Performance Limiting Effects in X-Band Accelerators

Acceleration gradient is a critical parameter for the design of future TeV-scale linear colliders. The major obstacle to higher gradient in room-temperature accelerators is rf breakdown, which is still a very mysterious phenomenon that depends on the geometry and material of the accelerator as well as the input power and operating frequency. Pulsed heating has been associated with breakdown for many years however there have been no experiments that clearly separate field and heating effects on the breakdown rate. Recently, such experiments have been performed at SLAC with both standing-wave and travelling-wave structures. These experiments have demonstrated that pulsed heating is limiting the gradient. Also, a dual-moded cavity has been designed to better distinguish the electric field, magnetic field and pulsed heating effects on breakdown.