Gradient-driven instabilities and the subsequent nonlinear evolution of generated vortices in sheared E X B flows are investigated for magnetized plasmas with and without gravity (magnetic curvature) and magnetic shear by using theory and implicit particle simulations. In the linear eigenmode analysis, the instabilities considered are the Kelvin-Helmholtz (K-H) instability and the resistive interchange instability. The presence of the shear flow can stabilize these instabilities. The dynamics of the K-H instability and the vortex dynamics can be uniformly described by the initial flow pattern with a vorticity localization parameter E. The observed growth of the K-H modes is exponential in time for linearly wIsEable modes, secular for the marginal mode, and absent until driven nonlinearly for linearly stable modes. The distance between two vortex centers experiences rapid merging while the angle 6 between the axis of the vortices and the external shear flow increases. These vortices proceed toward their overall coalescence, while shedding small-scale vortices and waves. The main features of vortex dynamics, the nonlinear coalescence and the tilt or the rotational instabilities of vortices, are shown to be given by using a low-dimension Hamiltonian representation for interacting vortex cores in the shear flow.938

The operating characteristics of a 140 GHz, pulsed gyrotron are reported. Total efficiencies of 36% and ouput powers of 175 kW have been obtained in single mode operation.Measurements of power and efficiency have been made for a variety of modes between 120 and 160 GHz, and these results are in good agreement with predictions based on nonlinear theory. The best results have been obtained with isolated, asymmetric modes, such as the TE 4 , 2 , 1 (127.3 GHz), TE 2 , 3 , 1 (136.7 GHz),and the TE 3 , 3 , 1 (155.6 GHz). Although mode competition was found to prevent the TEO, 3 , 1 mode (139.5 GHz) from reaching the optimum operating conditions, an output power of 138 kW and total efficiency of 29% were achieved with this mode. A variety of new, highly accurate diagnostic techniques that have been developed to measure the power, frequency, and mode content of the output radiation will be reviewed. In addition, the operating characteristics of both laminar and nonlaminar magnetron injection guns will be compared. The high powers and efficiencies obtained in this experiment are promising for the extension of gyrotron output powers to the megawatt range.2

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