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Title: Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT)

Abstract

Cold pure electron plasmas confined in Penning-Malmberg traps with mirror fields are known to exhibit density variations along field lines, such that the density is roughly proportional to the magnetic field strength, n{approx}B. The Columbia Nonneutral Torus (CNT) is the first stellarator designed to study pure electron plasmas, and exhibits substantial mirroring, with B{sub max}{approx_equal}1.8B{sub min}. However, results of a three-dimensional equilibrium solver, presented in this Letter, predict a factor of 5.3 increase in density from the minimum-field cross section to the maximum-field cross section along the magnetic axis, for a 1.5 cm Debye length plasma (a{approx_equal}15 cm for CNT). In this Letter, it is shown that the density variation of electron plasmas in mirror traps can be significantly enhanced in a device that has a cross section that varies from cylinder-like to slab-like, such as the CNT. A simple analytic expression is derived that describes the axial density variation in such a device, and it is found to agree well with the computational predictions for CNT.

Authors:
;  [1]
  1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027 (United States)
Publication Date:
OSTI Identifier:
20860416
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 12; Other Information: DOI: 10.1063/1.2405341; (c) 2006 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; DEBYE LENGTH; ELECTRONS; EQUILIBRIUM; MAGNETIC FIELDS; MAGNETIC MIRRORS; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; RADIATION TRANSPORT; STELLARATORS; TRAPS; VARIATIONS

Citation Formats

Lefrancois, Remi G., and Pedersen, Thomas Sunn. Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT). United States: N. p., 2006. Web. doi:10.1063/1.2405341.
Lefrancois, Remi G., & Pedersen, Thomas Sunn. Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT). United States. doi:10.1063/1.2405341.
Lefrancois, Remi G., and Pedersen, Thomas Sunn. Fri . "Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT)". United States. doi:10.1063/1.2405341.
@article{osti_20860416,
title = {Large density variation predicted along the magnetic axis for cold electron plasmas in the Columbia Nonneutral Torus (CNT)},
author = {Lefrancois, Remi G. and Pedersen, Thomas Sunn},
abstractNote = {Cold pure electron plasmas confined in Penning-Malmberg traps with mirror fields are known to exhibit density variations along field lines, such that the density is roughly proportional to the magnetic field strength, n{approx}B. The Columbia Nonneutral Torus (CNT) is the first stellarator designed to study pure electron plasmas, and exhibits substantial mirroring, with B{sub max}{approx_equal}1.8B{sub min}. However, results of a three-dimensional equilibrium solver, presented in this Letter, predict a factor of 5.3 increase in density from the minimum-field cross section to the maximum-field cross section along the magnetic axis, for a 1.5 cm Debye length plasma (a{approx_equal}15 cm for CNT). In this Letter, it is shown that the density variation of electron plasmas in mirror traps can be significantly enhanced in a device that has a cross section that varies from cylinder-like to slab-like, such as the CNT. A simple analytic expression is derived that describes the axial density variation in such a device, and it is found to agree well with the computational predictions for CNT.},
doi = {10.1063/1.2405341},
journal = {Physics of Plasmas},
number = 12,
volume = 13,
place = {United States},
year = {Fri Dec 15 00:00:00 EST 2006},
month = {Fri Dec 15 00:00:00 EST 2006}
}
  • Significant variations in the density and potential along the axis of a pure electron plasma in the Columbia Non-neutral Torus (CNT) stellarator have now been measured. Large variations along the magnetic field are predicted by three-dimensional equilibrium reconstructions of CNT plasmas and by simple electrostatic and geometric arguments [Lefrancois and Pedersen, Phys. Plasmas 13, 120702 (2006)]. The density variation, n{sub axis,{phi}}{sub =0} {sub deg.}/n{sub axis,{phi}}{sub =90deg.}, is measured directly for several different plasma equilibria, and has a median value of 7.8, consistent with the predicted density variation of 4.4, because the error bars are large. The associated variation in potentialmore » predicted from the Boltzmann relation, e{delta}{phi}/T{sub e}=ln(4.4)=1.5, was also measured experimentally. The median measured, e{delta}{phi}/T{sub e}, was 1.1, which is of the predicted sign and in rough agreement with the measurements, but smaller than predicted. The difference is statistically significant, but might be related to the imperfect numerical modeling of the complicated electrostatic boundary conditions in CNT. The measured variations reconfirm that the Debye lengths of these plasmas are small.« less
  • The Columbia Nonneutral Torus is a new stellarator experiment being built at Columbia University, New York, to study the confinement of nonneutral and electron-positron plasmas. It will be a two-period, ultralow aspect ratio classical stellarator configuration created from four circular coils. The theory of the confinement and transport of pure electron plasmas on magnetic surfaces is reviewed. The guiding principles behind the experimental design are presented, together with the actual experimental design configuration.
  • For the first time, nonconstant space potential {phi}{sub s} and electron density n{sub e} on magnetic surfaces of helical nonneutral plasmas are observed experimentally. The variation of {phi}{sub s} grows with increasing electron injection energy, implying that thermal effects are important when considering the force balance along magnetic field lines. These observations confirm the existence of plasma equilibrium having nonconstant {phi}{sub s} and n{sub e} on magnetic surfaces of helical nonneutral plasmas.
  • The first studies of pure electron plasmas confined on magnetic surfaces in the Columbia Non-neutral Torus are overviewed. The electron plasma is created by a thermionic emitter filament and similar filaments mounted on ceramic rods are used as Langmuir and emissive probes. The equilibrium density, temperature and potential profiles are experimentally measured. Numerical calculations of the equilibrium agree well with measurements and also predict a toroidal density variation of a factor of four. The confinement time is found to decrease with increased neutral pressure and emitter bias voltage, and it is presently limited to 20 ms by the insulated emittermore » and probe rods. A retractable electron emitter and external diagnostics will be used to determine the confinement time in the absence of rods. Ion driven instabilities are observed at high neutral pressure and low magnetic field strength. Further research of these instabilities will be carried out.« less