Collector failures on 350 MHz, 1.2 MW CW klystrons for the accelerator production of tritium (APT)

At Los Alamos we have built and are operating the front end of the APT accelerator. We are currently operating a 7 MeV radio frequency quadrapole (RFQ) using three, 1.2 MW CW klystrons. These klystrons are required to dissipate the full beam power in the collector and were designed for this operating condition. The klystrons have operated for a maximum of 1500 HV hours and one collector has failed and all collectors show varying degrees of damage on the cone of the collector as well as sag in the overall collector structure. This paper will present and discuss the collector damage. It will also discuss the difficulties of diagnosing the collector failure. After the collector failure, we began utilizing a hydrophonic system to diagnose collector heating. The collectors are designed to allow for mixed-phase cooling and with the hydrophonic test equipment we were able to observe normal, single-phase cooling, mixed-phase cooling, as well as the hard boil on a collector with significant damage. These data will be presented and discussed. We also discovered that the worst case beam profile from a collector heating standpoint was not the full power beam but was instead a beam with the same perveance at about 55% of the maximum beam power. These results will be presented. The paper will also discuss some of the immediate steps taken to halt the collector damage on the remaining 350 MHz klystrons and design changes that are being implemented to solve the problem.