Stephen Vincena

Experimental observation of Alfven wave cones

The spatial evolution of the radial profile of the magnetic field of a shear Alfven wave launched by a disk exciter with radius on the order of the electron skin depth has been measured. The waves are launched using wire mesh disk exciters of 4 mm and 8 mm radius into a helium plasma of density about 1.0×1012 cm−3 and magnetic field 1.1 kG. The electron skin depth δ=c/ωpe is about 5 mm. The current channel associated with the shear Alfven wave is observed to spread with distance away from the exciter. The spreading follows a cone‐like pattern whose angle is given by tan θ=kAδ, where kA is the Alfven wave number. The dependence of the magnetic profiles on wave frequency and disk size are presented. The effects of dissipation by electron–neutral collisions and Landau damping are observed. The observations are in excellent agreement with theoretical predictions [Morales et al., Phys. Plasmas 1, 3765 (1994)].

Shear Alfvén waves in a magnetic beach and the roles of electron and ion damping

¯ e/2. The wave thus propagates from a region where Landau damping is significant to where ion-cyclotron damping dominates. Detailed two dimensional measurements of the wave magnetic field morphology are presented. The measured wavelength decreases in accord with WKB solutions of a modified wave equation. Wave damping is also observed and dissipation by both ions and electrons is required in the WKB model to fit the data. Suppression of the damping via electrons in the model results in a predicted wave magnetic field amplitude twenty times larger at the ion-cyclotron resonance point than observed.

Laboratory Measurements of Electrostatic Solitary Structures Generated by Beam Injection

Electrostatic solitary structures are generated by injection of a suprathermal electron beam parallel to the magnetic field in a laboratory plasma. Electric microprobes with tips smaller than the Debye length (λDe) enabled the measurement of positive potential pulses with half-widths 4 to 25λDe and velocities 1 to 3 times the background electron thermal speed. Nonlinear wave packets of similar velocities and scales are also observed, indicating that the two descend from the same mode which is consistent with the electrostatic whistler mode and result from an instability likely to be driven by field-aligned currents.

Spectral gap of shear Alfvén waves in a periodic array of magnetic mirrors

A multiple magnetic mirror array is formed at the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] to study axial periodicity-influenced Alfven spectra. Shear Alfven waves (SAW) are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. Alfven wave spectral gaps and continua are formed similar to wave propagation in other periodic media due to the Bragg effect. The measured width of the propagation gap increases with the modulation amplitude as predicted by the solutions to Mathieu’s equation. A two-dimensional finite-difference code modeling SAW in a mirror array configuration shows similar spectral features. Machine end-reflection conditions and damping mechanisms including electron-ion Coulomb collision and electron Landau damping are important for simulation.

Lithium ion sources for investigations of fast ion transport in magnetized plasmas

In order to study the interaction of ions of intermediate energies with plasma fluctuations, two plasma immersible lithium ion sources, based on solid-state thermionic emitters (Li aluminosilicate) were developed. Compared to discharge based ion sources, they are compact, have zero gas load, small energy dispersion, and can be operated at any angle with respect to an ambient magnetic field of up to 4.0 kG. Beam energies range from 400 eV to 2.0 keV with typical beam current densities in the 1 mAcm(2) range. Because of the low ion mass, beam velocities of 100-300 kms are in the range of Alfven speeds in typical helium plasmas in the large plasma device.

Experimental observations of shear Alfvén waves generated by narrow current channels

Alfven waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Shear Alfven waves radiated from sources with cross-field scale size of the order of the electron inertial length, δ=c/ωpe, have properties which differ considerably from planar magneto-hydrodynamic waves. Currents of cross-field size δ are common in space plasmas. A series of experiments in the large plasma device (LAPD) at UCLA is presented which illustrates that waves generated by small scale fluctuating currents radiate across magnetic field lines and are associated with complex three-dimensional currents. Waves generated by two sources are observed to constructively interfere to produce large magnetic fields in spatial regions away from source field lines. Volume data sets will be presented and discussed in the light of space plasma applications.

Chaos in magnetic flux ropes

Magnetic flux ropes immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Each collision results in magnetic field line reconnection and the generation of a quasi-separatrix layer. Three-dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. Conditional averaging is possible for only a number of rotation cycles as the field line motion becomes chaotic. The permutation entropy can be calculated from the time series of the magnetic field data (this is also done with flows) and is used to calculate the positions of the data on a Jensen–Shannon complexity map. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The Lyapunov and Hurst exponents are calculated and the complexity and permutation entropy of the flows and field components are shown throughout the volume.

Turbulent transport of fast ions in the Large Plasma Device

Strong drift wave turbulence is observed in the Large Plasma Device [H. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] on density gradients produced by a plate limiter. Energetic lithium ions orbit through the turbulent region. Scans with a collimated ion analyzer and with Langmuir probes give detailed profiles of the fast ion spatial distribution and the fluctuating fields. The fast ion transport decreases rapidly with increasing fast ion gyroradius. Unlike the diffusive transport caused by Coulomb collisions, in this case the turbulent transport is nondiffusive. Analysis and simulation suggest that such nondiffusive transport is due to the interaction of the fast ions with stationary two-dimensional electrostatic turbulence.

Measurement of ion motion in a shear Alfvén wave

In this study, the technique of laser-induced fluorescence (LIF) has been used to meausure Ti and the E×B0 and polarization drifts of shear Alfven waves in the Large Plasma Device at UCLA [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. The waves were launched by an antenna located at the end of the device and were observed to propagate along the axis of a 9 m long, 40 cm diameter cylindrical argon plasma in the kinetic regime [βe≈9.5(me∕mi)], with fwave∕fci≈0.8. Care was taken to record the measurements from various diagnostics at the same spatial positions on four cross-sectional planes along the length of the plasma. Two-dimensional LIF measurements of the ion drifts perpendicular to B0 were undertaken. Ion drifts were observed to be as large as 14% of the ion thermal speed. The ion polarization and E×B0 drifts were distinguished by their phase relation to Bwave. The measured drifts are compared to kinetic theory. E⊥ (the transverse component...

Generation of whistler waves by a rotating magnetic field source

The paper discusses the generation of polarized whistler waves radiated from a rotating magnetic field source created via a novel phased orthogonal two loop antenna. The results of linear three-dimensional electron magnetohydrodynamics simulations along with experiments on the generation whistler waves by the rotating magnetic field source performed in the large plasma device are presented. Comparison of the experimental results with the simulations and linear wave properties shows good agreement. The whistler wave dispersion relation with nonzero transverse wave number and the wave structure generated by the rotating magnetic field source are also discussed. The phase velocity of the whistler waves was found to be in good agreement with the theoretical dispersion relation. The exponential decay rate of the whistler wave propagating along the ambient magnetic field is determined by Coulomb collisions. In collisionless case the rotating magnetic field source was found to be a very efficient radiation sourc...