263 GHz gyrotron tuning curves: (a) output power (blue circles) and efficiency (red squares) as function of electron beam current at 12 kV cathode voltage, V0. (b) Output power (blue circles) and frequency (red circles) versus beam voltage for fixed cavity inlet coolant temperature (20 °C), magnetic field, and filament power. The beam current, I0, varied as indicated on the plot. (c) Frequency (blue circles) and output power (red circles) as a function of inlet cavity coolant temperature with all other parameters fixed. (d) Output power (blue circles) and frequency (red squares) versus beam voltage for fixed magnetic field and filament power. The cavity inlet coolant temperature is adjusted to maintain a constant frequency, 263.544 GHz.