Time-domain analysis of reflections influence on gyrotron operation

A time-domain modal analysis of gyrotron operation in the presence of refle ctions is presented. An eigenfunction expansion method, along with a convol ution technique, are used to accurately represent the time-dependent electr omagnetic field in the gyrotron cavity, taking into account a mismatched te rmination 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-st ep integration scheme. The influence of reflections of different kind on gy rotron operation has been investigated numerically for a wide range of para meters. Numerical results for a 1 MW, 140 GHz, TE22,6-mode gyrotron develop ed at Forschungszentrum Karlsruhe, Germany, are compared with experimental results. Generally, the comparison shows quite good agreement. Moreover, fo r a specific parameter range, the simulations show unstable operation of th e gyrotron. In that case, the output signal has a broad spectrum up to abou t 12% of relative width. Finally, different mechanisms of output power redu ction and frequency pulling are discussed. (C) 2001 American Institute of P hysics.