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(FT/P1-14) Performance of a Compact Four-Strap Fast Wave Antenna

S.J. Wukitch1), R.L. Boivin2), P.T. Bonoli1), J.A. Goetz3), J. Hosea4), Y. Lin1), M. Porkolab1), I.H. Hutchinson1), E. Marmar1), G. Schilling4), J.R. Wilson4)
1) MIT Plasma Science and Fusion Center, Cambridge, USA
2) present address: General Atomics, San Diego, CA, USA
3) present address: University of Wisconsin, Madison, WI, USA
4) Princeton Plasma Physics Laboratory, Princeton, NJ, USA

Abstract.  Ion cyclotron range of frequency (ICRF) is expected to be a primary auxiliary heating source in future experiments and fusion reactors. Compact antennas with high power density able to withstand large disruption forces present significant challenges to ICRF antenna design. A compact four-strap antenna has been installed in Alcator C-Mod and its performance has been compared with a pair of two-strap antennas. The key design features are the long vacuum strip line feeds, folded current strap configuration, use of ceramic insulators in the Faraday screen, and open Faraday screen. The heating efficiency and impurity generation are nearly identical to the other antennas while the loading is $ \sim$ 2.5 higher. The power handling of the antenna was limited by arcing at relatively low maximum voltage. The strip line and antenna strap had arc damage localized to regions where the RF E-field was parallel to the tokamak B-field. For E||B, the breakdown voltage was determined to be $ \sim$15 kV/cm. Redesign of the strip line has resulted in an increase in the maximum voltage from 17 kV to 25 kV. Finally, the current strap is being modified to increase the maximum voltage to $ \sim$35 kV.

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IAEA 2003