PERFORMANCE OF RF AMPLIFIERS FOR ISAC-II MEDIUM BETA LINAC OPERATION

A heavy ion superconducting linac at TRIUMF to increase the final energy of radioactive beams from the existing room temperature ISAC accelerator is now operational. The linac consists of twenty quarter wave cavities operating at 106 MHz to produce an accelerating voltage gain of 20 MV. This paper describes the operational experience of the 106 MHz rf tube amplifiers which were installed and commissioned for the medium beta linac in October 2005. Total rf power installed is 16 kW where one quarter power is used for regular linac operation. During the initial commissioning of the amplifiers, only one power tube was found to be noisy and was replaced. The first major tube failure occurred in April 2007 after 9000 operating filament hours. Soon thereafter five more tubes showed signs of loss of gain. It was evident that most of the tubes reached their tube life by this time and all were replaced with new tubes. The extension of tube life, and prediction of tube failure are the main concerns of these tube amplifiers. Efforts are being made to incorporate reduced filament power operation in order to get longer tube life. Systematic check of the amplifier performance during scheduled maintenance and shutdown period is undertaken. Reduction of tube emission and hence gain degradation from their nominal value causes excessive drive power from the rf control system to keep the cavity voltage constant under closed loop. Hence monitoring the drive power is useful to predict early tube failure. Input drive power and gain of all the 20 amplifiers are available at the EPICS data archive which can be monitored and plotted. This will allow early warning of tube failure so that tubes can be replaced before they actually fail. The failure modes of the tubes and diagnostics to predict tube failure will be described.