Non-Uniform Cathode Emission Studies of a Mig Gun

Summary form only given. We present results of emission non-uniformity studies by 3D modeling and experiments on a 96 kV, 40 A magnetron injection gun (MIG) for use in a 1.5 MW 110 GHz gyrotron. Non-uniform beam current density is attributed to emitter surface roughness, surface temperature inhomogeneity, and variation of the work function along the emitting cathode ring. The emission nonuniformity can lead to increased mode competition and an overall decreased efficiency of the device. The gun is modeled from the cathode to the start of the interaction cavity using a 3D gun code, Michelle 3D developed by SAIC. A nonuniform emitter model is incorporated in Michelle to model the measured nonuniform emission from the cathode and to compute its impact on the velocity spread and the beam pitch factor. The designs of capacitive probes to measure the velocity pitch factor at four azimuthal sections of the beam and a rotating probe diagnostic to measure the emission nonuniformity at the cathode are also presented. Experimental results from these diagnostics will be presented and compared with Michelle 3D simulations