Cryogenic testing of the RF input waveguide for the CEBAF upgrade cryomodule

In order to support the planned CEBAF upgrade at the Jefferson Lab a new cryomodule has been designed. A key component of the new cryomodule is the upgraded RF input waveguide, which may couple as much as 10 kW of RF power to superconducting radio frequency (SRF) cavities contained in a bath of superfluid helium. The coupler consists of a straight copper plated stainless steel waveguide as a thermal transition between 2 K and 300 K and one ceramic window at the warm end of the waveguide. The waveguide interior shares a common vacuum with the cavity. Initial testing of the upgraded coupler has been conducted at Jefferson Lab in a representative cryomodule. During testing, data was obtained regarding waveguide temperature profiles as well as coupler arcing and multipacting. Predicted temperature profiles were used to determine the optimum location of the 60 K heat intercept on the waveguide, and were found to be comparable to the actual measured profiles. The coupler was found to be free of multipacting up to 1.7 kW. No arcing occurred during multiple eight-hour runs in heavy field emission with typical radiation levels of 0.5 to 1.0 R/hr outside the cryostat