High magnetic fields in couplers of X-band accelerating structures

Increasing the accelerating gradient is an important issue for linear accelerators. Among phenomena that limit the gradient is rf breakdown in accelerating structures. Breakdowns have been frequently observed in coupler cells of accelerating structures and have been attributed to the electrical field enhancement noted in simulations. Several solutions have been proposed to reduce this enhancement. For example, increasing the group velocity in the coupler and adjacent cells, and shaping of the coupler cell to reduce maximum surface electric fields below the fields in the structure. Little attention was paid to enhancement of the magnetic field in the couplers although the possibility of damage due to pulse heating. The limit imposed by rf pulse heating and thermal fatigue was discussed, but the connection between the high rf magnetic field and coupler breakdowns was realized only recently in high gradient experiments with traveling wave (TW) and standing wave (SW) 11.4 GHz accelerating structures. These experiments are part of extensive experimental and theoretical program underway at SLAC, FNAL and KEK to develop structures that reliably meet the Next Linear Collider and Japanese Linear Collider (NLC/JLC) requirement of 50 MV/m loaded (65 MV/m unloaded) gradient operation.