Coupling a Boltzmann plasma and BCA surface for the kinetic treatment of plasma-material interactions

Keniley, Shane M

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https://hdl.handle.net/2142/98432 Copy

Description

Title

Coupling a Boltzmann plasma and BCA surface for the kinetic treatment of plasma-material interactions

Author(s)

Keniley, Shane M

Issue Date

2017-07-20

Director of Research (if dissertation) or Advisor (if thesis)

Curreli, Davide

Committee Member(s)

Uddin, Rizwan

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Boltzmann solver
• Model coupling
• Plasma-surface interactions
• Binary collision approximation

Abstract

A near-wall plasma and material surface form a volatile region involving surface erosion, impurity ionization, and redeposition, creating a far-from-equilibrium system of mutually interacting plasma and impurity species. As impurity recycling is expected to play a major role in the long-term performance of plasma-facing components in magnetic fusion devices, modeling of the plasma- surface interface is required to predict the behavior of both the material surface and the near-wall plasma. In this work, a method of simulating plasma-material interactions by dynamically coupling a continuum Boltzmann plasma model to a Monte-Carlo surface model is presented. The model is based on a multi-species Boltzmann solver for the plasma using finite difference methods. Von Neumann stability analysis of the Runge-Kutta time discretization with upwind-biased numerical schemes are detailed up to fourth-order accuracy, and the errors associated with each scheme are quantified. A modification to the classical binary-collision approximation code TRIDYN is utilized to model the surface, which was treated as a boundary condition to the plasma model. The Boltzmann solver calculates the ion energy-angle distribution and density of ions striking the surface that are needed as input to the BCA code, and density estimation is used to reconstruct a velocity distribution to be passed back to the Boltzmann solver. Both plasma ions and impurities are treated as Boltzmann kinetic species, allowing high resolution even at very disparate densities, particle fluxes, drift velocities, and energy fluxes. The plasma model is shown to be capable of resolving features of Landau damping with matching theoretical and calculated damping rates of 0.1534, and the two-stream instability is shown to have an energy peak at 18 tωp. Convergence of the plasma sheath problem is established utilizing the fourth-order upwind finite difference method. Numerical density estimation techniques are applied to construct velocity distributions from discrete data samples provided by TRIDYN, and a sputtered particle sample size of 1000 is shown to constrain the mean integrated squared error of the density-estimated velocity distribution to O(10^−1). As a proof-of-concept of the coupling method, an example calculation of a helium plasma facing a beryllium wall is reported in both unmagnetized and magnetized conditions, recording the evolution of the phase spaces of ions, neutrals, and material impurities in the near-wall region at nominal ITER conditions.

Graduation Semester

2017-08

Type of Resource

text

Permalink

http://hdl.handle.net/2142/98432

Copyright and License Information

Copyright 2017 Shane Keniley

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