TRIUMF, Vancouver
PAVAC, Richmond, B.C.
The ISAC-II superconducting heavy ion linac, commissioned in 2006, consists of twenty quarter wave cavities housed in five cryomodules. An upgrade of the linac is ongoing that will see the addition of twenty more quarter wave cavities in three cryomodules by the end of 2009. The status of the upgrade will be reported.
University of Pennsylvania, Philadelphia, Pennsylvania
TRIUMF, Vancouver
The quality factor of superconducting radio-frequency niobium cavities decreases with the applied RF field in the medium field range. The medium field Q-slope effect has been investigated by many authors but models, most commonly including thermal feedback, do not fully explain experimental evidence. In this contribution we analyze medium field Q-slope data measured on ISAC-II low beta quarter wave cavities at 4.2K and 2K. The data is compared with a model that, in addition to thermal feedback of the surface temperature, takes into account the reduction of the critical temperature with the applied magnetic field. This hypothesis is tested adding two surface heaters on the LHe side of the cavity in the high magnetic field region and obtaining Q-curves at different heater power levels. The temperature increase of the RF surface due to the heater power is directly measured using a test niobium chamber. A model to explain the experimental results is presented with conclusions concerning the thermal feedback mechanism and on the relationship between the critical temperature field dependence and the Q-slope.