Plasma Collection by an Obstacle

Citation

Vu, Hoanh Xuan (1990) Plasma Collection by an Obstacle. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ctcn-eq94. https://resolver.caltech.edu/CaltechETD:etd-11152007-131939

Abstract

The problem of plasma collection by an obstacle is investigated systematically to identify potentially important physical effects. In the absence of an ambient toroidal plasma flow, plasma collection by a two-dimensional obstacle of half-width d extending to infinity in the y-direction (slab geometry) is studied in detail. The transport process is taken to be classical. The plasma is assumed to be strongly magnetized. The external magnetic field is assumed to be uniform, and is perpendicular to the obstacle's surface. Our numerical results suggest that both ion viscosity and ion viscous heating can be important in the regions where the ion velocity possesses sharp gradients, e.g., the region near the obstacle's tip. A two-dimensional, semi-empirical, model is proposed to account for the effect of anomalous transport due to a low-frequency, microscopic, electrostatic fluctuation of the poloidal electric field. The obstacle has a half-width of d, and is assumed to extend to infinity in the y-direction. The plasma is assumed to be strongly magnetized. The external magnetic field is assumed to be uniform, and is perpendicular to the obstacle's surface. In general, our proposed model suggests the following: 1. Contrary to that which has been suggested in the literature, the cross-field ion viscosity coefficient (as well as the cross-field ion thermal conductivity) is not enhanced because the cross-field transport is dominated by the fluctuation induced convection. 2. Viscous heating may have an important effect on the ion temperature when there exists a large velocity gradient. Furthermore, in the presence of anomalous transport, the physical mechanism by which viscous heating is generated is quite different from the case where the transport process is classical. In the absence of an ambient toroidal plasma flow, our numerical results suggest that for realistic plasma parameters, the peaked ion temperature is up to 85% higher than the ambient ion temperature due to ion viscous heating. A numerical code based on the above model is developed to deal with the case where the ambient toroidal plasma flow is finite. Such a situation arises in connection with experimental data obtained by the so-called Janus probe or Mach probe. Our numerical results indicate that near the obstacle's tip, the ions on the downstream side are hotter than those on the upstream side.

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