Coupler design for NLC/JLC accelerating structures

In the framework of the NLC/JLC collaboration, Fermilab is working on developing the technology for X-band accelerating structures. One key element of a structure is the input/output coupler. The coupler should provide not only good power transmission from the waveguide to the structure, but also should not limit the high gradient performance of the accelerating structure itself. There are other special requirements for designing high power input/output couplers: reliability; cost; sensitivity to errors; trapped HOM; etc. In this paper we discuss the HFSS simulation of different types of couplers for JLC/NLC accelerating structures. We compare the surface electric and magnetic fields, field asymmetries, pulsed heating, presence of trapped modes and the complexity of production. Results of RF measurements and high gradient tests show good agreement with the design parameters.