Multimegawatt gyrotrons for plasma heating

The gyrotron is under development as a high power source for plasma heating at electron cyclotron resonance. For heating large scale plasmas, such as the DIII-D machine at General Atomics, it is advantageous to have high unit power heating sources to reduce the cost and complexity of the system. The authors will present preliminary designs of 1.5 and 2 MW gyrotrons at a frequency of 110 GHz. The gyrotron designs are based on previous successful results at the 1 MW level at frequencies from 110 to 170 GHz. The baseline design is for a TE28.8 mode cavity with an electron beam of 80 to 110 kV and a current of up to 80A. The expected efficiency exceeds 30% but it should increase to over 50% with a depressed collector. The output beam will be a Gaussian TEM00 mode in free space. The gyrotron will be investigated experimentally in short pulse operation (approximately 3 microseconds) at MIT and, if successful, will be developed in a 10s pulsed or CW version by industry. There are two competing approaches for the design of multimegawatt gyrotrons: conventional, cylindrical cavity gyrotrons and coaxial cavity gyrotrons. The conventional cavity approach is being considered as an extension of present day gyrotron research at 110 GHz. The coaxial cavity gyrotron is under investigation at MIT with the goal of output powers of 3 MW at 140 GHz. Recent experimental results from the coaxial cavity gyrotron at power levels in excess of 1 MW will be presented