Characteristics of vacuum breakdowns under high gradient pulsed operation

We have been studying the feasibility of electron-positron linear collider using normal conducting X-band high gradient acceleration in its mail linac. CERN compact linear collider, CLIC, has been aiming at a 100 MV/m acceleration with 2-beam acceleration scheme. In collaboration with CLIC, we have been studying the high gradient characteristics on the CLIC prototype accelerator structures. We proved that a CLIC damped structure prototype could be operated at such level with meeting the required breakdown rate for CLIC. Through the study on four types of prototype structures in the breakdown rate and associated damage to the accelerator structure, we have started studying the basic mechanism of the vacuum breakdowns under high gradient. We found that the breakdown rate keeps decreasing exponentially over several thousand hours of operation. The breakdowns show the statistically random appearance and governed by the parameters of the pulse. Some of these basic natures are described in this