First Experience with in situ Helium Processing of the LEP Superconducting Modules

The helium processing technique has been used at CERN to improve the performance of the accelerating RF cavities with respect to field emission. It was extensively used in the vertical bare cavity tests to improve the superconducting cavities for the LEP Project. However this technique was rarely used on fully equipped production modules because of the risk of damaging the power couplers and its ceramic window by a possible glow discharges. First experience with helium processing on finished modules equipped with additional vacuum interlocks to protect the power couplers was acquired in the test area in October 1996. He processing was applied to three modules limited by field emission. Their overall performance was increased up to about 7 MV/m. Confident with these results, one module was processed in the LEP tunnel just before the shutdown 1996. The results obtained confirmed the feasibility of this method on a module already installed in the LEP tunnel and its efficiency in improving the performances. During the winter shutdown 1996-97, helium processing has been done separately on seven installed in LEP.