Doctoral Dissertations

Date of Award

12-1990

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Electrical Engineering

Major Professor

Igor Alexeff

Committee Members

E. Harris, G. Hoffman, J. Rochelle, J. Tillman, D. Rosenberg

Abstract

A high power microwave transmission and launching system has been developed for Electron Cyclotron Heating (ECH) of plasmas in the Advanced Toroidal Facility (ATP) fusion confinement experiment at Oak Ridge National Laboratory. Microwave power is generated by two 53 GHz, 200 KW cw gyrotron tubes. A waveguide transmission and launching system for each tube has been designed and built with the goal of depositing the maximum amount of power at the center of the plasma. Centralized deposition of the microwave power is possible at high frequencies by use of a launcher with a narrow radiated beamwidth and carefully controlled polarization to couple to electrons at the cyclotron resonant surface. In order for the transmission systems to operate at this high frequency and power level, highly over-moded waveguides have been used to reduce losses and arcing. To produce a narrow, polarized beam, the waveguide system was designed for minimum parasitic mode conversion so that the launcher can operate with nearly a single input mode. Several waveguide components were developed for the waveguide system including: a waveguide mode analyzing directional coupler, a rippled- wall mode converter, improved miter bends, and vacuum pumpout sections. To determine the mode purity of these components and efficiency of the system, laboratory measurement techniques for over-moded waveguide component evaluation were developed. A polarization controlled beam launcher was developed which launches a ~12 cm (-20 dB) beamwidth linearly polarized beam. The plane of polarization can be rotated to allow optimum coupling to either extra-ordinary or ordinary plasma waves. The transmission and launching system performed reliably. Modeling of electromagnetic wave propagation in the ATF plasma and measurement of beam absorption and plasma parameters were performed to determine the overall effectiveness of the ECH system. A coupled-mode wave propagation code was written to investigate the effect of magnetic field shear on the plasma wave modes in ATF. It was shown, under medium plasma density operation, that power absorption is nearly complete and that polarization rotation due to magnetic field shear is influential and must be accounted for in order to maximize first pass absorption in the plasma.

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