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High ionisation fraction plasmas in a low temperature, multidipole cusp plasma

Published online by Cambridge University Press:  19 June 2018

V. Désangles ,
J. Milhone ,
C. Cooper ,
D. B. Weisberg ,
M. D. Nornberg  and
C. B. Forest Open the ORCID record for C. B. Forest [Opens in a new window]
V. Désangles*
Affiliation:
Univ Lyon, Ens de Lyon, Univ Claude Bernard Lyon 1, CNRS, Laboratoire de Physique, F-69342 Lyon, France
J. Milhone
Affiliation:
Department of Physics, University of Wisconsin, Madison, WI 53706, USA
C. Cooper
Affiliation:
Department of Physics, University of Wisconsin, Madison, WI 53706, USA
D. B. Weisberg
Affiliation:
Department of Physics, University of Wisconsin, Madison, WI 53706, USA
M. D. Nornberg
Affiliation:
Department of Physics, University of Wisconsin, Madison, WI 53706, USA
C. B. Forest
Affiliation:
Department of Physics, University of Wisconsin, Madison, WI 53706, USA
*
†Email address for correspondence: [email protected]
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Abstract

The depletion of neutral helium atoms has been studied in an unmagnetised spherical plasma created by DC discharge in a multidipole confinement field. Knowing the neutral density profile is critical to predicting the equilibrium flow of such plasmas. A model of the emissivity due to electron-impact excitation of neutral atoms in the plasma has been derived and used to fit radiance measurements of several neutral transitions to extract the radial profile of neutral density for plasmas of varying temperature and density. We report a depletion of the core neutral density varying between negligible levels to 80 % of the edge neutral density depending on the input power and fuelling. The corresponding ionisation fraction varies between 30–80 % in the plasma core. A simple neutral diffusion model is sufficient to describe the shape of neutral density profile implied by the radiance measurements. We have used the measurements to include a drag force due to neutral charge-exchange collisions in simulations of driven plasma flow. The simulation predicts a better fit to Mach probe flow measurements when this neutral drag is accounted for. This work shows that accounting for a realistic neutral profile is important to predict the plasma flow geometry and its magnetohydrodynamics (MHD) stability.

Keywords

plasma diagnosticsplasma dynamicsplasma flows
Type
Research Article
Information
Journal of Plasma Physics , Volume 84 , Issue 3 , June 2018 , 905840312
Copyright
© Cambridge University Press 2018 

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