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

We report experimental results on a two frequency gyrotron, operating in the TE_22,6 mode at 110 GHz and the TE_24,7 mode 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 TE_24,7 mode were limited by the electron beam quality. At both frequencies, excellent Gaussian beam content was found: 1) 99% for the TE_22,6 mode and 2) 97% for the TE_24,7 mode. Both output beams had waist radii of 2.65 cm, in very good agreement with theory.