Modeling of high power gyrotrons: comparison with experiments

Abstract

Summary form only given, as follows. The operation of gyro devices is, in principle, described by Maxwell's equations, along with the relativistic equations of motion of electrons which are injected into and pass through an interaction region (an electrodynamical system) immersed in a strong applied magnetic field. One would like to simulate the operation of gyro devices using particle in cell (PIC) computer codes. Unfortunately, the PIC simulations are not yet at a stage where they can be used effectively for gyrotron modeling. However, successful modeling of gyro-devices has been carried out over the years using reduced descriptions of the interaction. The key step in deriving the reduced description is representing the radiation field as a superposition of a small number of modes of the interaction waveguide whose amplitudes and phases evolve slowly in time and space on the scale of the frequency and wavelength of the radiation. The basic time scale for updating the radiation field is increased from a fraction of the waveperiod, as it is in PIC codes, to a fraction of the cavity fill time, which is essentially the wave period times the quality factor, Q, of the cavity (typically of the order of 1000). Furthermore, the structure of the field in the direction transverse to the applied magnetic field is expressed as a sum of normal modes of the symmetric waveguide. This reduces the solution of Maxwell's Equations to the solution of a coupled system of a relatively small number of one dimensional partial differential equations. As the amplitudes and phases of the cavity modes change on the cavity fill time scale, one can achieve a savings in computation time by periodically launching ensembles of beam particles, which transit the interaction region in a short time compared with the cavity fill time, and then updating the cavity fields taking relatively large time steps.