Time-domain analysis of reflections influence on gyrotron operation

A time-domain modal analysis of gyrotron operation in the presence of reflections is presented. An eigenfunction expansion method, along with a convolution technique, are used to accurately represent the time-dependent electromagnetic field in the gyrotron cavity, taking into account a mismatched termination of the output port. The relativistic equations of motion for the particles associated with the electron beam and the equations for the field expansion coefficients are solved self-consistently by means of a multi-step integration scheme. The influence of reflections of different kind on gyrotron operation has been investigated numerically for a wide range of parameters. Numerical results for a 1 MW, 140 GHz, TE22,6-mode gyrotron developed at Forschungszentrum Karlsruhe, Germany, are compared with experimental results. Generally, the comparison shows quite good agreement. Moreover, for a specific parameter range, the simulations show unstable operation of the gyrotron. In that case, the output signal has a broad spectrum up to about 12% of relative width. Finally, different mechanisms of output power reduction and frequency pulling are discussed.