Study of the Radio-frequency Driven Sheath in the Ion Cyclotron Slow Wave Antennas

The electrostatic Faraday screen(FS) is generally used to suppress the strong electrostatic interaction between the ICRF antenna and edge plasmas near the antenna. It is recognized that FS suppresses the electrostatic field parallel to the magnetic field(E//), and particle and heat fluxes into the antenna current strap effectively. Even with FS, however, impurity generation from FS is a severe problem in high power ICRF heating experiments.[1],[2] Recent researches on the ICRF antenna-edge plasma interaction have shown that the RF driven sheath on the FS has a key role on the impurity generation form FS by ion sputtering. [3]~[6] The ion flow accelerated in the RF driven sheath causes an additional heat load on the antenna structure and gives a power loss in the RF system, transferring energy the RF field to the antenna material surface through the ion bombardment. In addition, the RF driven sheath may bias the edge plasma potential near the ICRF antenna and drive steady state convective cells in the scrape-off layer. These induced convective cells are possible to modify the particle and heat transport in the scrape-off layer .[7],[8] So far, the effects of RF driven sheath on the ICRF antenna-plasma interaction are studied theoretically and analyzed using experimental data of high power ICRF heating in large tokamaks. In the present experiment using a linear plasma device, both the DC and RF voltages on the antenna current strap and FS of the loop antenna for ICRF slow wave heating are measured and compared with theoretical estimation. Simultaneously the plasma heat load to the antenna current strap and FS is measured with calorimetry to obtain the additional RF power dissipation by the RF induced sheath. The net radiated RF power for the excitation of ICRF slow waves is estimated from the conventional RF power measurement and the antenna heat load.