Low-power fast wave antenna loading as a radio-frequency sheath diagnostic

Abstract

Radio-frequency (RF) sheaths induced by the RF E∥ component are a ubiquitous feature of Ion Cyclotron Range of Frequency (ICRF) heating and current drive. An important consequence of RF sheaths is the power dissipation, Psh, caused by ions accelerated in the rectified sheath potential and flowing into material boundaries, such as the antenna structure and nearby limiters. It is shown that the RF sheath-power dissipation yields an antenna loading resistance RL larger than the usual fast wave (FW) loading at very low RF power PRF (typically below 100 kW), because Psh scales as ‖Erf‖, whereas the FW coupled power scales as ‖Erf‖2. The curve RL(PRF) has a maximum at PRF=0 and rapidly decreases with power until asymptoting at the usual FW loading. The ratio RL(0)/RL(∞) is a measure of the average RF sheath voltage on the antenna and nearby limiter surfaces. It is suggested that this technique could be used to measure the RF sheath properties of different antennas or operational regimes (e.g., different phasings) before attempting high-power operation. A generalized loading model is applied to interpret recent measurements on the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 69] for a four-strap antenna with and without a Faraday screen. The possible use of this effect as a diagnostic for the scrape-off layer density is also discussed.