Behavior of air exposure of medium beta (= 0.45) niobium sc cavity

Abstract

The superconducting medium beta structure is being developed to upgrade proton energy from 400 MeV to 600 MeV in the KEK/JAERI joint project (phase 2). In this development, it is required to establish a reliable final horizontal cavity assembly procedure in order to realize the excellent performance in the accelerator. So far it is known with beta = 1 sc cavities that the performance of the niobium sc cavity electropolished, high pressure water rinsed, and then baked, has no degradation with a long term air exposure followed by high pressure water rinsing. Recently it was tested whether this finding is true for the medium beta (= 0.45) structure using a very excellent cavity with E(sub p) = 68 MV/m (H(sub rf) = 1,750 Gauss). A preliminary experimental result shows a different behavior from beta = 1 cavities. Q(sub 0) was lowered by factor 3 or 4 after air exposure. The Q(sub 0)-degradation was improved by additional baking, but the achievable field gradient was limited to E(sub p) = 50 MV/m. In this paper, the experimental results will be presented.