Low-power fast wave antenna loading as a radio-frequency sheath diagnostic
- Lodestar Research Corporation, Boulder, Colorado 80301 (United States)
Radio-frequency (RF) sheaths induced by the RF {ital E}{sub {parallel}} 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, {ital P}{sub sh}, 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 {ital R}{sub {ital L}} larger than the usual fast wave (FW) loading at very low RF power {ital P}{sub RF} (typically below 100 kW), because {ital P}{sub sh} scales as {vert_bar}{ital E}{sub rf}{vert_bar}, whereas the FW coupled power scales as {vert_bar}{ital E}{sub rf}{vert_bar}{sup 2}. The curve {ital R}{sub {ital L}}({ital P}{sub RF}) has a maximum at {ital P}{sub RF}=0 and rapidly decreases with power until asymptoting at the usual FW loading. The ratio {ital R}{sub {ital L}}(0)/{ital R}{sub {ital L}}({infinity}) 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 [{ital Plasma} {ital Physics} {ital and} {ital Controlled} {ital Nuclear} {ital Fusion} {ital 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. {copyright} {ital 1996 American Institute of Physics.}
- OSTI ID:
- 254917
- Journal Information:
- Physics of Plasmas, Vol. 3, Issue 1; Other Information: PBD: Jan 1996
- Country of Publication:
- United States
- Language:
- English
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