SM51B-01
Role of DC Electric Fields and Wave Heating in Cavity Profiles of Depleted Density and Transverse Ion Acceleration in Laboratory and Space Plasmas
Laboratory experiments on the WVU Q Machine, test-particle simulations, and Monte Carlo simulations are shown to provide evidence for explaining the inhomogeneity in both the plasma-density profile and the ion- temperature profile associated with cylindrically symmetric lower-hybrid cavities observed by the GEODESIC sounding rocket, the OEDIPUS-C sounding rocket, and the Freja satellite. Two potential contributions to the inhomogeneous profiles are identified. Both mechanisms (one dc and the other ac) rely on finite values of the Larmor radius and can result in nonlocal effects that deplete ion density within the cavity and enhance ion density immediately outside the cavity to form ion-gyroradius-scale shoulders encircling the cavity perimeter. In the absence of waves, a cylindrically symmetric, radial, DC electric field can be responsible for a polarization shift that produces such inhomogeneity in the density profile [1]. In the presence of waves, wave-induced transverse ion acceleration occurring within the cavity can produce such inhomogeneity in the density profile [2]. In combination, the two effects are shown to be comparable, necessitating an interpretation that includes both mechanisms for quantitative agreement. For the lab data, laser-induced fluorescence techniques provide high resolution in coordinate space and velocity space. [1] Reynolds et al., Inhomogeneity scale lengths in a magnetized, low temperature, collisionless, Q-machine plasma column containing perpendicular-velocity shear, Phys. Plasmas 13, 092106 (2006). [2] Knudsen et al., Lower-hybrid cavity density depletions as a result of transverse ion acceleration localized on the gyroradius scale, J. Geophys. Res. 109, A04212 (2004). This research is supported by NSF.
SM51B-02
Space-plasma campaign on UCLA's Large Plasma Device (LAPD)
Knudsen [JGR, 1996] describes a potential role for stationary Alfvén (StA) waves in auroral arcs' frequency dependence. Magnetized plasmas are predicted to support electromagnetic perturbations that are static in a fixed frame if there is uniform background plasma convection. These stationary waves should not be confused with standing waves that oscillate in time with a fixed, spatially varying envelope. Stationary waves have no time variation in the fixed frame. In the drifting frame, there is an apparent time dependence as plasma convects past fixed electromagnetic structures. We describe early results from an experimental campaign to reproduce in the lab the basic conditions necessary for the creation of StA waves, namely quasi-steady-state convection across magnetic field-aligned current channels. We show that an off-axis, fixed channel of electron current (and depleted density) is created in the Large Plasma Device Upgrade (LAPD) at UCLA, using a small, heated, oxide-coated electrode at one plasma-column end and we show that the larger plasma column rotates about its cylindrical axis from a radial electric field imposed by a special termination electrode on the same end. Initial experimentation with plasma-rotation-inducing termination electrodes began in May 2006 in the West Virginia Q Machine, leading to two designs that, in January 2007, were tested in LAPD. The radial profile of azimuthal velocity was consistent with predictions of rigid-body rotation. Current-channel experiments in LAPD, in August 2006, showed that inertial Alfvén waves could be concentrated in an off-axis channel of electron current and depleted plasma density. These experimental results will be presented and discussed. This research is supported by DOE and NSF.
SM51B-03
Stationary Alfven Waves
A nonlinear, quasineutral, collisional, two-fluid model of uniform plasma convection across field-aligned current sheets is developed to describe stationary Alfven (StA) waves. Previously, Knudsen [JGR, 1996] showed that, in the absence of collisions, stationary inertial Alfven (StIA) waves can accelerate electrons parallel to the magnetic field and cause time-independent plasma-density variations having spatial periodicity in the direction of the convective flow over a broad range of spatial scales and energies. StA waves may play a role in auroral arc formation. Here, Knudsen's model has been generalized for warm, collisional plasma. For StIA waves, dissipative effects of ion collisions are shown to alter the perpendicular ac and dc structure of parallel electron drift velocity, and electron collisions are shown to either increase or decrease the field-aligned electron energy, depending on the initial electron drift speed. Non-zero temperatures are shown to affect the perpendicular periodicity of the StIA wave structure and have minimal effect on the wave amplitude. Changes to the perpendicular structure of parallel electron drift velocity are shown to be less sensitive to temperature effects than to collisional effects. Similar to StIA waves, stationary kinetic Alfven (StKA) waves carry a field-aligned component of electric field, which is found to modulate the field-aligned electron velocity, and accelerate electrons to speeds in excess of the local Alfven speed. Unlike StIA waves, StKA waves modulate the direction of field aligned electron flow. Finite electron resistivity supports the StKA wave field and causes the enhancement of field-aligned electron energy. Ion collisions either increase or decrease the field-aligned electron energy, depending on the initial electron drift speed. This research is supported by NSF and NSERC (Canada).
SM51B-04
Magnetic Turbulence in Colliding Laser-Produced Plasmas
The expansion and interaction of dense plasmas in the presence of a magnetized background plasma is important in many astrophysical processes, among them coronal mass ejections and the many examples of plasma jets from astrophotography. Turbulence is expected to be present in many such configurations. We describe a series of experiments which involve the collision of two dense (initially, n > 1015cm-3) laser-produced plasmas within an ambient, highly magnetized background plasma. The laser-produced plasmas form diamagnetic cavities in which a large percentage of the background magnetic field (600G) has been expelled. First-stage observations of these structures have been completed using a fast (3ns exposure) camera. The photographs indicate complicated structure at late times, in addition to coherent corrugated structures on the bubble surfaces. The data hint at the presence of turbulence in the interaction. The second stage of observation will consist of direct investigation of the magnetic field using probes. A novel diagnostic system composed of small (300-500 micron) 3-axis differential magnetic field probes in conjunction with a ceramic motor system capable of extremely fine (sub-micron) positioning accuracy is currently under development. An ensemble of magnetic field data from fixed and movable probes makes possible the calculation of the cross-spectral function. Initial data from photography and a prototype probe will be presented.
SM51B-05 INVITED
Current Filament Merging Driven by Cross-Field Plasma Flows
The study of the penetration and mixing of plasmas with differing density, temperature,
and species composition has wide-ranging applicability to space plasma systems
such as coronal mass ejections, magnetic clouds, galactic jets, and super novae.
In these laboratory experiments, two high-beta plasmas are created
using a pair of 1.5J, 8ns lasers which strike facing solid carbon targets at right angles
to the background magnetic field. The targets are
immersed within a low-beta, helium plasma and the lasers are aimed to
produce head-on, or glancing collisions. The cylindrical background plasma is 17 m long
(10 parallel Alfven wavelengths) by 60 cm wide (300 ρi or 175 c/ωpe).
The laser-produced
plasmas (LPPs) expand as diamagnetic cavities, become polarized, and
then E× B drift at speeds of Mach 10 (v/cs) across the field. As they do so,
the ambient plasma facilitates charge separation between energetic LPP electrons
and relatively unmagnetized 1keV LPP ions. One of the many resulting dynamic features
is the release of a continuous stream of electrons from each LPP.
Downstream from the LPP merging, the fast electron current filaments
come together with reconnection-like X-line field patterns and eventually
merge with a broadband spectrum of electromagnetic (whistler wave) fluctuations.
Near-miss LPP collisions result in elongated current sheet formations and
the shedding of magnetic field eddies. Current sheet thicknesses are
a few electron inertial lengths and the width is approximately one ion inertial length.
These results will be presented along with 3D measurements of the magnetic fields
and the underlying current systems.
These experiments are conducted at the Basic Plasma Science Facility, in the upgraded Large Plasma Device
(LAPD)
located at the University of California, Los Angeles, USA.
This work is funded by the United States Department of Energy and the
National Science Foundation.
http:plasma.physics.ucla.edu/bapsf
SM51B-06
Dynamics of Magnetic Flux Ropes in a Laboratory Plasma
The behavior and interaction of magnetic flux ropes has long been a topic of interest to solar and space plasma
physicists. (Gekelman, et al. IEEE Trans. Plasma Sci. 20, 614. Furno, et al. Phys. Plasmas 12,
055702.) Very few laboratory experiments have been performed as it is necessary to have a relatively
collisionless plasma and currents with significant self-generated fields. Movable lanthanum hexaboride
(LaB6) cathodes have been developed to study the 3D dynamics of flux ropes in the Large Plasma Device
(LaPD). The background plasma (n ~ 2 × 1012 cm -3, d ~ 60 cm, L ~ 18 m, and
τrep