Improvement of gyrotron by suppression of undesirable waves

Undesirable waves inside a gyrotron are factors that disturb the improvement of performance such as high power, long pulse and high efficiency operations of a gyrotron. It was found that undesirable waves inside the gyrotron were followings, (1) a parasitic oscillation in a beam tunnel, (2) a parasitic oscillation in an RF radiator, (3) stray RF inside of the gyrotron. The Parasitic oscillation in the beam tunnel was suppressed completely by application of SiC to inner surface of the beam tunnel. As a result, improvement of efficiency of main mode oscillation at a cavity from ∼20% to >30% was achieved. The parasitic oscillation in the RF radiator was reduced by increasing oscillation efficiency at the cavity. The power of the stray RF due to diffraction loss on mirrors inside of the gyrotron was estimated more than 10% of output power. However, no trouble was observed by extraction of the stray RF through DC break. Long pulse operations such as 1MW - 10s level (0.9MW-9.2s) were achieved by suppression of parasitic oscillation, effective extraction of the stray RF to the outside of the gyrotron and reinforcement of the cooling at the inside of the gyrotron which overheat due to the stray RF. (author)