Design of a Magnetron Injection Gun for a 670-GHz 300-kW Gyrotron

A 300-kW 670-GHz gyrotron, operating with a pulsed coil at the fundamental cyclotron harmonic, is designed at the University of Maryland for an application of detecting concealed radioactive materials. The design of a low-spread diode-type magnetron injection gun for this gyrotron is presented. Constraints due to the pulsed coil design and the limitation of maximum electric field at the cathode result in a steep tilting angle of the cathode surface, 74°, along with a Pierce-type focusing section and a high magnetic compression ratio larger than 170. A pitch ratio of 1.34 and a low pitch ratio spread of 2.5% for a cold beam and 9.2% due to emitter surface temperature of 0.1 eV and 1-μm roughness were obtained. The results were benchmarked with three simulation codes: EGUN, TRAK, and MICHELLE. Numerical results were calculated for beam currents up to 19 A, accelerating voltage of 50-90 kV, and magnetic field of 25-30 T. A sensitivity analysis with respect to critical parameters such as the depth of the focusing section and the internal simulation parameters is provided.