A gyrotron is under development to produce 1 MW CW output at 140 GHz. The design will be briefly reviewed and recent test results will be presented. A power output of 930 kW with pulse length of 5 ms was demonstrated. In that case the pulse length was limited by the capabilities of the existing power supply. Pulse lengths of 700 s at 500 kW were also demonstrated. This power level is determined by the long pulse limitation of the power supply. The paper will discuss more details of the operation including the effect of various parameters on power output and efficiency, calorimetric measurements of how the mm‐wave power is distributed throughout the gyrotron, and measurements of output frequency and vacuum level versus time for long‐pulse operation. © 2003 American Institute of Physics