Single-ended Q-machine Research Articles

Nonlinear effects in the current-voltage characteristic of a low-density Q-machine plasma: II. Related to the electrostatic ion-cyclotron instability

In a single-ended Q-machine the current-voltage characteristic of a small collector shows a sudden jump of the current at the onset of the electrostatic ion-cyclotron instability. This jump was also accompanied by hysteresis. This is explained as an indication that the well known nonlinear potential structures that appear periodically in front of the collector during one cycle of a high-amplitude electrostatic ion-cyclotron instability, originate from collective effects of excitation and ionization interactions produced in front of the cold plate where accelerated electrons suffer inelastic collisions with the alkaline vapour present in the plasma column.

Nonlinear effects in the current-voltage characteristic of a low-density Q-machine plasma: I. Related to the potential relaxation instability

We report on an investigation in the magnetized alkaline plasma of a single-ended Q-machine where the current-voltage characteristic of the cold end plate showed some new nonlinear features. The most interesting phenomena are sudden jumps of the electron saturation current which are partly accompanied by hysteresis. These new phenomena are best visible for low densities. The current jumps are explained as an indication that the well known oscillating nonlinear potential structures, which appear in the plasma column when an electron current is drawn through it, and which are known as the potential relaxation instabilities, originate from collective effects of excitation and ionization interactions produced in front of the cold plate where accelerated electrons suffer inelastic collisions with the alkaline vapour present in the plasma column.

The effect of the collector sheath on the potential relaxation instability

The potential relaxation instability (PRI) can be excited in the magnetized plasma column of a single-ended Q-machine by drawing an electron current to a collector. This can either be the terminating tungsten plate (the cold plate (CP) or a grid (G) inserted in between. The frequency of the PRI, which according to the standard model should be solely determined by the length of the system, in fact also depends on the collector bias, the plasma density and the magnetic field. If, however, the reduction of the system length due to the electron-rich sheath in front of the collector is taken into account, in a certain range the PRI frequency becomes more independent of the external parameters.

Potential relaxation instability and ion acoustic waves in a single-ended Q-machine dusty plasma

The potential relaxation instability (PRI) is excited when the cold endplate of a single-ended Q-machine is biased positively. If a sufficient concentration of negatively charged dust is present, the PRI is eliminated and the current-driven ion acoustic (IA) instability is observed. As the concentration of the negatively charged dust is increased, the frequency of the IA waves also increases, in agreement with theoretical predictions.

Confinement of dust particles in a double layer

Negatively charged, micron-sized dust grains have been electrostatically confined in the high-potential region of an anode double layer formed in a single-ended Q-machine plasma. The levitated dust grains are large enough to be visually observed in the scattered light from the plasma source. The various forces on a dust grain in a plasma, i.e., gravitational, electric, ion drag, and neutral drag are estimated based on the measured plasma and dust parameters. The possibility of observing a Coulomb lattice in such a setup is discussed.

Effects of a radial electric field on low frequency instabilities in a magnetized plasma

In a single-ended Q-machine the effect of a radial electric field on current-driven low frequency instabilities is investigated experimentally. The field is produced by biasing a cylindrical wall surrounding the plasma column. When the wall potential is positive, the low frequency potential relaxation instability is effectively suppressed, and the current is not limited by anomalous resistivity. In that regime a new type of instability is found with a higher frequency that increases proportionally to the wall potential. When the wall potential is negative, the potential relaxation instability is enhanced, and a strong modulation and periodic limitation of the current occurs. The results show that the potential relaxation instability is well controlled by the wall potential, an effect which might be relevant for achieving a stable high current in a plasma. The investigations also shed more light on the mechanism of the one-dimensional potential relaxation instability.

Electrostatic ion cyclotron waves driven by a radial electric field

An experiment was conducted in a single-ended Q-machine in which a voltage was applied to an electrode of variable diameter. For electrode diameters D<10 rho i, field-aligned current-driven ion cyclotron waves were destabilized. If the electrode diameter was increased, a higher-frequency, lower-amplitude ion cyclotron wave was observed. The dispersion, mode amplitude and propagation characteristics of this mode were measured. The mode, which propagates azimuthally, was found to be driven by the radial electric field which exists at the edge of the current channel created by the biased electrode.

Potential relaxation instability and ion acoustic waves in a single-ended Q-machine plasma with negative ions

The potential relaxation instability (PRI) is excited when the cold endplate of a single-ended Q-machine is biased positively. If a sufficient concentration of negative ions (SF - 6) is present, the PRI is eliminated and the current-driven ions acoustic (IA) instability is observed. As the negative ion concentration is increased, the frequency of the IA waves increases, and the critical electron drift needed for their excitation decreases.

Quenching and amplification of the potential relaxation instability by control of the radial electric field

The effect of a radial electric field on the potential relaxation instability (PRI), excited by a positively biased electrode in a single-ended Q-machine, is investigated experimentally. When the radial electric field E r in the boundary region between the bulk of the plasma and the thin plasma outside is directed inward, the PRI is suppressed, and a maximum stable axial current flows. However, an outward pointing E r leads to an amplification of the PRI due to the enhancement of the radial ion loss, giving rise to a strong current chopping.

Space potential measurements with a continuously emitting probe

Space potential measurements with a continuously emitting probe suffer from the difficulty of a potential drop across the probe tip. In this paper we derive a simple method to ensure that the measurements are accurate in spite of this potential drop. Experimental results obtained in a single-ended Q-machine confirm the theoretical findings and provide reassuring evidence that the potential drop does not disturb the space potential unduly.

An experimental investigation on the influence of neutral collisions on the current-driven electrostatic ion-cyclotron instability

An experimental investigation on the influence of ion/electron-neutral collisions on the electrostatic ion-cyclotron instability was performed in a single-ended Q-machine plasma. The collision frequency was controlled by varying the neutral back-ground pressure between 10−4 Pa and 1 Pa. Various noble gases (He, Ar, Kr and Xe) and nitrogen were employed. In all cases the instability was almost quenched when the neutral background pressure passed a critical value which increased for decreasing cross-section of ion-neutral collisions. The ion-neutral collision frequency at this critical pressure was much smaller than the ion gyrofrequency. However, the mean free path at critical pressure was shorter than the length of the plasma column. Under these conditions the results are consistent with the interpretation of the ion-cyclotron instability as a two-dimensional potential relaxation oscillation.

Electrostatic ion-cyclotron instability and potential relaxation instability excited by a ring-button electrode

Instabilities are studied which are excited by drawing an electron current to a button, surrounded by a concentric, axially moveable, floating ring in a single-ended Q-machine. Only the electrostatic ion-cyclotron instability is observed as long as the ring is behind the button or not farther than about two ion pitches in front of it. A strong enhancement of the ion-cyclotron instability occurs if the distance between button and ring is exactly one ion pitch. For larger distances the potential relaxation instability appears between the ring and the button.

The Electrostatic Ion-Cyclotron Instability Excited by a Current to a Strip Collector

The electrostatic ion-cyclotron instability is excited in a single-ended Q-machine by drawing an electron current to a strip collector crossing the entire plasma column. The instability is most easily excited when the width of the strip is a few ion Larmor radii. The instability is confined within the current channel, defined by the projection of the strip collector.

A comment on 'interaction of lower hybrid waves with the current-driven ion-acoustic instability'

Majeski et al. (1984) have investigated the interaction between the current-driven 'ion-acoustic' instability and high frequency lower hybrid waves. The 'ion-acoustic' instability was excited by drawing an electron current through the plasma column of a single-ended Q-machine by means of a positively biased cold plate. Schmittwieser et al. do not believe that the observed instability is of the ion-acoustic type but that it is rather the so-called potential relaxation instability.

Determination of the frequency-controlling region of the current-driven electrostatic ion-cyclotron instability

A small, axially moveable test coil is used to locally increase the magnetic field of a single-ended Q-machine. The influence of this additional field on the current-driven electrostatic ion-cyclotron instability (EICI) depends on its distance from the collector. Three different regions are found, and in the middle region, at a distance of several cm from the collector, the local magnetic field determines the instability frequency and amplitude along the entire column. When the local increased magnetic field is in the region nearest to the collector, however, the EICI is damped and its frequency is decreased.

Electrostatic ion-cyclotron instability driven by a slow electron drift

The electrostatic ion-cyclotron instability can be excited in a single-ended Q-machine by a positively biased collector. According to infinite plasma theory the instability is triggered by the drifting electrons. Recently this explanation has been questioned since experiments by another group have shown that the excitation takes place solely near the collector. The excitation has been ascribed to the collector sheath which has strong axial and radial gradients. The authors try to prove that drifting electrons are able to excite the ion-cyclotron instability. They produce an electron drift without the formation of a localized two-dimensional sheath. To that end the plasma electrons are delivered by a source which is spatially separated from the ion source. If the central section of these electrons is slightly accelerated by a grid they observe the instability.

Can the electrostatic ion-cyclotron instability by driven by a two-dimensional sheath?

An experiment in a single-ended Q-machine was conducted in which a voltage was applied between two concentric floating electrodes which terminated the plasma column. When the inner electrode was more than 200 V positive with respect to the outer one the electrostatic ion-cyclotron instability was observed. In the usual case the electron current is drawn between the grounded (plasma generating) hot plate and a positively biased electrode, with only a small voltage needed to trigger the instability. However, the current threshold is the same for both cases, indicating that it is a critical electron drift speed, not a sheath effect, which is necessary for the instability.

Interaction of lower hybrid waves with the current-driven ion acoustic instability

Experiments performed in a single-ended Q-machine examine the effect of a large-amplitude electrostatic lower hybrid wave on the dispersion of the current-driven ion acoustic instability. Nonlinear coupling of the modes provides a mechanism for suppression of the ion acoustic instability through a reduction in ion-acoustic mode phase velocity and an associated increase in ion Landau damping. An analysis which includes the effects of electron collisional or transit-time damping on the interaction is presented. This analysis demonstrates the resonant character of the interaction and allows quantitative evaluation of the lower hybrid power levels necessary for stabilization of the ion acoustic mode.

About localization and suppression of the so-called ion-acoustic instability in a low-density single-ended Q-machine

The so-called ion-acoustic instability in a collisionless single-ended Q-machine is suppressed when the (positive) dc bias of the exciting electrode exceeds a certain threshold value. The latter increases with increasing plasma density and/or distance between the hot plate and the exciting electrode.

Axial equilibria, disruptive effects, and Buneman instability in collisionless single-ended Q-machines

The plane-diode model is used to study the equilibrium properties of a collisionless single-ended Q-machine over the most relevant parameter regions. A method is given for determining parameter points, or points of operation, which enables straightforward application of the theoretical results to specific experimental situations. A new disruptive effect is predicted which manifests itself in discontinuous changes in the equilibrium properties with a continuous variation of the neutral flux. As an example of application, a simplified analysis of the strong instability occurring at positive cold-plate biases is presented.