SRF Cavity
 
  • Introduction

Dedicated to investigate the possibility to fabricate 1.3 GHz superconducting RF cavities from large grain niobium and to achieve high gradient, SRF cavities made from large grain niobium has been fabricated and a series of vertically tests have been carried out.
      In 2006 , the Chinese Ning Xia large grain cavity research for ILC started. The cavities will be welded in March 2007 in Beijing. A centrifugal barrel polishing (CBP) machine is manufactured and mounted in the RF Superconducting Laboratory at IHEP. The vertical test will begin this summer at IHEP. At the same time, IHEP also works in collaboration with KEK on three Chinese large-grain niobium cavities. Xu Qingjin, Zong Zhanguo, Zhai Jiyuan and Ge Mingqi from IHEP successively worked for this research with Kenji Saito's group. The results of the single cavity are exciting and the highest accelerating gradient achieved in vertical test is 47.92 MV / m with a high Q0 of 1.00E+10.

CBP machine
CBP machine
 

cavity
China Large Grain Cavities

 
  • Cavity Fabrication

Half-cells are produced by one-step deep drawing. To prepare the deep drawing, subsequent to an annealing, a 10 um surface layer was removed by chemical etching. As the grain sizes of the large grain sheets were not uniform, tearing at the iris, and strong earing and grain steps at equator region occurred. After deep drawing, the half-cells were trimmed to the final size for electron-beam welding. To avoid making a hole or non-fusion caking at the region of high surface current, 40% welding from the inside (equator RF-side) was adopted firstly and then from outside with 10% overlap. Then the beam pipe/flange assembly made from fine grain material was electron beam welded.

 
  • Surface Treatment

The cavities both were subjected to the same surface treatments which consisted of centrifugal barrel polishing (CBP), light chemical polishing (CP), annealing, electro-polishing (EP), high pressure rinsing (HPR) and baking.

treatment

 
  • Results

Although the nonuniform characteristics of mechanics brought some complexities to the fabrication, such as deep-drawing and electron beam welding, the cavity manufacture was completed successfully. The standard surface treatments which is effective for polycrystalline material is still suitable for the large grain material. When the two cavities were removed 90 micron by CBP, 10 micron by CP and more than 80 micron by EP, the performance both reached the high gradient of above 43 MV/m and the maximum was 47.9 MV/m with a quality factor of 1.00E+10. The table below summarizes the results of the tests of the two large grain cavities.

V.T.

EP+Baking+HPR

Eacc,max
(MV/m)

Rres(nohm)

Limitation

1st

EP(30+3)+Baking(12hrs)

24.02

6.08

Field Emission

2nd

EP(30+3)+Baking(120*48hrs)

22

3.15

Field Emission

3rd

EP(30+3)+Baking(130C*48hrs)

40.76

5.79

Quench

4th

HPR+Flashing(3 hrs)

47.9

3.05

Quench

1st

EP(80+3)+Baking(12hrs)

43.8

1.95

Quench

2nd

Additional Baking(12 hrs)

43.2

2.65

Quench

 

Z.G. Zong, IHEP & TIPC