Development of a Second Harmonic Multi-Frequency Gyrotron with Gaussian Beam Output

A sub-THz second harmonic multi-frequency gyrotron with Gaussian beam output is under development. This gyrotron can change the frequency stepwise in the range from 270 to 420 GHz. For multi-frequency Gaussian-beam output, all design modes should be converted to Gaussian beams with one mode converter. To achieve this, nine oscillation modes with close values of the transverse propagation angle were selected. Oscillation test of the selected modes was carried out with a linear type gyrotron equipped with the designed cavity. Oscillations of seven second-harmonic modes were successfully observed among the nine selected modes. Operation condition for each mode was experimentally investigated. An internal mode converter to radiate Gaussian beam from each mode was designed.