Design and Calculation of an Electron Gun for Centimeter-band Megawatt-level Gyrotron TWTs

In recent years, the megawatt-level magnetron injection gun for traveling-wave tubes has attracted extensive attention due to its high-power output and precise electron beam control, yet challenges such as non-uniform cathode emission and velocity spread induced by surface roughness remain to be solved. To address these issues, this paper proposes an electron gun design method based on a dual-anode structure, which first optimizes electron beam parameters using 3D particle simulation via CST and then derives an expression for velocity spread considering surface roughness based on the Maxwell - boltzmann distribution; we further construct a cathode array emission model comprising approximately 1 000 regions with Gaussian noise to simulate surface roughness, and experimental results show that under a 120 kV/30 A operating condition, the method achieves a transverse-to-longitudinal velocity ratio of 1.1, a guiding center radius of 7.1 mm, and a velocity spread of 4.9%, with a 1 μm surface roughness increasing the velocity spread by about 3%.