Measurement of the effects of Faraday shields on ICRH antenna coupling

Compact loop antennas are being applied to several fusion experiments, including DIII-D, the Advanced Toroidal Facility, Tore Supra, and the Tokamak Fusion Test Reactor. Although individual configurations vary, all of these antennas generally comprise a current strap in a recessed box and a Faraday shield. The coupling characteristics of cavity antennas that have current straps with the previously evaluated cross-sectional shapes are tested with several Faraday shields. The coupling is purely a measure of the magnetic flux linkage at relevant ion-cyclotron resonance heating (ICRH) frequencies and does not include plasma spectral effects. Impedances and relative fields are measured for various combinations of the current strap and Faraday shield. The experiments show that the fractional reduction in the magnetic flux linkage to the plasma resulting from the addition of any particular Faraday shield is virtually independent of the shape of the current strap. This is true in spite of the fact that the same mechanism which is responsible for the reduction in flux is also responsible for a significant redistribution of the antenna current on the current strap. Thus the process of optimizing antennas is reduced to that of separately optimizing the current strap and Faraday shield