Design, simulation, and experimental results of a 110 GHz high-power gyrotron mode converter

Summary form only given. We present the design, computer simulation, and experimental results of a quasi-optical mode converter for use in a 110 GHz high-power gyrotron. The particular gyrotron under test generates a right-handed TE/sub 22.6/ circular waveguide mode, and is designed to produce megawatt power levels over pulse durations on the order of 5 s. This high power level provides a rigorous constraint on the output-beam shape; namely, that the peaking factor (the ratio of peak to average intensity-a measure of the power distribution across a cross-section of the beam) be approximately 2.0. Such a peaking factor corresponds to a nearly flat power distribution across the output window to allow for proper cooling of the window. To that end, the mode converter employs a rippled-wall waveguide mode transformer/beam launcher, and a pair of toroidal focusing mirrors to produce a Gaussian-like beam. Two shaping mirrors follow. and convert the Gaussian beam into a quasi-optical beam with a flat power distribution. The rippled-wall launcher was designed using standard coupled mode theory, and the toroidal mirrors derive from Gaussian beam analysis. The shaping mirrors result from a vector diffraction synthesis procedure developed at UW. Simulations of the resulting system were carried out independently at MIT, UW, and CPI, using both physical optics and vector diffraction theory to estimate the field profiles on the launcher, the mirrors, and the output window plane. These tests showed that in theory the field at the output window indeed has a flat-top power distribution with a peaking factor of 2.0.