Ph.D. Proposal Presentation by Dingkang Zhang

Thursday, May 6, 2004

(Dr. Weston M. Stacey, Chair)

"Neutral Particle Transport in Plasma Edge Using Transmission/Escape

Probability (TEP) Method "

__Abstract__

Transmission/Escape probability (TEP) method has been demonstrated to be an efficient and accurate calculation for neutral transport in the tokamak edge and divertor configuration. The obvious advantages of the TEP method are ability to simulate neutral transport problems with complicated configurations, widely varying mean free path and strong particle density gradient. However, the comparisons with Monte Carlo for specially designed problems have also shown that a number of approximations to simplify the TEP methodology limit the applications of TEP method, especially for very short or very large mean free path cases.

In this proposed research, the original TEP methodology, which assumed isotropic
inward and outward neutral distributions (DP-0) in each computation region for
the purpose of calculating transmission probabilities across the region, is
extended to calculate linearly (DP-1) and quadratically (DP-2) anisotropic distributions
incident at each interface for the purpose of calculating transmission probabilities.
To address the non-uniformity of change-exchange neutral sources when the mean
free path is small compared to the dimension of the computation region, two
different methods will be developed to take into account that the escape of
scattered or charge-exchange neutrals is preferentially across the incident
surface. One approach is to use more than one spatial expansion functions per
region, so that local flux gradients can be taken into account. The alternative
approach is to use diffusion theory to calculate the total and directional escape
probabilities. The accuracy and effectiveness of these two methods will be investigated
by comparison with Monte Carlo method. In addition, A linear approximation will
be used to take into account of the gradient of angular flux at interfaces.
The research is expected to provide a fast and accurate method for calculation
of neutral particle transport in the complex tokamak edge and divertor configuration.