Experimental Results for a Pulsed 110/124.5-GHz Megawatt Gyrotron

We report experimental results on a two frequency gyrotron, operating in the TE22,6mode at 110 GHz and the TE24,7mode at 124.5 GHz. The gyrotron uses the same electron gun as a previous single frequency 110-GHz gyrotron, with a new cavity and internal mode converter designed for optimized performance at the two frequencies. For a 98 kV, 42-A electron beam operating in 3-μs pulses, an output power of 1.25 MW was obtained at 110 GHz (30% efficiency) and 1.0 MW at 124.5 GHz (24% efficiency). The highest power obtained was 1.4 MW with a 96 kV, 45-A beam (32% efficiency) at 110 GHz. In both modes, mode competition was minimal around the high-power operating point and operation was extremely stable. The output power and efficiency in the TE24,7mode were limited by the electron beam quality. At both frequencies, excellent Gaussian beam content was found: 1) 99% for the TE22,6mode and 2) 97% for the TE24,7mode. Both output beams had waist radii of 2.65 cm, in very good agreement with theory.