Pushing past a megawatt: development of long-pulse high-power gyrotrons

Summary form only given. High-power, long-pulse or CW millimeter-wave gyrotrons are used for electron cyclotron heating, current drive, and instability suppression in magnetic fusion devices. Fusion devices typically require unit power levels on the order of 1-2 MW and pulse lengths from 10 seconds to CW. CPI has developed and delivered megawatt-class gyrotrons at 110 GHz and 140 GHz, and is currently fabricating a prototype 1.5 MW gyrotron at 110 GHz. The basic design of the 110 GHz, 1.5 MW gyrotron is similar to that of the earlier 110 GHz, 1 MW devices, but employs a depressed collector to boost overall electrical efficiency. The initial configuration employs single-stage collector depression. Subsequent installation and testing of a multi-stage depressed collector developed by Calabazas Creek Research is also planned. The gyrotron employs a diode magnetron injection gun, a TE/sub 22,6,1/ cavity interaction mode, and an internal converter (consisting of a dimpled-wall launcher and a set of four steering and focusing mirrors) to deliver a TEM/sub 00/ output beam through an 88 mm aperture CVD diamond output window. While the interaction mode for the 1.5 MW gyrotron is the same as for CPI´s 110 GHz 1 MW design, and while the same superconducting magnet system can be employed with either gyrotron, the 1.5 MW system represents a major redesign, including a new electron gun optimized for higher-voltage operation (96 kV instead of 80 kV), an interaction circuit modified to yield optimal efficiency and acceptable ohmic losses at 1.5 MW, an improved mode converter design to reduce internal losses, a larger output window, and a new single-stage depressed collector design for enhanced efficiency, easier fabrication, and improved reliability.