Absence of Magnetic Insulation in Electron MHD
Abstract
In a large laboratory plasma (ne ~= 10^12 cm-3, k Te ~= 2 eV, B0 ~= 30 G, 1 m ⊥ B_0, 2.5 m allel B_0), cross-field currents are collected by a positively biased plane electrode of size large compared to the electron Larmor radius. For rapidly pulsed voltages (ω_ce-1 << t_rise << ω_ci-1), transient electron currents in excess of the electron saturation current are collected. The peak current is essentially independent of the angle between the electrode surface normal and the magnetic field. The current density, J = nabla × B / μ_0, in the vicinity of the electrode is measured with magnetic probes. The large cross-field current is found to be an electron Hall current driven by a space-charge electric field parallel to the electrode surface outside the sheath. The electric field accelerates ions, causes a density depletion in the current channel, and a collapse of the large current on the time scale of an ion transit time across the electrode. In contrast to electron collection along B_0, the dc current exhibits no relaxation oscillations and there are no heating or ionization phenomena since E ⊥ J. These results may be relevant to pulsed probes, tethered electrodes in space, and plasma switches.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- November 1996
- Bibcode:
- 1996APS..DPP..4Q36L