Influence of background plasma on electromagnetic properties of "Cold" gyrotron cavity

A simple approximate theory of background plasma influence on the electromagnetic properties of high-power gyrotron cavities is presented. It is shown that a background plasma slightly reduces the operational frequency of gyrotrons, increases the quality factors of operational and neighbouring modes, and modifies the longitudinal field distribution in the gyrotron cavity. The calculated downshift of the operational frequency is compared with long-pulse measurements performed in 140-GHz 1-MW continuous wave gyrotron experiments at Forschungszentrum Karlsruhe