Multimode Theory and Simulation of Quasioptical Gyrotrons and Gyroklystrons

Very high power sources for millimeter wavelength radiation are desired for cyclotron heating of magnetic fusion devices. To meet this goal a quasioptical gyrotron has recently been proposed and analysed using a single mode approach. In this work, a more realistic analysis of the quasioptical gyrotron is carried out using multimode time-dependent simulations to study mode competition in a highly overmoded cavity. It is shown that with suitable contouring of the DC magnetic field, single mode operation at 30% or greater efficiency is possible. To further improve on efficiency, an alternative configuration, the quasioptical gyroklystron, is also analysed using multimode dynamic simulations. The gyroklystron is shown to lead to a significant increasing in efficiency (≈ 45%), particularly at moderate values for the high frequency electric field. Similar analyses of the quasioptical gyrotron and gyroklystron operating on the second harmonic have shown promising results ; efficiencies ≈ 30% have been obtained for the gyroklystron configuration.