The operating characteristics of a 140 GHz, pulsed gyrotron are reported. Total efficiencies of 36% and ouput powers of 175 kW have been obtained in single mode operation.Measurements of power and efficiency have been made for a variety of modes between 120 and 160 GHz, and these results are in good agreement with predictions based on nonlinear theory. The best results have been obtained with isolated, asymmetric modes, such as the TE 4 , 2 , 1 (127.3 GHz), TE 2 , 3 , 1 (136.7 GHz),and the TE 3 , 3 , 1 (155.6 GHz). Although mode competition was found to prevent the TEO, 3 , 1 mode (139.5 GHz) from reaching the optimum operating conditions, an output power of 138 kW and total efficiency of 29% were achieved with this mode. A variety of new, highly accurate diagnostic techniques that have been developed to measure the power, frequency, and mode content of the output radiation will be reviewed. In addition, the operating characteristics of both laminar and nonlaminar magnetron injection guns will be compared. The high powers and efficiencies obtained in this experiment are promising for the extension of gyrotron output powers to the megawatt range.2