Excitation of Several Axial Eigenmodes in Gyrotrons with Mismatched Window - Simulations and Experimental Studies

Summary form only given. Recently the regime of simultaneous excitation of several axial eigenmodes of the gyrotron cavity was studied in relativistic gyrotron with output power of hundreds of kW. It was shown that such regimes exist predominantly with a proper reflector position, when the Q-factors of neighboring eigenmodes are close to each other. The paper is devoted to theoretical and experimental studies of this phenomena in 10-20 kV sub-relativistic gyrotrons. Experimental studies were carried out at the Ka-band technological gyrotrons at the Forschungszentrum Karlsruhe (in the regime of 20 ms electron pulse duration) and the Institute of Applied Physics (CW operation), which were operated at the TE₁₂ mode when the millimeter-wave output window was strongly mismatched and was used as external reflector. Two different situations were realized experimentally. In the first, when the Q-factors of axial modes were quite different, only the steady-state oscillations at two different frequencies were observed. In the second, when the Q-factors were close, the regime of two frequency oscillations was realized. These results are in a good agreement with "cold" electrodynamic studies and non-stationary PIC-code simulations. The possibilities of increasing the efficiency in the non-stationary regimes are discussed