Study of an Axis-Encircling Large-Orbit Cusp Electron-gun for Ka-band Gyrotron Traveling Wave Amplifiers

An axis-encircling large-orbit cusp electron beam can keep stable work in high power and high frequency gyrotrons traveling wave amplifiers (gyro-TWAs) at high efficiency. However, the CUSP magnetic field is difficult to obtain in engineering realization. A gradually-varying magnetic field produced by a solenoid magnet is obtained for the axis-encircling large-orbit cusp electron beam, which is designed for a Ka-band gyro-TWAs. And a magnetic cusp gun (MCG) is designed, which can work with low parameters spread at the beam voltage of 65kV and beam current of 5A. The calculated results show that the beam can be generated with average radius 1.1 mm, average beam pitch ratio $\alpha=\mathbf{1.0}$ , and the axial magnetic field $\mathbf{B}_{\mathbf{z}\mathbf{0}}=\mathbf{0.641}\mathbf{T}$ is designed as the magnetic field in the interaction region. At the same time the spread of the transverse velocity and the longitudinal velocity are 2.3% and 4.3% respectively. The designed can be used for Ka-band gyro-TWAs.