Can AN Insulator Replace the Faraday Shield of AN RF Antenna?

The Faraday shield (FS) of a fast-wave antenna can be replaced by boron nitride plates (BN) while still obtaining RF heating and reduced RF-edge interaction. BN plates replaced an FS on Phaedrus-T, a medium-sized tokamak using frequencies (f) near the ion cyclotron frequency (f_{ci}). Data taken with several different antenna configurations and under a variety of operating conditions show that the BN plates function better than an FS in terms of reducing the direct interaction of the antenna with the edge plasma. When operating with an FS-covered antenna: large Scrape-Off Layer modifications occur during RF, the loading resistance is high (which indicates parasitic loading) and a large amount of iron is present in the plasma. In contrast, when operating with a BN-surrounded antenna: only minor edge modifications are seen when the RF turns on, the loading resistance is lower and the iron impurity level is reduced by a factor of 20 as compared to the FS-covered antenna. The BN-surrounded antenna coupled RF power to the core plasma. After BN side limiters replaced the FS, eigenmodes were seen in the loading resistance as the line density was ramped up for f > f_{ci }. For the first time in a tokamak, apparent Alfven-wave heating (f < f_{ci}) was observed, thus demonstrating experimentally, at least in this case, that the functions of an FS (which reduces antenna coupling to the edge while coupling to the core) can be better accomplished with BN side limiters around the antenna.