Computation of the effect of neutral collisions on ion current to a floating sphere in a stationary plasma
Abstract
The SpecializedCoordinate Electrostatic Particle and Thermals in Cell (SCEPTIC) code [I. H. Hutchinson, Plasma Physics and Controlled Fusion 45, 1447 (2003)] is used to compute the ion flux to a floating sphere in a stationary plasma, including the effects of chargeexchange collisions with neutrals. The results agree well with the available prior calculations at very low and very high collisionality thus validating the code and those prior calculations. In addition, SCEPTIC provides quantitative results at intermediate collisionalities where no analytic theory is available. Reasonable agreement in the floating potential is found there with some prior numerical results. For {lambda}{sub De} much greater than the sphere radius, enhancements of the ion collection above the orbital motion limited value by factors as high as 5 occur, enough to exceed slightly the radial motion value, but only over a restricted range of collisionality. An explicit analytic fit to SCEPTIC's results is provided for easy evaluation over the entire range of collisionality.
 Authors:
 Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
 Publication Date:
 OSTI Identifier:
 20960126
 Resource Type:
 Journal Article
 Resource Relation:
 Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 1; Other Information: DOI: 10.1063/1.2431584; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
 Country of Publication:
 United States
 Language:
 English
 Subject:
 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; CALCULATION METHODS; CHARGE EXCHANGE; COLLISIONS; COORDINATES; CURRENTS; IONS; PLASMA; PLASMA SIMULATION; SPHERES
Citation Formats
Hutchinson, I. H., and Patacchini, L. Computation of the effect of neutral collisions on ion current to a floating sphere in a stationary plasma. United States: N. p., 2007.
Web. doi:10.1063/1.2431584.
Hutchinson, I. H., & Patacchini, L. Computation of the effect of neutral collisions on ion current to a floating sphere in a stationary plasma. United States. doi:10.1063/1.2431584.
Hutchinson, I. H., and Patacchini, L. Mon .
"Computation of the effect of neutral collisions on ion current to a floating sphere in a stationary plasma". United States.
doi:10.1063/1.2431584.
@article{osti_20960126,
title = {Computation of the effect of neutral collisions on ion current to a floating sphere in a stationary plasma},
author = {Hutchinson, I. H. and Patacchini, L.},
abstractNote = {The SpecializedCoordinate Electrostatic Particle and Thermals in Cell (SCEPTIC) code [I. H. Hutchinson, Plasma Physics and Controlled Fusion 45, 1447 (2003)] is used to compute the ion flux to a floating sphere in a stationary plasma, including the effects of chargeexchange collisions with neutrals. The results agree well with the available prior calculations at very low and very high collisionality thus validating the code and those prior calculations. In addition, SCEPTIC provides quantitative results at intermediate collisionalities where no analytic theory is available. Reasonable agreement in the floating potential is found there with some prior numerical results. For {lambda}{sub De} much greater than the sphere radius, enhancements of the ion collection above the orbital motion limited value by factors as high as 5 occur, enough to exceed slightly the radial motion value, but only over a restricted range of collisionality. An explicit analytic fit to SCEPTIC's results is provided for easy evaluation over the entire range of collisionality.},
doi = {10.1063/1.2431584},
journal = {Physics of Plasmas},
number = 1,
volume = 14,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}

The ion flux on a small attractive sphere (the sphere radius is much less than the Debye length) and the electric potential distribution around the sphere charged in a stationary weakly ionized plasma are calculated by the direct solution of the kinetic equation with the BhatnagarGrossKrook collision term. Only a weak dependence of the ion flux on the electron temperature is found. An analytical approximation of the calculated ion currentvoltage characteristics is proposed for intermediate collisionalities, where neither a collisionless theory nor a continuum fluid approach is valid. This approximation was used for calculation of the floating sphere potential. Amore »

Effect of ion current in the collisionless theory of floating ac probe measurements
Previous treatment of the theory of floating ac probes has considered only the electron current voltage dependence. In this paper the effect of including the voltage dependence of the ion current is examined, and equations are derived which allow the use of the calculations by Laframboise of the ion current to cylindrical probes having arbitrary ratios of probe diameter to Debye length. The error in electron temperature measurements introduced by neglecting the ion current variation, and the range of usefulness of the technique, is discussed. For example, in the normally expected range of floating potential, a measurement of the electronmore »