Varian is in the process of developing gyrotrons capable of generating power levels of 1 MW CW at a frequency of 140 GHz. The first experimental gyrotron in this program has been designed to generate short-pulse power levels of 1 MW and up to 400 kW CW. In recent tests on this gyrotron, a short-pulse power level of 820 kW was obtained at 21% efficiency. While operating at peak power levels of 350-400 kW, the duty factor was raised to 10% (30 pps, 3.33 ms-pulse duration). Power losses in various parts of the tube have been measured and are in agreement with theoretical predictions. Tests at higher average powers, up to the 400 kW CW goal for the tube, are underway. Design efforts aimed at increasing the average power capabilities of the gyrotron up to 1 MW are in progress.© (1988) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.