Recent progress in producing megawatt gyrotrons for ECH applications

High-power, long-pulse or CW millimeter-wave sources are required in electron cyclotron heating or current drive for fusion experiments. Gyrotron oscillators have served as effective sources in this area for many years. To reduce the cost per watt of ECH power generated, megawatt gyrotrons are required. Recent developments in using diamond for the output window has eliminated one of the obstacles to demonstrate a megawatt, CW device. At CPI, various programs are underway to deliver long pulse or CW megawatt gyrotrons. In particular, we are building a number of 110 GHz, 10 sec, 1 MW gyrotrons. Two of these have been tested to 550 kW, 10 sec and 1 MW, short pulse. These gyrotrons will be tested to 1 MW for ten second pulses at the customer's site. A third gyrotron with the same specifications is currently being fabricated at CPI. In addition, we have designed and are currently fabricating a 1 MW CW gyrotron at 140 GHz. In addition to features that are similar to the 110 GHz gyrotrons, like the internal converter, diode gun, and diamond window, the 140 GHz gyrotrons also include a single-stage depressed collector. A similarly-configured 500 kW, 3 sec gyrotron at 70 GHz was recently tested and delivered. In this paper, we summarize the basic design of the 110 GHz and 140 GHz gyrotrons and report on the experimental results of the megawatt, 10 sec, 110 GHz gyrotrons.