Radial Ion Transport in a Limited Axisymmetric Ecr Plasma.
Abstract
An experimental study of the radial transport of ions in the presence of a limiter for an axisymmetric plasma sustained by the resonant absorption of 2.45 GHz whistler waves is presented. In steady state, approximately 30% of the particles ionized in the core plasma flow radially into the scrape-off layer. The magnitude of radial ion flux is observed to be bounded from below by the radial electron transport rate and from above by the classical collisional ion transport rate of the cold edge ions. The system proves to be susceptible to a Kelvin-Helmholtz instability. The transport experiments were performed at a set input power of 300 W and over the neutral pressure range of 1-3 mTorr in Argon gas. The bulk ions possess a typical temperature of 1eV and do not obey a diffusion equation in the core plasma. Strong radial electric fields are observed and appear to provide the principal means by which radial ion diffusion is controlled. The presence of strongly sheared electric fields in this system excites a hydrodynamic instability of the Kelvin-Helmholtz type. The most frequently observed mode is driven by a region of depressed potential at the plasma edge. A theory for instability in the presence of a localized inverted Gaussian radial potential is presented and compared with experimental measurements. The convection of edge plasma into the core under influence of this mode leads to an increased edge temperature and, hence, enhanced axial losses from the scrape-off layer, thereby modifying the radial profile of the scrape-off layer. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).
- Publication:
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Ph.D. Thesis
- Pub Date:
- January 1995
- Bibcode:
- 1995PhDT........33G
- Keywords:
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- Physics: Fluid and Plasma