Magnetized Laboratory Plasmas Research Articles

On the role of ion temperature anisotropy in the growth and propagation of shear‐modified ion‐acoustic waves

Oblique ion‐acoustic waves, excited by the combination of magnetic field‐aligned (parallel) electron drift and sheared parallel ion flow, are investigated in magnetized laboratory plasma that is characterized by ion temperature anisotropy. Direct measurements of the parallel and perpendicular ion temperatures, parallel and perpendicular ion drift velocities, electron temperature and parallel electron drift velocity, parallel and perpendicular wave vector components, and mode frequency and growth rate are used to elucidate the shear‐modified ion‐acoustic (SMIA) instability mechanism and document an observed correlation between ion temperature anisotropy and wave propagation angle. Experimental measurements show that anisotropy significantly influences the growth rate and propagation angle of oblique ion‐acoustic waves. These results support the ion‐acoustic wave interpretation of broadband waves in the auroral energization region where shear and anisotropy are known to exist and may have ramifications for many space plasmas in which anisotropy exists in the electron temperature or ion temperature.

Observation of inverse ion-cyclotron damping induced by parallel-velocity shear.

The generation of broadband multiharmonic spectra of electrostatic ion-cyclotron waves is demonstrated in a magnetized laboratory plasma in which shear in the magnetic-field-aligned (parallel) ion flow and a relative parallel electron drift are present. Shear correlates with an increased number of harmonics and a decreased electron drift speed. Wave and particle measurements indicate that cyclotron damping is reduced and even becomes negative. The fluctuations in the time domain are spiky, similar to electric-field fluctuations observed both in Earth's auroral zone and in numerical simulations.

Experimental verification of the shear-modified ion-acoustic instability.

The predicted shear-induced shift of the wave phase velocity, the essence of the shear-modified ion-acoustic (SMIA) instability mechanism that reduces ion Landau damping for otherwise damped ion-acoustic waves [V. Gavrishchaka et al., 80, 728 (1998)], is verified with direct measurements in a strongly magnetized laboratory plasma. The SMIA growth rate is shown to increase with increasing shear, as predicted. SMIA wave propagation is shown to be possible at both small and large angles to the magnetic field, consistent with space observations of ion-acoustic-like waves.

Interest of electron–ion bremsstrahlung for plasma physics

The achievements of Prof. Pratt are astounding, even when restricting the attention to electron bremsstrahlung. He has worked out refined theories and codes to determine the cross-sections in the relativistic regime, including the dependences on angles and polarization. He has discussed the validity of simpler calculations. His deep understanding and the data he and his coworkers have tabulated have paved the way to many studies in plasma physics. The features of bremsstrahlung of highly dense plasmas were identified. For less dense and most often non-Maxwellian plasmas, the spectra can be analyzed to extract the electron distribution function. Examples are taken mostly from laser plasmas and magnetized laboratory plasmas.

Dynamical study and control of drift waves in a magnetized laboratory plasma

The various dynamical regimes of collisional drift waves in a magnetized plasma column are experimentally studied. These unstable low-frequency electrostatic waves are related with strong modulations of the ion and electron density. The angular velocity of the rotating plasma column is the control parameter of the dynamics: regular, chaotic and turbulent regimes are obtained. The spatial extension of the system allows for the occurrence of spatiotemporal chaos. The time-delay auto-synchronization method of controlling chaos [K. Pyragas, Phys. Lett. A 170, 421 (1992)] though purely temporal is successfully applied. A numerical study using coupled nonlinear oscillators exhibiting chaos is compared to the experimental results. The control method is tested on this model.

Control of the chaotic regimes of nonlinear drift-waves in a magnetized laboratory plasma

Nonlinear drift-waves are experimentally studied in a cylindrical magnetized laboratory plasma. Low-dimensional chaotic regimes are controled using the time-delay autosynchronization method [K. Pyragas, Phys. Lett. A 170, 421 (1992)]. A significant reduction of spectral width in the power spectrum is obtained.

Laboratory Observations of Shear Alfvén Waves Launched from a Small Source

Alfven waves are known to be very important in the dynamics of magnetized plasmas in space and in laboratories on earth [1]. They transport electromagnetic energy, may accelerate plasma particles, and are produced in large, dense, magnetized laboratory plasmas or in almost any space plasma environment where there are changes in plasma currents or magnetic field configuration. The experiments described in this Letter are concerned with the shear mode of the Alfven wave launched from sources with transverse scale comparable to the electron collisionless skin depth, d s› cyvped. The shear wave propagates at frequencies below the ion cyclotron frequency svcid, and the associated currents are divergenceless s= j › 0 d with the currents parallel to the background magnetic field, B0, carried by electrons while ions carry the currents transverse to the field through the polarization drift. In a plasma where the ion temperature is much smaller than the electron temperature sTi ? Ted as is typical of the Large Plasma Device (LAPD) discharge plasma, the dispersion relation for shear Alfven waves is [2]

Pulsed currents carried by whistlers. IX. In situ measurements of currents disrupted by plasma erosion

In a magnetized laboratory plasma described in the companion paper [Stenzel and Urrutia, Phys. Plasmas 4, 26 (1997)], a large positive voltage step (V≫kTe/e) is applied to electrodes. The current front propagates in the whistler mode in the parameter regime of electron magnetohydrodynamics. The topology of the current density is that of nested helices. Large transient currents in excess of the electron saturation current can be drawn. A transient radial electric field associated with the current rise, excites a compressional, large amplitude, radially outgoing sound wave, which leaves the current channel depleted of plasma. The current collapses due to the density erosion. Electric field reversal excites a rarefaction wave which leads to a partial density and current recovery. Periodic plasma inflow and outflow cause the current to undergo strong relaxation oscillations at a frequency determined by the electrode diameter and the sound speed. In addition, a broad spectrum of microinstabilities is observed in regions of high current density. For drift velocities approaching the thermal speed, the spectrum extends beyond the ion plasma frequency (ωpi) up to the electron plasma frequency (ωpe). Correlation measurements above ωpi reveal modes propagating along the electron drift at speeds above the sound speed but well below the electron drift speed.

Pulsed currents carried by whistlers. VIII. Current disruptions and instabilities caused by plasma erosion

In a large magnetized laboratory plasma (n≃1012 cm-3, kTe⩾1 eV, B0⩾10 G, 1 m × 2.5 m), the transient processes of switch-on currents to electrodes are investigated experimentally. The current rise time lies between the ion and electron cyclotron periods (electron magnetohydrodynamics). The initial current scales linearly with applied voltage and is not limited by the electron saturation current of the positive electrode, but by the ion saturation current of the return electrode. The collection of electrons in the flux tube of the positive electrode gives rise to a space charge electric field, which expels the unmagnetized ions, erodes the density, and disrupts the current. Repeated current oscillations arise from a feedback between current, density, and potential oscillations. The dependence of the transient and unstable electrode currents on externally variable parameters is investigated in the present paper. A companion paper [Urrutia and Stenzel, Phys. Plasmas 4, 36 (1997)] presents in situ measurements of plasma currents, plasma parameters, and microinstabilities. These results are relevant to the physics of pulsed Langmuir probes, current collection from tethered electrodes in space, and plasma erosion switches.

Nonreciprocal magnetoplasma sheath waves

We present theoretical and experimental results relevant to azimuthal magnetoplasma sheath waves in the case of the magnetic field parallel to the axis of a thick conducting cylinder. For the theory, a planar geometry is used with the ambient magnetic field parallel to the conducting plane and propagation perpendicular to the magnetic field. The results show two low‐frequency transverse magnetic (TM) modes propagating in opposite directions with different propagation constants and also different cutoff frequencies separated by the cyclotron frequency. At frequencies above the upper hybrid frequency, one unidirectional TM mode is found. No transverse electric (TE) modes have been found. The experimental setup consists of a metallic cylinder with two protruding radial monopole antennas separated by an angle of 90°, placed in an axially magnetized laboratory plasma. Measurements of the two‐port S parameters clearly show nonreciprocity caused by the propagation of different wave modes in opposite directions.

Pulsed currents carried by whistlers. VI. Nonlinear effects

In a large magnetized laboratory plasma (n≂1011 cm−3, kTe≥1 eV, B0≥10 G, 1 m × 2.5 m), current pulses in excess of the Langmuir limit (150 A, 0.2 μs) are drawn to electrodes in a parameter regime characterized by electron magnetohydrodynamics (ωci≪ω≪ωce). The transient plasma current is transported by low-frequency whistlers forming wave packets with topologies of three-dimensional vortices. The generalized vorticity, Ω, is shown to be frozen into the electron fluid drifting with velocity v, satisfying ∂Ω/∂t≂∇×(v×Ω). The nonlinearity in v×Ω is negligible since v and Ω(r,t) are found to be nearly parallel. However, large currents associated with v≥(2kTe/me)1/2 lead to strong electron heating which modifies the damping of whistlers in collisional plasmas. Heating in a flux tube provides a filament of high Spitzer conductivity, which permits a nearly collisionless propagation of whistler pulses. This filamentation effect is not associated with density modifications as in modulational instabilities, but arises from conductivity modifications. The companion paper [Stenzel and Urrutia, Phys. Plasmas 3, 2599 (1996)] shows that, after the decay of the transient wave magnetic field, magnetic helicity remains in the plasma due to temperature-gradient driven currents.

Nonlinear Phenomena Associated with Large Amplitude Whistler Pulses

In a magnetized laboratory plasma (n 10 11 cm -3 , kT e ≥ 1 eV, B 0 ≥ 10 G, 1 m diam x 2.5 m) large amplitude current pulses (150 A, 0.2 μs) are excited in the parameter regime described by Electron MHD (EMHD ; ω ci << ω << ω ce ). The currents are transported by low-frequency whistlers forming wave packets with topologies resembling 3D spherical vortices. The generalized vorticity, Ω = ⊇ x (v + eA/m e ), is shown to be frozen into the electron fluid, ∂Ω/∂t ⊇ x (v x Ω). The nonlinearity in v x Ω is negligible since v and Ω(r,t) are found to be nearly parallel. Thus, large amplitude pulses [Ω(r, t) ≤ Ω 0 = eB 0 /m e ] show the same behavior as small amplitude pulses. However, the associated large currents with v drift ≥ (kT e /me) 1/2 lead to strong electron heating which can modify the damping of whistlers in collisional plasmas (ω ∼ v ei T e -3/2 ). Observations show that a heated flux tube provides a filament of high Spitzer conductivity which permits a nearly collisionless propagation of whistler pulses. This filamentation effect is not associated with density modifications as in modulational instabilities but arises from conductivity modifications. The heated flux tube also generates a quasi-dc magnetic field driven by thermoelectric currents. These assume self-consistently a Taylor state with ⊇ x B kB.

Computer controlled probe diagnostic system and applications in a magnetized laboratory plasma

A computer controlled acquisition and evaluation system for electrostatic probes is presented. Fully automatic evaluation of the probe characteristics yields electron temperature, plasma potential, and density during the recording. Two-dimensional probe positioning is accomplished with stepping motors. The system is characterized by 12-bit probe current resolution and extremely wide probe voltage range (±130 V). It is tested in a collisionless argon magnetoplasma and compared to standard wave diagnostic techniques for magnetized plasmas, i.e., the resonance cone technique. The influence of the probe radius on the electron current in a magnetized plasma is studied experimentally. The two techniques yield identical density and temperature values within 20% accuracy and therefore show the reliable applicability of the automatic evaluation system.

Interaction of a density modulated electron beam with a magnetized plasma: Emission of whistler waves

Whistler wave excitation in a magnetized laboratory plasma by a density modulated electron beam is studied for frequency modulation below, but in the range of, the electron gyrofrequency. When the beam velocity differs from the phase velocity of the whistler at the frequency modulation of the beam, a whistler wave, and, by Čerenkov emission, a beam wave at the same frequency are excited as single poles. When both velocities are equal, a resonance occurs leading to a maximum in the emission of whistler waves. The experiment performed in a magnetized plasma (np≊1011 cm−3, Te≊0.2 eV, B≊100 G) with a modulated electron beam (nb≊109 cm−3, Eb≊300 eV) is in agreement with a simple model which predicts the characteristics of the excited waves.

Currents between tethered electrodes in a magnetized laboratory plasma

Laboratory experiments on important plasma physics issues of electrodynamic tethers are performed. These include current 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 × 200 cm, ne ≤ 1012 cm−3, kTe ≤ 3 eV, Bo &lt; 100 G, Ar, pn ≈ 3×10−4 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 kTe/e. The total current density in space and time is obtained from complete measurements of the perturbed magnetic field, J = ▽ × B(r,t)/µo 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 “Alfvén 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 neutrals are ionized near the anode. The anode sheath emits high‐frequency electron transit‐time oscillations at the sheath‐plasma resonance. The beam generates Langmuir turbulence, ion sound turbulence, electron heating, space charge fields, and Hall currents. An insulated, perfectly conducting transmission line embedded in the plasma becomes lossy due to excitation of whistler waves and magnetic field diffusion effects. The implications of the laboratory observations on electrodynamic tethers in space are discussed.

Whistler wings from moving electrodes in a magnetized laboratory plasma

In a large laboratory plasma the current pattern set up by moving electrodes has been measured. It is observed that the current flows oblique to the magnetic field at an angle given by the electrode speed across and the current penetration along which is controlled by whistler waves. The current pattern, characterized as “whistler wings”, occurs irrespective of whether the electrodes collect electrons/ions or emit fast electron beams. These results are relevant to active experiments in space involving electrodynamic tethers, beam injections and large space stations.

Direct oscilloscopic analysis of wave processes in a magnetized plasma

The paper deals with an experimental analysis of oscillations existing in a magnetized laboratory plasma. A simple arrangement allows the direct photographing of a diagram from a CRO screen which shows the main features of wave processes in the whole range of the applied magnetic field. Examples of results obtained in the magnetized positive column of glow discharges in neon and helium are presented.

Propagation of Transverse Electromagnetic Waves in a Magnetized Laboratory Plasma

The collisional, cold plasma, dispersion relation adequately describes the propagation of electromagnetic waves along the magnetic field in a laboratory plasma. The collision frequency is estimated and its linear dependence on electron density is demonstrated by a new method.