Analysis of the output mode from 60 GHz, 200 kW pulsed and CW gyrotrons

A Varian 60 GHz, CW gyrotron has recently been operated at power levels up to 200 kW CW. The tube employs a TE011/TE021 complex interaction cavity which inhibits mode competition by the TE221 mode with the desired TE021 mode. The output mode of the cavity is nominally in the TE02 circular waveguide mode in overmoded waveguide. Since the output waveguide also serves as the gyrotron collector, mode conversion occurs in the taper sections and gaps incorporated into the collector. Measurements of the mode conversion on a 60 GHz, 200 kw, 100 ms pulse tube, have been made and compared with similar measurements on the 60 GHz, 200 kW, CW gyrotron. These measurements indicated that the CW tube had significantly more conversion of the TE02 mode into other TEon modes than did the pulse tube. Current design work is aimed at reducing the mode conversion of the CW design to the same levels as the pulse design (≲10% mode conversion).