Separation of thermal expansion and beam charge neutralization effects in high power 140 GHz CW gyrotrons

During the first few seconds of longer pulses in high-power gyrotrons, the main mode frequency decreases over a few hundred MHz, until a stable operation is reached. This is due to the combination of two effects, namely the thermo-mechanical expansion of the cavity and a shift in the effective electron energy through ionization of residual gas in the tube. Most investigations focus on the quasi-stationary “short-pulse” range of the first few milliseconds, and the long-pulse stationary range after both tuning effects have settled.