R. L. Stenzel

Whistler wave propagation in a large magnetoplasma

The propagation of linear whistler waves is investigated in a very large quiescent collisionless discharge plasma in the parameter regime 0.05<ω /ωc<1, ωc/2π≃250 MHz, ωp/ωc≳10, where ωc and ωp are electron cyclotron and plasma frequency, respectively. Wave dispersion, damping, and polarization are determined from interferometer measurements. Group velocities are measured by propagating phase‐coherent wave bursts. Low frequency wave packets disperse into nose whistlers. The phase and amplitude distribution of antenna‐excited whistler waves is mapped along and across the magnetic field. The different directions of group and phase velocities are clearly demonstrated. Highly oblique whistlers are observed. In weakly nonuniform plasmas (ne/∇⊥ne≫λ∥) ray tracing is performed directly verifying the theoretical picture of ducting in density troughs. Perfectly ducted undamped whistlers are observed in field aligned troughs of width d≃λ∥. The profound effect of ray divergence from finite size antennas and of wave re...

Filamentation instability of a large amplitude whistler wave

A filamentation instability of a large amplitude whistler wave is observed in a laboratory experiment. The wave is launched in a large uniform magnetoplasma from antennas which produce a diverging energy flow in the linear regime. At large amplitudes (Bw/B0≃1%) the radiation pressure gives rise to a density depression originating from the antenna. The wave refracts into the density minimum which enhances the pressure and a filamentation instability develops. It saturates when a long field‐aligned density trough is formed in which the wave is almost perfectly ducted. The temporal and spatial evolution of the self‐ducting process are shown. The duct formed by the high power whistler wave also guides small amplitude whistlers over a wide range of frequencies.

Particle dynamics and current‐free double layers in an expanding, collisionless, two‐electron‐population plasma

The expansion of a two‐electron‐population, collisionless plasma into vacuum is investigated experimentally. Detailed in situ measurements of plasma density, plasma potential, electric field, and particle distribution functions are performed. At the source, the electron population consists of a high‐density, cold (kTe≂4 eV) Maxwellian, and a sparse, energetic ( (1)/(2) mv2e≂80 eV) tail. During the expansion of plasma, space‐charge effects self‐consistently produce an ambipolar electric field whose amplitude is controlled by the energy of tail electrons. The ambipolar electric field accelerates a small number (∼1%) of ions to streaming energies which exceed and scale linearly with the energy of tail electrons. As the expansion proceeds, the energetic tail electrons electrostatically trap the colder Maxwellian electrons and prevent them from reaching the expansion front. A potential double layer develops at the position of the cold electron front. Upstream of the double layer both electron populations exist...

Directional velocity analyzer for measuring electron distribution functions in plasmas

A directional velocity analyzer has been developed for measuring electron distribution functions in plasmas. It contains a collimating aperture which selects particles from a narrow cone in velocity space and a retarding potential analyzer. The distribution function f(v, θ, φ) is obtained from a large number of analyzer traces taken at different angles θ, φ. In addition, the small analyzer can be moved in space and the measurements are time resolved so as to obtain the complete phase space information f (v, r, t). The large data flow of this seven‐variable function is processed with a high‐speed digital data‐acquisition system. The new electron velocity analyzer is applicable over a wide parameter range in electron energies and densities. Various cases of anisotropic distributions such as beams, shells, tails, and drifts have been successfully investigated.

Novel directional ion energy analyzer

A new ion energy analyzer with high angular resolution (⩽3×10−4 sr) is described. It consists of a microchannel plate followed by a retarding‐grid type analyzer. The microchannel plate is not used for charge multiplication but as a geometric filter with narrow angular passband (ϑ≃0.6°) yet high transparency (T≃60%). The energy analyzer is used to measure the true velocity space distribution of low‐energy ion beams (Eb = 10–100 eV) in a double plasma device. Its superior performance over the conventional gridded energy analyzer is demonstrated. Applications to the study of beam wakes are shown.

Whistler waves in space and laboratory plasmas

An overview of whistler wave phenomena in space and laboratory plasmas is given. Common features and different approaches between laboratory and space plasma research are pointed out. Both research activities have discovered a rich variety of whistler wave effects. Many useful applications have emerged. Open research topics and interactions between lab and space research on whistlers are pointed out.

Dynamics of fireballs

Fireballs are discharge phenomena on positively biased small electrodes in plasmas. The discharge arises from electron energization at a double layer. Fireballs can collect relatively large electron currents from the ambient plasma. Fireballs can become unstable to relaxation oscillations. This paper addresses the space–time evolution of pulsed fireballs. Growth and collapse of fireballs produce large density and potential variations near the electrode which couple into the background plasma production. Unstable fireballs emit bursts of fast ions and ion acoustic waves. High-frequency emissions near the electron plasma frequency have been observed and associated with the sheath–plasma instability rather than electron beam–plasma interactions. New shapes of fireballs have been observed in dipole magnetic fields.

Pulsed currents carried by whistlers. Part I: Excitation by magnetic antennas

Time‐varying plasma currents associated with low‐frequency whistlers have been investigated experimentally. Pulsed currents are induced in the uniform, boundary‐free interior of a large laboratory plasma by means of insulated magnetic antennas. The time‐varying magnetic field is measured in three dimensions and the current density is calculated from R∇×B(r,t)=μ0J, where J includes the displacement current density. Typical fields B(r,t) and J(r,t) induced by a magnetic loop antenna show three‐dimensional helices due to linked toroidal and solenoidal field topologies. Constant amplitude and phase surfaces assume conical shapes since the propagation speed along B0 is higher than oblique to B0. The wave vector is highly oblique to B0 while the energy flow is mainly along B0. The electric field in the wave packet contains both inductive and space‐charge contributions, the latter arising from the different dynamics of electrons and ions as explained by physical arguments. The dominant electric field in a whistl...

High-frequency instability of the sheath-plasma resonance

Coherent high‐frequency oscillations near the electron plasma frequency (ω≲ωp) are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath–plasma resonance and is driven by a negative rf sheath resistance associated with the electron inertia in the diodelike electron‐rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening, and copious harmonics. The fundamental instability is a bounded oscillation caused by wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres), which excludes other radiation mechanisms such as orbitrons or beam–plasma instabilities. The line broadening mechanism has been identified as a f...

Magnetic Field Line Reconnection Experiments

A laboratory experiment concerned with the basic physics of magnetic field line reconnection will be discussed. Stimulated by important processes in space plasmas and anomalous transport in fusion plasmas the work addresses the following topics: Dynamic magnetic fields in a high beta plasma, magnetic turbulence, plasma dynamics and energy transport. First, the formation of magnetic neutral sheets, tearing and island coalescence are shown. Nonstationary magnetic fluctuations are statistically evaluated displaying the correlation tensor BB in the ω-k domain for mode identification. Then, the plasma properties are analyzed with particular emphasis on transport processes. Although the classical fluid flow across the separatrix can be observed, the fluctuation processes strongly modify the plasma dynamics. Direct measurements of the fluid force density and ion acceleration indicate the presence of an anomalous scattering process characterized by an effective scattering tensor ν*. Turbulence also enhances the plasma resistivity η* by one to two orders of magnitude. Measurements of the three-dimensional electron distribution function fe(vx, vy, vz) using a novel energy analyzer exhibit the formation of runaway electrons in the current sheet. Associated microinstabilities are observed. Finally, a macroscopic disruptive instability of the current sheet is observed. Excess magnetic field energy is converted at a double layer into particle kinetic energy and randomized through beam-plasma instabilities. These laboratory results will be compared with related observations in space and fusion plasmas.