Probe measurements of electron energy distributions in a strongly magnetized low-pressure helium plasma

Abstract

Electron energy distributions (EED) in strongly magnetized (∼0.3 T), low-pressure (∼0.2 Pa) helium plasmas of the toroidal device “Blaamann” [K. Rypdal et al., Plasma Phys. Controlled Fusion 36, 1099 (1994)] have been measured. In the analysis of measurements, one applies a simplified expression for the limit of a strongly magnetized plasma relating the electron energy distribution to the first derivative of electron probe current with respect to the probe potential. It is shown that for the conditions investigated this approximation gives the same electron densities and slightly lower temperatures (up to 10%) as the kinetic theory for arbitrary magnetic field strength. Cylindrical probes, which are oriented along and perpendicular to the magnetic field, are used in the measurements. It is shown that these probes give nearly identical results. However, the probe that is oriented perpendicular to the magnetic field can give the electron energy distribution in a wider energy range and with better accuracy. The cross-field diffusion coefficient of electrons near the probe is estimated and shown to be classical.

Non-equilibrium quasi-stationary states in a magnetized plasma

“…The potential profile calculated from Eq. (4) agrees very well with the one measured experimentally when electron density measurements (needed to estimate ρ(r )) are made by cylindrical Langmuir probes employing kinetic theory for the electron saturation current in a magnetized plasma (Demidov et al, 1999). This actually serves as a validation of this method of electron density measurement, which yields values more than 3 times higher than results based on the conventional Langmuir formula for unmagnetized plasmas.…”

Modification Of The Electron Energy Distribution Function During Lithium Experiments On The National Spherical Torus Experiment

“…5 can be used such that the first derivative is found to be directly proportional to the distribution function [12,25]. This yields the result that: This equation is a simplification of the full non-local interpretation where it was found that the error in using the approximation is less than 5% when the diffusion parameter is suitably large (ψ 0 > 100) [12].…”

“…In order to address the I-V characteristic in the region beyond the floating potential, a more comprehensive probe theory is sought. In certain circumstances, it can be shown that the non-local approach is usable [12,23,24,25].…”

Study of a scalable large-area radio-frequency helicon plasma source

“…Time-averaged plasma parameters are measured with Langmuir probes compensated against RF potential fluctuations [17]. Plasma density, electron temperature and plasma potential are extracted from the probe characteristics using a kinetic probe theory that takes into account the ambient magnetic field B 0 [18]. For the high density (helicon) discharges, a 160 GHz-interferometer is used to crosscalibrate the plasma density as obtained from the probe characteristics.…”