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Development and validation of a two-fluid plasma-neutral model

Author: Eric T. Meier
Requested Type: Consider for Invited
Submitted: 2011-06-10 10:59:40

Co-authors: U. Shumlak, R. D. Milroy, D. Kirtley, J. Slough

Contact Info:
University of Washington
3205 Franklin Ave. E
Seattle, WA   98102

Abstract Text:
A tractable, useful model for capturing neutral effects in plasmas has been a long-standing goal of the PSI-Center. To this end, a two-fluid model for interacting and reacting plasma and neutral fluids has been derived by taking moments of the Boltzmann equation. Separate density, momentum, and energy equations for the two (plasma and neutral) fluids are derived in addition to a generalized Ohm’s law for electromagnetic evolution. The approach is similar to that taken by Braginskii [1] to produce the single-fluid MHD model, but is generalized to include a neutral fluid and associated resonant charge exchange, electron-impact ionization, and radiative recombination reactions. Electron mass is neglected. A single-species, optically thin partially ionized gas is assumed. Excited states are not tracked. Temperature-dependent ionization and recombination reactions are modeled. The resonant charge exchange implementation accurately accounts for the statistical average of the relative ion-neutral particle velocities for each fluid moment. Braginskii closures are used for the plasma, and Chapman-Enskog hard sphere closures are used for the neutral gas. This plasma-neutral model is implemented in the HiFi implicit spectral element code [2,3]. Validation testing has been conducted by applying the implementation to the Electrodeless Lorentz-Force (ELF) thruster experiment. In ELF, an RMF-formed FRC is accelerated, and ejected from a conical vessel. ELF is intended to have a long operational lifetime while operating at high power density and efficiency. An AFOSR program, Neutral Entrainment, is investigating the downstream interaction of this FRC with an injected neutral gas. As the FRC is accelerated down a cylinder by the peristaltic action of external coils, it couples strongly, primarily through charge exchange, to a field of neutral gas that has been injected ahead of the FRC. The tight coupling of the FRC to the neutral gas allows efficient acceleration of neutral gas, enhancing thruster efficiency by providing more thrust for a given ionization energy investment than a conventional electric propulsion device. Five key parameters for the ELF thruster are: FRC velocity, FRC temperature, FRC density, neutral gas profile, and neutral gas density. The modeling effort, involving simulations of neon FRC interaction with neon neutral gas, has provided insight for optimizing these parameters to provide good coupling of the FRC to the neutral gas while limiting wall damage. Quantitative comparison to experimental results will be presented.
[1] S.I. Braginskii, Rev. Plasma Phys., 1 (1965) 205
[2] A.H. Glasser, X.Z. Tang, Comput. Phys. Commun. 164 (2004) 237
[3] V.S. Lukin, PhD thesis, Princeton University, 2008

Characterization: A7


University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011