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 whistler packet is a radial space-charge field. Neither the field topology nor the propagation characteristics are sensitive to the induced magnetic field amplitude up to Bwave≲B0. The results are relevant to both the basic properties of whistlers and to applications such as large loop antennas and electrodynamic tethers in space plasmas.
A low frequency, oblique whistler wave packet is excited from a single current pulse applied to a magnetic loop antenna. The magnetic field is mapped in three dimensions. The dominant angle of radiation is determined by the antenna dimensions, not by the resonance cone. Topological properties of the inductive and space charge electric fields and space charge density confirm an earlier physical model. Transverse currents are dominated by Hall currents, while no net current flows in the parallel direction. Electron-ion collisions damp both the energy and the helicity of the wave packet. Landau damping is negligible. The radiation resistance of the loop is a few tenths of an Ohm for the observed frequency range. The loop injects zero net helicity. Rather, oppositely traveling wave packets carry equal amounts of opposite signed helicity.
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
Plasma bubbles are created in an ambient discharge plasma. A bubble is a plasma volume of typically spherical shape, which is separated from the ambient plasma by a negatively biased grid of high transparency. Ions and electrons from the ambient plasma flow into the bubble volume. In steady state the flow of particles and currents is divergence-free, which is established by the plasma potential inside the bubble. The grid has two sheaths, one facing the ambient plasma, the other the bubble plasma. The inner sheath is observed to become unstable, causing the plasma potential in the bubble to oscillate. The instability arises from an excess of ions and a deficiency of electrons. Its frequency is in the range of the ion plasma frequency but depends on all parameters which influence the charge density in the sheath. When the grid voltage is very negative, electrons cannot enter the outer sheath, and the inner sheath becomes a virtual anode which reflects ions such that the bubble interior is empty. When an electron source is placed into the bubble it can neutralize the ions and the bubble refills. Without plasma sources or sinks the bubble plasma is extremely sensitive to perturbations by probes. Modified current-voltage characteristics of Langmuir and emissive probes are demonstrated. A sequence of papers first describes the basic steady-state properties, then the time evolution of bubbles, the effects of electron sources in bubbles, and the role of the grid and bubble geometry. The physics of plasma bubbles is important to several fields of basic plasma physics such as sheaths, sheath instabilities, diagnostic probes, electrostatic confinement, and current and space charge neutralization of beams. V C 2012 American Institute of Physics. [http://dx.
The transport of time-dependent current between electrodes in contact with a large laboratory magnetoplasma is examined experimentally. Single electrodes biased with respect to the chamber wall or pairs of electrically floating electrodes are used to produce pulsed currents (ωci≪2π/Δt≪ωce). The associated magnetic field vector, B(r,t), is measured in space and time, and the total current density is calculated from J(r,t)=∇×B(r,t)/μ0. The current front is found to propagate at a characteristic wave speed, which does not depend on current amplitude or polarity. The transient current spreads across B0 within a conical region, which depends on source geometry and plasma parameters. It is shown by Fourier transforming B(r,t) into B(k,ω) that the transient fields consist of a spectrum of oblique low-frequency whistler waves. In Fourier space, the inductive and space charge electric fields are calculated from Faraday’s law and the assumption that Etot=Eind+Esc along B0 is negligible. Inverse transforming yields E(r,t). The transient wave fields (B,J,E) exhibit multiple induction effects and the formation of space charges. The results are relevant to pulsed Langmuir probes, beams, and antennas as well as moving steady-state magnetic/current sources such as particle collectors on spacecraft and magnetized asteroids (e.g., Gaspra).
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