1-cell (Japan Nb + N-doping)
7th week: 3.9 GHz 1-cell (baseline), 3.9 GHz 1-cell (N-doping), 3.9 GHz 1-cell (LG + N-doping)
Higher frequency, better N-doping performance.
Two 1.3 GHz 1-cell cavities would be transported to KEK for EP, N-doping and test in August.
Martina Martinello, FCC week 2018.
IHEP, CAS 31

32. Goal within 1 year
Research on N-doping/N-infusion, increase the Q value (650 MHz, 1.3 GHz) by N-doping/N-infusion and EP further.
Complete the development of test cryomodule and begin experiments.
Complete eight 1.3 GHz 9-cell cavities for SCLF, target: Besides, make 8 couplers for these cavities, target: 14kW (traveling wave).
IHEP, CAS 32

33. 3~5 years
2021: Exceed CEPC target (650MHz 2-cell: 1.3GHz 9-cell: N-doping/N-infusion technology to HE-RACING TECHNOLOGY. Meanwhile: 600 cavities for SCLF start batch production.
2023: Master Nb3Sn coating technology. Complete coating with 1.3 GHz cavity. Goal: (at 4.2K).
2023: Preliminarily master key technology of Fe-pnictides coating with curved surface.
IHEP, CAS 33

34. Acknowledgements: PKU and KEK. Thanks for your attention!
IHEP, CAS