High-gradient tests on S-band 2m-long accelerating structures for KEKB injector linac

In this paper, we describe high-gradient test results performed on S-band 2m-long accelerating structures for KEKB injector linac. High-gradient tests were performed on three types of accelerating structures: (1) a conventional accelerating structure for KEKB injector linac, (2) an accelerating structure whose input and output couplers were replaced by ones without a crescent-shaped cut, and (3) a conventional accelerating structure which was rinsed with high-pressure ultrapure water. As a result of the tests, an average accelerating gradient of more than 40 MV/m was obtained in all structures. Especially, 45 MV/m was achieved in a structure rinsed with high-pressure ultrapure water with a short rf processing time and a low breakdown rate (this gradient was limited by the klystron output power). Black patterns were observed at a crescent-shaped cut and the iris of the coupler cells. The simulated surface current densities were higher by about 10 times than that of the other surfaces of the cells. The measured momentum spectra were reproduced qualitatively by a simulation. It was found that the breakdown rate decreased significantly after conditioning.