A 60 GHz, 200 kW CW gyrotron with high output mode purity

For the first time, a Varian 60 GHz gyrotron, designed specifically to generate microwaves in a single output mode, has been operated at power levels up to 200 kW CW. High output mode purity is required for the efficient utilization of gyrotrons as high power microwave sources for electron cyclotron resonance heating (ECRH) in magnetic fusion plasmas. Using mode-specific directional couplers, measurements of the output mode content indicated that greater than 95% of the microwave output was in the desired TE02mode, with only small percentages in the neighboring TE01and TE03circular electric modes. The pure mode CW design uses a 2.5-inch diameter collector. The collector has a magnetic field arrangment which capably avoids excessive heating by distributing the 640 kW CW beam over a sufficient collector area. With pure mode operation, window temperatures are 10°C to 15°C higher than with mixed mode operation.