A 2D axisymmetric PIC code has been developed to investigate the adverse effects of the electron cyclotron instabilities on the electron beam quality in the gyrotron. Both electrostatic and electromagnetic interactions are considered. The main result is that the electrostatic instability can induce energy spreads, of an order of magnitude larger than can be expected from beam optics, and can thus lead to degradation of the gyrotron efficiency.
The large beam density was identified as the main cause for large energy spreads, which can be quickly estimated using Fig. 6. In high power gyrotrons which usually operate at high current, and high frequency quasi-optical gyrotrons in which the beam cross-section should be kept small in order to couple efficiently with the standing wave of the Fabry-Perrot resonator, a small beam density constraint should, indeed, be taken into account. However, this may be difficult to achieve.