Excitation of the Potential Relaxation Instability in a Weakly Magnetized Discharge Plasma by a Voltage Step

Abstract

An instability is triggered in a weakly magnetized discharge plasma by the application of a positive voltage step to a planar collector immersed into the plasma column with its surface perpendicular to the magnetic field lines. Time developement of the plasma densitytafter the application of the pulse is measured by a Langmuir probe. Radial and axial velocity of the plasma density perturbation are measFed. Radial velocity is consistent with the increase of the plasma potential in the current channel: Axial velocity is very high. It is interpreted as phase velocity of radial quasiperie dic motion of the plasma in and out of the current channel. Response time of the collector current to the applied voltage step is measured versus different parameters. Experimental results are in agreement with a qualitative model presented in previous work [l] where the observed instability is modeled as a two dimensional potential relaxation instability (PEU). Minor improvements of the previous model are proposed. A rarefaction pulse that moves towards the collector is found as the initial stage of the instability.

Experimental Study of the Creation of a Fire-rod I: Temporal Development of the Electron Energy Distribution Function

“…Something similar as described in [33] and [31] happens also in the present experiment. When a positive voltage step is applied to the collector, electrons are instantaneously drawn out of the current channel.…”

“…This is very similar to the mechanism of the so-called potential relaxation instability (PRI) [29,30,34]. The PRI was studied also by our group in our previous experiments [28,31]. We studied also the excitation of the PRI [31] in a weakly magnetized discharge plasma column by the application of positive voltage steps to a collector.…”

“…The PRI was studied also by our group in our previous experiments [28,31]. We studied also the excitation of the PRI [31] in a weakly magnetized discharge plasma column by the application of positive voltage steps to a collector. The experimental set-up was very similar to the one described in this paper, only the amplitude of the applied potential steps was lower, the plasma density was bigger and the neutral gas pressure was lower.…”

“…Similar, only in the axial direction, was observed also by Schrittwieser and Rasmussen [35] in a Q machine. It was found [31] that the velocity of this motion was proportional to √ V a , but would then eventually saturate at the velocity, which was of the order of the ion sound velocity c s = kT e /m i . In the region close to the collector (z ≈ 10 cm) this radial motion was also axially shifted, so that the density depression moved towards the collector.…”

“…The electron saturation current to the collector I a often oscillates around some average value. These oscillations are caused by various plasma instabilities that are triggered when electron an current is drawn through the plasma column [28]- [31]. The initial value of I a is much higher than the later constant value.…”