1. Peng Sha Institute of High Energy Physics, CAS
CEPC SRF CAVITY WORK OF 2016
Peng Sha
Institute of High Energy Physics, CAS
Institute of High Energy Physics, Chinese Academy of Sciences

2. Institute of High Energy Physics, Chinese Academy of Sciences
Outline
High-performance Research
Other Work
Institute of High Energy Physics, Chinese Academy of Sciences

3. Institute of High Energy Physics, Chinese Academy of Sciences
Introduction
Till now, most SRF cavities used at accelerators are made from bulk niobium, except copper coating with niobium film (LHC/CERN).
At present: bulk Niobium + ILC/XFEL standard recipe of post processing.
Request in future: high Q and gradient, low cost (construction and operation), high reliability et al.
N-doping, et al.
Niobium cavity coating with Nb3Sn, et al.
Institute of High Energy Physics, Chinese Academy of Sciences 3

4. Institute of High Energy Physics, Chinese Academy of Sciences
Advantage of N-doping
N-doping was discovered in 2012 at FNAL.
It can increase Q by one time for 1.3 GHz 9-cell cavity, then reduce the cavity power by 50%.
Till now, there have been over 100 vertical tests of cavity N-doped successfully at FNAL, JLAB and Cornell, et al.
It has been adopted by LCLS-II.
Institute of High Energy Physics, Chinese Academy of Sciences 4

5. Institute of High Energy Physics, Chinese Academy of Sciences
Advantage of Nb3Sn
Niobium cavity coating with Nb3Sn can work at 4.2K, which save much cost than 2K.
Nb3Sn is predicted to have 2x magnetic field limit of niobium
Institute of High Energy Physics, Chinese Academy of Sciences 5

6. 2/6 Recipe of N-doping
The experiments of N-doping began with 1.3 GHz cavities firstly. And the recipe have been optimized.
Finally, the 2/6 recipe of N-doping is adopted by 1.3GHz 9-cell cavities of LCLS-II, which is transferred to industrial vendors (RI and Zanon).
2/6 recipe adopted by LCLS-II
Step
Temperature (℃), Pressure (Pa)
Duration (min)
Hydrogen degassing
800 ± 10, 0
180 ± 5
N-doping
800 ± 10, 3.5 ± 10
2 ± 0.1
Vacuum annealing
6 ± 0.1
Institute of High Energy Physics, Chinese Academy of Sciences 6

7. Principle research of N-doping
N-doping of niobium can create a niobium nitride layer, which is about 2-micron deep and harmful to Q value. So this layer is removed by 5-micron electro polishing.
Then, the diffraction pattern of transmitted electron microscope shows only clean niobium phase without nitride. It indicates that the interstitial nitrogen contributes to the increase of Q value.
Institute of High Energy Physics, Chinese Academy of Sciences 7

8. Vertical test results of 1.3 GHz 9-cell cavities
All cavities meet the design target of at 2K. Average Q is at 2K.
Institute of High Energy Physics, Chinese Academy of Sciences 8

9. Preliminary and latest results for 1st LCLS-II cryomodule at FNAL
Institute of High Energy Physics, Chinese Academy of Sciences 9

10. N-doping: decrease of surface resistance and quench field
N-doping has been proven to prevent Q-slope at medium accelerating field for superconducting cavities, which is found to reduce the BCS surface resistance compared to ILC/XFEL standard by 50%.
R 𝑚𝑎𝑔 increases, more sensitive to ambient magnetic field (1 nOhm/mGs), while 0.35 with out N-doping. So magnetic shielding is important: 10mGs ~3mGs.
𝑅 𝑆 = 𝑅 𝐵𝐶𝑆 + 𝑅 𝑚𝑎𝑔 + 𝑅 𝑒𝑡𝑐
Institute of High Energy Physics, Chinese Academy of Sciences 10

11. CEPC cavity requirements
Qualification
Normal Operation
Max. Operation
650 MHz 2-cell Cavity
VT 22 MV/m
HT 20 MV/m
MV/m
20 MV/m
1.3 GHz 9-cell Cavity
VT 25 MV/m
23 MV/m
Very high Q, N-doping or other technique is necessary!
Institute of High Energy Physics, Chinese Academy of Sciences 11

12. N-doping experiments of Nb sample
Secondary ion mass spectrometry (SIMS) of niobium samples
Niobium samples for N-doping in the furnace
Institute of High Energy Physics, Chinese Academy of Sciences 12

13. SIMS result before N-doping
Institute of High Energy Physics, Chinese Academy of Sciences 13

14. SIMS result after N-doping
Institute of High Energy Physics, Chinese Academy of Sciences 14

15. Vertical test of 1.3GHz cavity N-doped
There’ve been 3 vertical tests during 2016 (very limited).
The furnace for N-doping is equipped with diffusion pump, which is dirty. Cryopump is more clean, which is adopted by FNAL, JLAB, et al.
After N-doping, following was Buffer Chemical Polishing, because we have no Electro Polishing facility.
Institute of High Energy Physics, Chinese Academy of Sciences 15

16. Institute of High Energy Physics, Chinese Academy of Sciences
Outline
High-performance Research
Other Work Related
Institute of High Energy Physics, Chinese Academy of Sciences

17. Design of 650MHz 2-cell Cavity
Position and dimensions of main coupler, HOM coupler and pickup have been determined. Mechanical analysis is undergoing.
Parameter
Unit
Main Ring
Number of cells - 2 5
R/Q Ω
213
514
Geometry factor
284
268
Epeak/Eacc
2.4
Bpeak/Eacc
mT/(MV/m)
4.2
Institute of High Energy Physics, Chinese Academy of Sciences 17

18. Institute of High Energy Physics, Chinese Academy of Sciences
Electro-Polishing
Design finished and under bidding now.
Institute of High Energy Physics, Chinese Academy of Sciences 18

19. Management of CEPC SRF System
Manage our program and group with Jiyuan together.
Contact man for MOST Program, lots of document work.
Prepare for Huairou Platform.
Visit FNAL for N-doping collaboration with Zhenghui Mi.
Attending 2 CEPC-SppC Workshop.
Institute of High Energy Physics, Chinese Academy of Sciences 19

20. Institute of High Energy Physics, Chinese Academy of Sciences
Thanks for your attention！
Institute of High Energy Physics, Chinese Academy of Sciences