Initial operation of a 1 MW, 170 GHz gyrotron with an internal mode converter

Summary form only given. We report the results from initial operation of a high frequency gyrotron suitable for ECH of the International Thermonuclear Experimental Reactor (ITER). The gyrotron is designed to operate in the TE/sub 28,8,1/ mode at 170 GHz. Our goal is to demonstrate efficient operation at a power level in excess of 1 MW with a Gaussian output beam. The experiment uses a new 7.5 T magnet with a 20 cm bore to provide the cavity magnetic field. The large bore is needed to accommodate the mode converter and mirror transmission line inside the gyrotron that will transform the RF power into a Gaussian beam. This experiment utilizes an 83 kV, 35 A annular electron beam and a tapered cavity. The cavity was designed to have a short interaction length and large input angle in order to avoid multimode oscillations. The cavity uses an iris to achieve a sufficiently large diffractive Q (1200) so that a high interaction efficiency is possible. Nonlinear simulations indicate that single mode emission can be achieved with total efficiencies up to 35%. However, the manner in which the gyrotron reaches its final operating parameters is critical. The electrode shapes of the electron gun were chosen so that the beam characteristics would be insensitive to machining errors, and to variations in the operating parameters resulting from misalignment or voltage fluctuations.