Currents between tethered electrodes in a magnetized laboratory plasma

Abstract

Laboratory experiments on important plasma physics issues of electrodynamic tethers are performed. These include'cunent propagation, formation of wave wings, limits of current collection, nonlinear effects and instabilities, charging phenomena, and characteristics of transmission lines in plasmas. The experiments are conducted in a large afterglow plasma (100 cm x 200 cm, ne -< 1012 C m-3, kT,, _< 3 eV, Bo < 100 G, Ar, Pn --3 x 10 -4 Torr). The current system is established with a small electron-emitting hot cathode tethered to an electron-collecting anode, both movable across the magnetic field and energized by potential differences up to V = 100 kTJe. The total current density in space and time is obtained from complete measurements of the perturbed magnetic field, J = V x B(r, t)/Bo. The fast spacecraft motion is reproduced in the laboratory by moving the tethered electrodes in small increments, applying delayed current pulses, and reconstructing the net field by a linear superposition of locally emitted wavelets. With this technique, the small-amplitude dc current pattern is shown to form "whistler wings" at each electrode instead of the generally accepted "Alfv6n wings." For the beam electrode, the whistler wing separates from the field-aligned beam which carries no net current. Large-amplitude return currents to a stationary anode generate current-driven microinstabilities, parallel electric fields, ion depletions, current disruptions, and time-varying electrode charging. At appropriately high potentials and neutral densities, excess

Low-frequency sheath instability stimulated by an energetic ion component

“…Basing on the characteristics of the observed instability, it has been assumed that the instability is stimulated by the energetic ion component produced during the resonant absorption of the microwave. It has been shown that this instability differs from the previously reported processes 2,6-10, 12,13,17,18,21 and is characterized by the following features: ͑i͒ oscillations in the probe current occur only when the probe is biased positively with respect to the plasma potential, ͑ii͒ the instability is driven by a flux of suprathermal ions entering the sheath area of the probe, and ͑iii͒ the instability amplitude exhibits saturation, when the energetic ion component is reflected inside the sheath and cannot reach the probe surface.…”

“…[6][7][8][9][10] This deep interest to sheath processes is stimulated mainly by their great importance for every device used in fundamental and applied plasma studies, such as plasma diagnostic techniques, [11][12][13][14] plasma diodes 15 and discharges, 2,9,[16][17][18] antennas in plasmas, 6,7,13,19 current systems in space, [20][21][22][23] and satellite charging processes. [23][24][25] High-frequency sheath-plasma processes, which occur in the vicinity of the local plasma frequency, have been widely studied in laboratory experiments.…”

Dynamics of positive probes in underdense, strongly magnetized, E×B drifting plasma: Particle-in-cell simulations

“…Experimentally, Stenzel et al investigated the plasma dynamics of positive probes (r p < r i ) in MDP in a pulsed non-drifting plasma apparatus. [9][10][11] By using the principle of superposition, they extrapolated their observation of a whistler pulse excited when a bias was applied between a positive probe and an electron emitter to MDP conditions and predicted whistler wings could be excited by positive probes in MDP. In three dimensional electrostatic particle-in-cell (PIC) simulations, Singh et al simulated positive probes (r p < r i ) in MDP and made observations of anomalous current collection, wing-like potential structures, and lower hybrid waves in the ion ram region that they identified as a free energy source for electron heating.…”

The radiation impedance of orbiting conductors

“…Note also that there is no whistler emission (part W on the FM branch of Figure 1); Stenzel and Urrutia's experiments showing whistlers do not apply to an ionospheric tether; they either fail to reproduce the steady condition u> = k x V [Vrrutia and Stenzel, 1989] or correspond to the opposite regime V ~ 2 x 10 7 cm/s>-K A -4x 10 5 cm/s [Stenzel and Urrutia, 1990]. Radiation occurs at the contactors, where V • j s / 0, because it depends on the source divergence kj £ , a fact first noticed by Esies [1988] and clearly arising from the quasi-electrostatic character of the field, E~ -iki/j or -tkj_<#.…”