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During commissioning of the CEBAF accelerator, it was found that cavities could not be operated reliably at the gradients achieved for short periods during individual cavity commissioning. The principal hypothesis for the cause of about two thirds the faults seen is charging of the cold ceramic RF window, which is 7.6 cm off the beam axis. Beginning in February 1995, most RF systems faults were automatically logged. Simple statistical analysis of the accumulated fault data was first applied in July 1995, with a substantial drop in fault rate recorded. The intent of the analysis was to predict the gradient for each cavity at which it would fault once every ten days, leading to a fault rate for the machine of about 33/day (330 cavities). This analysis method was pursued through July 1996 with substantial benefit. Cavity gradients were increased thereafter to obtain information for an upgrade to 6 GeV, with concomitant fault rate increases. In late 1996 and early 1997, in situ helium discharge processing was employed in 88 cavities to reduce field emission. The methods used for the analysis of 30000+ faults recorded between February 1995 and December 1997 are presented. Comparisons of performance before and after helium processing are presented.
Date of Publication: June 1999