Phased operation of the DIII-D FWCD antenna array with a single power source

Abstract

Abstract A phasing and matching system has been designed and implemented for the four-element fast wave current drive (FWCD) antenna array on DIII-D, This system permits phased operation using a single transmitter. Coupled power levels of 1.1 Mw have been reached with relative phasing of ±π/2 and equal magnitudes of current and voltage on all four lines. Use of a single power source requires the achievement of amplitude and phase control at high power, without feedback control of these quantities. The system uses only standard components consisting of transmission lines, unmatched tees, and manually controlled phase shifters and stubs. Phasing, matching, and amplitude control for all four current straps are done using a total of five tuning elements. This simplification is achieved through the use of two resonant loops, each connecting a pair of straps. A tuning algorithm developed for the system produces accurate matching, phasing, and amplitude balance within a small number of shots (≤ 5) in cases where the loading is sufficiently high, that is, when the resonant series load resistance (RSLR) > 2 ω, at values of kQ approaching 1. A coupled transmission line model of the antenna array and resonant loops has been created and used to determine changes in resistive and reactive loading, as well as changes in coupling between array elements during plasma shots. The design and modeling of this system and the operating experience are reviewed.