Horizontal assembly method for the high gradient superconducting RF cavities

Fabrication technique has been established to make high gradient superconducting RF cavities with the acceleration field gradient of 40 MV/m, however there is an issue to realize such a high gradient operation in a real accelerator. We have to develop the horizontal cavity assembly method without any performance degradation in order to install a pair of cavities into the horizontal cryo-module. We have started a basic R and D to solve this issue. The performance degradation in the horizontal cavity assembly in the TRISTAN (Transposable Ring Intersecting Storage Accelerators in Nippon) sc project can be explained well by the appearance of two points multipacting due to opening cavity vacuum with nitrogen gas after the vertical test. The nitrogen gas exposure or air exposure brings the serious performance degradation. High pressure water rinsing can recover the excellent performance with Eacc=40 MV/m. In this paper, we will present the results of the basic study. (author)