Design of a low-voltage, axially modulated, cusp-injected, third harmonic, X-band gyrotron amplifier experiment

We present the design of a prebunched, third harmonic small-orbit gyrotron experiment that utilizes a 45 kV, 8 A beam to produce over 185 kW of amplified power at 9.9 GHz in a 1.23 kG magnetic field. We detail the design of the electron gun, which produces a moderately compressed axially streaming annular beam. The beam is velocity modulated by a short TM₀₁₀ coaxial input cavity and energy is extracted in a right-circular TE₀₂₁ output cavity. Perpendicular energy is imparted to the beam via a nonadiabatic magnetic transition at the end of a 23 cm drift region between the two cavities. A nonlinear single particle code is used to predict an electronic efficiency of 52% and a large signal gain of 25.5 dB. This code generates the cavity field profiles via a scattering matrix formalism and uses the output from the electron gun code to model the beam. This improved model allows the potentially important effects of leakage fields and finite beam thickness and spread to be investigated