Frequency Measurements of a Complex-Cavity Gyrotron for 400 GHz Second-Harmonic Oscillation

A complex cavity was fabricated to improve the mode selection property of a gyrotron for the 400 GHz second-harmonic oscillation. A combination of the TE 8,3 and the TE 8,5 modes was selected to provide resonant coupling. In the experiment, many fundamental and several second-harmonic modes oscillated independently in either the first or the second cavity. However, no fundamental oscillation was observed in the operation magnetic field region of the TE 8,3 /TE 8,5 mode. The fabrication error of the cavity radii was successfully controlled to be smaller than 1 $\mu\mathrm{m}$ . The oscillation of the design mode was observed by the frequency measurements. However, a mode competition with the TE3,5 mode occurred.