Study Of Mode Locking In Tapered Cavity Gyrotron Oscillators

Abstract

It has been shown via quasilinear theory that a gyrotron oscillator with a highly dispersive waveguide structure can be mode locked by compensating with dispersion due to the electron beam. It is perhaps easier to modify the cavity geometry. A nonlinear investigation of mode locking is carried out using the MAGIC code. A closed, tapered cylindrical cavity gyrotron is used with an operating frequency of 20 GHz. A taper section of 11.25 cm with taper angle of 0.6{degree} is included. The cavity supports azimuthally symmetric TE{sub 01n} modes. The electron beam parameters are 50 kV beam voltage with 10 A beam current, and 0.5 cm guiding center radius. The external axial magnetic field is 7.46 kG. A beam velocity or beam density modulation is imposed on the input electrons to model the effect of a prebunching cavity. In the case of a 5%, 280 MHz density modulation the simulation results show the frequency interval between adjacent modes is almost identical to modulation frequency, indicating locking between adjacent modes. The output radiation consists of a train of narrow pulses (FFWHM {approximately}1 ns) at a 280 MHz repetition rate. An open-end tapered cavity connected to a waveguide is under investigation. Numerical simulationmore » results will be presented and an experimental design will be described.« less