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, was operated at power levels up to 200 kW CW. High output mode purity is required for the efficicent 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 made content indicated that greater than 95% of the microwave output was in the desired TE sub O2 mode, with only small percentages in the neighboring TE sub O1 and TE sub O3 circular electric fields. The pure mode CW design uses a 2.5-inch diameter collector. The collector has a magnetic field arrangement 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.