A Review of Vacuum Breakdown in High-Gradient Accelerators

Accelerating gradients above 100 MV/m are now routinely achieved during operation of prototype accelerating structures in the CLIC high-gradient test stands. At such accelerating gradients surface electric fields are in excess of 200 MV/m for pulse lengths in the range of 200 ns. About twenty structures have been tested for a few billion pulses each under controlled conditions. The data taken during the tests and subsequent analysis has given important insights into different aspects of vacuum breakdown. These include the dependence of gradient on structure geometry, the dependence of breakdown rate on gradient, the nature of the conditioning process, the statistics of breakdown and collectively lead to an improved understanding of the fundamental processes of breakdown at the surface field limit of metal surfaces.