J. M. Urrutia

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...

Pulsed currents carried by whistlers. V. Detailed new results of magnetic antenna excitation

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.

Pulsed currents carried by whistlers. III. Magnetic fields and currents excited by an electrode

Detailed measurements and analysis of electromagnetic fields asociated with pulsed plasma currents are reported. The objective is to demonstrate the properties of plasma currents in the electron magnetohydrodynamic regime and their relation to low frequency whistler waves. Short current pulses (fce−1≪Δt≪fci−1) are injected from an electrode into a large, uniform magnetoplasma. The dynamic fields, B(r,t), are measured with probes in three‐dimensional space and time, and are observed to propagate as wave packets predominantly along the guide magnetic field, B0. Four‐dimensional fast Fourier transformation of B(r,t) to B(k,ω) verifies that the wave fields fall on the dispersion surface of low‐frequency oblique whistlers. The magnetic field topology of the packets consists of linked toroidal and solenoidal contributions in force‐free configurations. The wave magnetic helicity is obtained quantitatively. Similarly, the topology of the current density field, J=∇×B/μ0, is explained by its components, characteris...

Pulsed currents carried by whistlers. IV. Electric fields and radiation excited by an electrode

Electromagnetic properties of current pulses carried by whistler wave packets are obtained from a basic laboratory experiment. While the magnetic field and current density are described in the preceding companion paper (Part III), the present analysis starts with the electric field. The inductive and space charge electric field contributions are separately calculated in Fourier space from the measured magnetic field and Ohm’s law along B0. Inverse Fourier transformation yields the total electric field in space and time, separated into rotational and divergent contributions. The space‐charge density in whistler wave packets is obtained. The cross‐field tensor conductivity is determined. The frozen‐in condition is nearly satisfied, E+ve×B≂0. The dissipation is obtained from Poynting’s theorem. The waves are collisionally damped; Landau damping is negligible. A radiation resistance for the electrode is determined. Analogous to Poynting’s theorem, the transport of helicity is analyzed. Current helicity is gen...

A new method for removing the blackout problem on reentry vehicles

Supersonic vehicles are surrounded by a plasma layer which produces a cutoff layer for electromagnetic waves. Methods to remove the layer by gas releases, dc magnetic fields, and E×B flows have been proposed earlier. The present work suggests a new approach which is based on laboratory observations. Ions are expelled by a time varying magnetic field which creates a Hall electric field. The ion expulsion opens up a window of transparency for wave communications.

Laboratory studies of magnetic vortices. III. Collisions of electron magnetohydrodynamic vortices

Magnetic vortices in the parameter regime of electron magnetohydrodynamics are studied in a large laboratory plasma. The vortices consist of magnetic field perturbations, which propagate in the whistler mode along a uniform dc magnetic field. The magnetic self-helicity of the spheromak-like field perturbations depends on the direction of propagation. Vortices with opposite toroidal or poloidal fields are launched from two antennas and propagated through each other. The vortices collide and propagate through one another without an exchange of momentum, energy, and helicity. The absence of nonlinear interactions is explained by the force-free fields of electron magnetohydrodynamic (EMHD) vortices.

Pulsed currents carried by whistlers. II. Excitation by biased electrodes

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 ...

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 measurement...

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 arise...

Electron magnetohydrodynamic turbulence in a high-beta plasma. I. Plasma parameters and instability conditions

The interaction of a dense discharge plasma with a weak external magnetic field has been studied experimentally. The electron pressure exceeds the field pressure and forms a magnetic hole in the plasma interior. The ions are unmagnetized, while the electrons are in a transition regime from none to full magnetization. The electron confinement changes from Boltzmann equilibrium to magnetic confinement. The pressure balance equation does not describe the diamagnetism because ambipolar E×B drifts oppose the diamagnetic drift. The net drift exceeds the sound speed by an order of magnitude and produces a strong two-stream cross-field instability. Although its spectrum is close to the lower hybrid instability, there are significant differences from the classical lower hybrid instability, e.g., the presence of strong magnetic fluctuations. These fall into the regime of electron magnetohydrodynamics (EMHD) with unmagnetized but mobile ions. While the EMHD turbulence is the main focus of the two following companion...