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

Summary form only given. 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. As the fusion gyrotron development is clearly headed towards higher output power and higher operating frequency, the corridors for operation parameters narrows with the desired cavity mode [1]. Furthermore, modern fusion reactors tend to demand gyrotron power modulation, which will cause operation in the mentioned non-stationary transition regime. In this work, the characterization and time constant separation of the named effects is attempted for the 1 MW, 140 GHz W7-X gyrotron. From the experimental side, this is done with the help of a transient spectrum measurement system [2], which with some modifications is capable of recording continuously in the transient pulse range. The measured time-dependent frequencies are used for the modeling of the two effects, which is supported through parameter studies using the self-consistent gyrotron simulation code EURIDICE. In addition, previous theoretical descriptions of the ionization processes in the device [3] were modified for the application to modern high power gyrotrons, and brought into context with the experimental and modeling data.