New Cavity Techniques and Future Prospects Peng Sha (shapeng@ihep.ac.cn) Institute of High Energy Physics, CAS 26th, Oct, 2016 eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences

Institute of High Energy Physics, Chinese Academy of Sciences Outline Introduction Research and development of N-doping N-doping at IHEP Research of Nb3Sn Summary eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences

Institute of High Energy Physics, Chinese Academy of Sciences Introduction In the recent decades, Superconducting cavities have been widely used to accelerate electron, proton, and ions. 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. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 3

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. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 4

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 eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 5

Institute of High Energy Physics, Chinese Academy of Sciences Outline Introduction Research and development of N-doping N-doping at IHEP Research of Nb3Sn Summary eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences

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 eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 7

Vertical test results of 1.3 GHz 9-cell cavities All cavities meet the design target of 2.7e10@16MV/m at 2K. Average Q is 3.5e10@16MV/m at 2K. But the average quench field is reduced to 23 MV/m, while over 35 MV/m without N-doping. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 8

Vertical test results of 650 MHz cavities The same recipe is adopted by 650 MHz cavities for PIP-II at FNAL, too. The Q value increases by 100% and reaches 7e10@17MV/m at 2K, which is the world record at this frequency. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 9

Recipe of post processing (LCLS-II and E-XFEL) eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 10

N-doping: successful transfer to industry for LCLS-II eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 11

Preliminary and latest results for 1st LCLS-II cryomodule at FNAL eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 12

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. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 13

Institute of High Energy Physics, Chinese Academy of Sciences eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 14

Institute of High Energy Physics, Chinese Academy of Sciences eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 15

Institute of High Energy Physics, Chinese Academy of Sciences eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 16

Institute of High Energy Physics, Chinese Academy of Sciences eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 17

Institute of High Energy Physics, Chinese Academy of Sciences NbxNy ~2 𝜇𝑚 eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 18

Only clean Nb lattice (no nitride phases). N must be interstitial! eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 19

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. It also causes the decrease of cavity quench field: from 35MV/m to 23 MV/m for 1.3 GHz 9-cell cavities, which need deeper research. 𝑅 𝑆 = 𝑅 𝐵𝐶𝑆 + 𝑅 𝑚𝑎𝑔 + 𝑅 𝑒𝑡𝑐 eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 20

Low Temperature N-doping In order to achieve both high Q and high field, low temperature (120C-160C) N-doping is being studied right now at FNAL, which realizes very high Q at both medium and high field. The quench field limitation ever reached 45 MV/m for 1.3 GHz 9-cell cavities. The actual regime of N-doping is still being studied. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 21

Institute of High Energy Physics, Chinese Academy of Sciences Outline Introduction Research and development of N-doping N-doping at IHEP Research of Nb3Sn Summary eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences

CEPC cavity requirements Qualification Normal Operation Max. Operation 650 MHz 2-cell Cavity VT 4E10 @ 22 MV/m HT 2E10 @ 20 MV/m 2E10 @16.5 MV/m 2E10 @ 20 MV/m 1.3 GHz 9-cell Cavity VT 3E10 @ 25 MV/m 2E10 @ 23 MV/m Very high Q, N-doping or other technique is necessary! eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 23

N-doping experiments of Nb sample Secondary ion mass spectrometry (SIMS) of niobium samples Niobium samples for N-doping in the furnace eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 24

SIMS result before N-doping eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 25

SIMS result after N-doping eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 26

Vertical test of 1.3GHz cavity N-doped 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. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 27

Institute of High Energy Physics, Chinese Academy of Sciences Outline Introduction Research and development of N-doping N-doping at IHEP Research of Nb3Sn Summary eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences

Vertical tests of Nb3Sn cavities Cornell has completed several vertical tests of Nb3Sn cavities. Nb3Sn has shown great potential for operation at medium gradients at 4.2 K. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 29

Institute of High Energy Physics, Chinese Academy of Sciences NB3Sn in future It still requires significant R&D for massive application. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 30

Institute of High Energy Physics, Chinese Academy of Sciences Summary There’ve been many new superconducting cavity technologies for the large accelerators under planning, while N-doping is developing very fast. It has been successfully applied in the LCLS-II project. For N-doping, it’s necessary to increase the accelerating field and quench field limitation. Besides, niobium cavity coating with Nb3Sn is also very promising. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences 31

Institute of High Energy Physics, Chinese Academy of Sciences Thanks for your attention！ Acknowledgments: FNAL, JLAB, Cornell, TRIUMF, CERN, DESY, Peking University, ANL, et al. eeFACT2016 Institute of High Energy Physics, Chinese Academy of Sciences