AbstractThe development of a high frequency, high harmonic gyrotron using a 17 T superconducting magnet is described. The design of this gyrotron was optimized for the second harmonic. The operation frequency of the gyrotron can be tuned from 160 to 440 GHz by the fundamental and extended up to 850 GHz by the second harmonic. One hundred per cent modulation of the output has been achieved under an anode voltage modulation level of only about 10%.