Operating Experience with High Power Beam Absorbers in the SLAC Beam Switchyard

In four years of operation at progressively increasing levels of average power new observations on the performance of slits, collimators, and beam dumps of various designs have been accumulated. The inherent longevity of high power beam absorbers based on low-Z or graded-Z design philosophy has been successfully demonstrated. Early operating experience at low power levels in the SLAC beam switchyard has been described previously. The performance of multiple protective interlock circuitry is described and system response patterns are given. Two types of beam line failures are discussed: The air burst resulting from the melting of soft-metal vacuum seals and stainless steel transport pipes at modest power densities; and the water burst resulting from the failure of heat transfer surfaces at high power densities. Problems of beam line restoration in a radiation environment are briefly discussed. Downtime to date attending beam-induced failure of the vacuum envelope has been of the order of 350 hours, averages less than 10 hours per event, and accounts for some 15% of the unscheduled downtime from all sources combined.