S-band J-type waveguide feeding accelerating structure for free electron laser

With the development of the required gradient of the waveguide coupled TW accelerating structure, for preventing the undesirable transverse momentum contributions being imparted to the beam during traversal of the linac coupler cavities, the use of symmetric dual feed cylindrical cavities which have diametrically opposed side wall coupling apertures has been widely accepted. As one type of the dual feed cylindrical cavities, the prototype of the S-band J-type waveguide feeding accelerating structure developed has got a top accelerating gradient of 30 MV per meter during RF conditioning. However, the transverse gradient of the amplitude and the phase of the longitudinal electric field caused by the existence of the quadrupolar field in the dual feed cylindrical cavity, will also degrade the beam emittance. So the J-type waveguide feeding racetrack cavity is studied theoretically. With comparison of the simulation results to the cylindrical cavity, the racetrack cavity can improve the rotational symmetry of the longitudinal electric field in the non-near axis area in the transverse plane very well, which will reduce the impact of the quadrupolar field. More importantly, the J-type waveguide feeding racetrack cavity is easier in the aspect of mechanical fabrication and measurement, which seems to be a very good direction for the future dual feed accelerating structures.