Theoretical Analysis and Numerical Simulation of Plasma Behavior in a Strongly Sheared Electric Field.
In experiments on linear plasma device PISCES -A, it is observed that the plasma behaves differently in a single electric shear layer and a double electric shear layer: the plasma fluctuations and the cross-field particle transport are considerably reduced in the latter case. This dissertation is an effort to understand the underlying physics. A preliminary analysis of the PISCES-A results is first performed, suggesting that the Kelvin-Helmholtz instability might be the dominant instability in PISCES -A. Following the preliminary analysis, a fluid analysis of the effects of the electric field geometry on the K -H instability is conducted which, however, indicates that the plasma would be more unstable in the double shear layer case, in contradiction to the experimental findings. Since the fluid analysis fails to explain the PISCES-A results, we employ a kinetic model and begin with the investigation of charged particle motion in a strongly sheared electric field. It is found that the particle orbits differ substantially from the vec Etimesvec B drift approximation when the spatial derivative of the vec Etimesvec B drift speed is comparable to the gyro-frequency of the particle. The particle can either loose its gyration motion, or it may gyrate around a drifting guiding center but with a strongly perturbed gyro-frequency and gyro-orbit as well as a very different guiding center drift. The equilibrium drift velocity of a plasma in a strongly sheared electric field is then investigated by particle simulations. The results show that the vec Etimesvec B drift approximation breaks down in a strongly nonuniform electric field and that the ion flow generated is affected by the shear of the electric field and by the spatial derivative of the shear. The plasma responds very differently in the two shear layer cases. In the double shear layer case, the plasma flow and flow shear are much weaker and the drift is different in magnitude from the expected vec Etimesvec B value. The stability of a plasma in a strongly sheared electric field is finally investigated, taking into account the fact that the actual equilibrium ion flow speed may differ from the vec Etimesvec B drift speed. The plasma is unstable and the behavior of the obtained instability is mainly affected by two factors: the Kelvin-Helmholtz effect and the Collisionless Simon-Hoh effect. In the double shear layer case, the Collisionless Simon-Hoh effect dominates and the resulting instability behaves like a Collisionless Simon-Hoh instability. In the single shear layer case, the instability behaves more like a Kelvin -Helmholtz instability. The growth rate in the single shear layer case is much higher. The growth rate increases significantly in the single shear layer case when electric field is increased while it changes little in the double shear layer case. This is in qualitative agreement with the observations in PISCES -A.
- Pub Date:
- Physics: Fluid and Plasma; Engineering: Nuclear