Inspection of inner surface of S-band accelerating structures of KEK electron-positron injector linac used for more than 30 years

The electron-positron injector linac of KEK has been operated for more than 30 years. It originally started with serving the 2.5 GeV photon factory ring and other missions were added requiring higher energy and higher charge. At present, it has been developing to inject to the SuperKEKB rings with more bunch charge (4 nC) and with much lower emittance (order of 20μm). However recently, the accelerating structures suffer from water leakage toward outside or sometimes into vacuum. They also show high field emission and frequent vacuum breakdowns. It is one of the urgent issues from now to identify such deteriorated accelerating structures and replace by good-behaving ones. In order to address to find the origin of such deteriorations and to search for the possible remedies, we have recently inspected some of the deteriorated accelerating structures. In this paper, we present the inspection results and discuss the weakness in design features and speculate possible mechanisms of deterioration. (author)