Initial design data for the second harmonic, 1 MW, 15 GHz gyrotron for plasma heating at the NSTX Tokamak

Summary form only given. In this work, we present an alternative solution, which is a gyrotron operating in the second harmonic. For operation in the second harmonic, a magnetic field of only about 3 kGauss is necessary. At frequencies as low as 15 GHz, 1 MW power can be produced even in the case of operation at low-order modes such as the TE/sub 02/-mode. Assuming 50% efficiency possible, an electron beam with nominal voltage and current of 80 kV and 25 A, respectively, is chosen to produce 1 MW of power. For a magnetron type electron gun the magnetic compression ratio based on the adiabatic theory should be close to 4.5, which is quite small for gyrotrons. Hence, the transition region between the gun and the resonator should be short. The beam produced for such short optics can be very stable with respect to the instabilities caused by the beam space charge. Based on the fact that the beam is stable as well as results from previous experiments with second harmonic gyrotrons at long wavelengths, an orbital-to-axial electron velocity of 2 is assumed. Detailed calculations are being done using the self-consistent code MAGY that include the effect of cavity wall profile and the magnetic field tapering on the gyrotron efficiency and power. It was found that second harmonic gyrotron with an optimized cavity wall profile can operate with more than 50% efficiency even in a constant magnetic field (about 2.925 kG).