CEBAF’s SRF Cavity Manufacturing Experience

Abstract

Construction of the Continuous Electron Beam Accelerator Facility (CEBAF) recirculating linac represents the largest scale application of superconducting rf (SRF) technology to date. The accelerating structures in CEBAF are 169 pairs of 1.5 GHz superconducting rf cavities - 9 pairs in an injector and 80 pairs each in two linacs. The beam is to be recirculated up to five passes through each linac. Data is presented on mechanical tolerances achieved by the industrial fabricator of the rf cavities (Siemens). Liquid helium leak rates integrated over 22 vacuum seals have been measured on over 110 cavity pairs. A roughly noilnal distribution of the log_1o (leak rate) is seen, centered about a rate of 10^-10.4 torr-1/s. Over 140 pairs of the cavities have been assembled and have completed sf testing at 2.0 K. Among these, 54% demonstrated usable accelerating gradients greater than 10 MV/in. Although the rf pel-formance characteristics well exceed the CEBAF baseline requirements of 5 MV/m at Q_o = 2.4×10⁹, the usual limiting phenomena are encountered: field emission, quenching, aiid occasional multipacting. A discussion of the occurrence conditions and severity of these phenomena during production cavity testing is presented. The frequency with which performance is limited by quenching suggests that additional material advances may be required for applications which require the reliable achievement of accelerating gradients of more than 15 MV/m.