C. L. Rousculp

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...

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 ...

Three‐dimensional currents of electrodynamic tethers obtained from laboratory models

Magnetic probe measurements in three dimensions (greater than 15,000 positions) and time in a large laboratory plasma (n(sub e) greater than or equal to 10(exp 11)/cc, kTe greater than or equal to 1eV, B(sub 0) = 20 G, 1 m diam. x 2.5 m length) reveal the plasma currents J = del(vector differential operator) x B/mu(sub 0) excited by a pulsed (delta-t = 100 ns), stationary, tethered pair of electrodes (approximately equals 1 cm diam., 20 cm spacing perpendicular to B(sub 0)). The plasma currents for a moving, dc-current carrying electrodynamic tether are obtained by a superposition of delayed pulses emitted at successively displaced tether positions. The transient plasma currents are carried by low-frequency whistlers instead of Alfven waves and form a 3D wing structure but no long phantom loop due to cross-field Hall current shunting.

Transport of time-varying plasma currents by whistler wave packets

In a large laboratory plasma the properties of time-varying current systems have been studied experimentally. The parameter regime of interest involves magnetized electrons and unmagnetized ions. In the laboratory complete measurements of three-dimensional, time-varying vector fields of the total current density are obtained from magnetic probe measurements. Pulsed currents are observed to propagate at the speed of whistler wave packets. Their field structure forms flux-rope-like configurations which are electromagnetically force-free. Moving sources induce eddy currents which excite waves and form Cerenkov-like whistler wings. The radiation patterns of moving magnetic antennas and electrodynamic tethers have been investigated. The current closure between tethered electrodes across B/sub 0/ has been mapped. Nonlinear effects of large-amplitude, antenna-launched whistler pulses have been observed. These involve a new modulational instability in which a channel of high conductivity is formed which permits the wave/currents to penetrate deeply into a collisional plasma. >

Diagnostics of magnetic antenna fields for low frequency whistlers in r-t and /spl omega/-k space

Summary form only given. In a large uniform laboratory plasma the magnetic field B/sub (r,t)/ of a shielded magnetic loop antenna (5 cm diam.) has been measured in both near and far zones. The antenna is driven with a single short current pulse which excites a whistler wave packet. With a movable magnetic probe and repeated pulses the vector field B/sub x/, B/sub z/ vs. x, y, z is mapped at 10,000 spatial positions with high time resolution. In the space-time domain the evolution of a dispersive wave packet from the antenna near-zone fields is observed. The wave packet consists of nested cones propagating with wave normals highly oblique to B/sub o/ but energy flow at a small angle to B/sub o/. In order to analyze the wave packet in terms of eigenmodes, the field B/sub (r,t)/ has been Fourier-transformed in time and 3-D space, B/sub (/spl omega/,k)/. At a selected frequency, the finite-size antenna launches a spectrum of k-vectors which, for an antenna dipole moment along B/sub o/, is highly axisymmetric. In the k/sub /spl perp - k/sub 1/ plane the wave energy is spread out along the refractive index curve of whistlers with the largest wave energy oblique to B/sub o/.