Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session S16: Laboratory Astrophysics II |
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Chair: Parvez Guzdar, University of Maryland Room: Hyatt Regency Dallas Landmark D |
Monday, April 24, 2006 3:30PM - 3:50PM |
S16.00001: Ion Heating During Reconnection Events in MST Invited Speaker: Magnetic reconnection is often accompanied by plasma heating in both laboratory and astrophysical situations, but the mechanism which converts the released magnetic energy to thermal energy remains unclear. In the Madison Symmetric Torus, spontaneous bursts of reconnection heat plasma ions, resulting in a doubling of the temperature in about one-hundred microseconds. Recent advances in diagnostics allow us to spatially and temporally resolve this heating for both majority deuterium and impurity ions. We find that the heat source is active throughout the plasma volume and appears stronger for impurities, implying a dependence on ion mass and/or charge. The heating is strongest when many nonlinearly coupled reconnection sites are active and weaker when only a few are active. Theoretical work to assess the importance of viscous damping of the flows associated with reconnection suggests they could be important. Work is supported by U.S.D.O.E. and N.S.F. [Preview Abstract] |
Monday, April 24, 2006 3:50PM - 4:10PM |
S16.00002: Ion Heating by Alfven Waves and Reconnection in NSTX Invited Speaker: The evolution of laboratory and astrophysical plasmas depends on the flow of energy between the ``equilibrium'' configuration, waves in the plasma and the thermal plasma. We explore two examples of this energy flow. In the first example, data from NSTX is examined for evidence that CAE in the frequency range from $\sim$ 0.2 fci to $\sim$ 1.2 fci excited by super-thermal ions might heat the thermal ions. Theory indicates that only a relatively small portion of the beam power would go into exciting the CAE on NSTX, and observations indicate that the amplitude of these waves, deduced from density fluctuations, is below the stochastic threshold for heating. Another example examines how internal magnetic reconnections can lead to heating of the thermal ions. One model postulates the excitation of a high frequency wave, which then damps on the ions. High frequency waves are indeed seen to follow some NSTX reconnection events. The second invokes direct acceleration of the thermal ions by the induced electric field [P. Helander, L.-G. Eriksson, R.J. Akers, et al.,Phys. Rev. Lett. 89 (2002) 235002-1]. \newline \newline In collaboration with S.S. Medley, Princeton Plasma Physics Laboratory. [Preview Abstract] |
Monday, April 24, 2006 4:10PM - 4:30PM |
S16.00003: Wave Experiments in Dusty Plasmas: Linear and Nonlinear Invited Speaker: A dusty plasma is an ionized gas containing small particles of solid matter, which are typically nanometer or micron size. These particles gain a large electrical charge by collecting electrons and ions from the ambient plasma. Examples of astrophysical dusty plasmas include the interstellar medium, comet tails and the rings of Saturn. The charged particles in a dusty plasma act like highly-charged super-massive ions, except that their charge is not fixed. Like ions, they can oscillate in response to electric fields, so that waves can propagate. These waves have very low frequencies, due to the heavy mass of the particles. Laboratory experiments are done by seeding microspheres into a glow-discharge plasma. In the presence of gravity, they can form a horizontal layer levitated by the electric field of a sheath. These particles are imaged directly, using video cameras, as they move about. Experiments have been performed to observe compressional and shear electrostatic waves. Experiments with low-amplitude linear waves as well as high-amplitude nonlinear waves will be described. At high amplitudes, compressional waves exhibit three-wave mixing. These experiments were performed in dusty plasmas that are strongly-coupled, with particles arranged in an ordered structure like molecules in a liquid or solid. Work supported by DOE and NASA. [Preview Abstract] |
Monday, April 24, 2006 4:30PM - 4:50PM |
S16.00004: Spontaneous Double Layer Formation in Expanding Plasmas Invited Speaker: A double layer (DL) is a localized region of electric field characterized by a strong potential gradient. Particle acceleration in DLs has been proposed as a mechanism for astrophysical phenomena such as energetic electrons in the auroral zone and energetic ions emitted by the Sun during solar flares. In laboratory plasmas, DLs are typically created at the interface of two different plasmas or by driving a current in a single plasma. Recently, DLs have been observed to spontaneously develop in three different, current-free, expanding helicon plasmas: Chi-Kung at ANU, MNX at PPPL, and HELIX at WVU. We will present measurements of the DL structure in HELIX and MNX. Both DLs have a total potential drop of 3-4 \textit{kT}$_{e}$ and length scales smaller than ion-neutral mean-free-path. The spatial structure of the DL and ion acceleration in the DL is consistent with the predictions of Monte-Carlo, Particle-in-Cell simulation. Based on the experimental parameters, we hypothesize that DL formation is triggered when the ion-neutral collisional mean free path exceeds the magnetic field gradient scale length. Similar to the DLs occurring in nature, those two helicon DLs, as well as the DL in Chi-Kung, occur in the region of strong magnetic field gradient. We will also present measurements of the effect of magnetic field strength on DL structure and show that the DL continues to strengthen after many tens of ms in a pulsed helicon plasma. [Preview Abstract] |
Monday, April 24, 2006 4:50PM - 5:10PM |
S16.00005: Laboratory Studies of Turbulence Associated with Localized Current Layers Invited Speaker: Localized current layers are a natural consequence of the interaction of the solar wind with the earth's magnetic field. In particular, field-aligned currents dynamically link the active magnetotail to the auroral ionosphere. Within these currents there may develop small-scale phenomena such as density-gradient or shear-driven instabilities, or electron solitary structures and micro-turbulence which may profoundly influence the larger-scale dynamics of the system. The Basic Plasma Science Facility (BaPSF) at UCLA offers a unique opportunity to model magneto/heliospheric phenomena, including current sheets. We present measurements from laboratory experiments of an electron current sheet which is several ion-gyroradii thick by up to ten Alfv\'{e}n wavelengths along the field(1cm by 20m). The current sheet leads to a depletion of the background plasma, forming a field-aligned density depression. Drift-Alfv\'{e}n waves are spontaneously excited and drive cross-field particle transport which relaxes the density gradient and modulates the current flow. We will also present initial results of small-scale electric field spikes within the current sheet using specially fabricated dipole probes with separation on the order of the Debye length---here 13 microns. These measurements are motivated by the observation of electron solitary structures throughout the magnetosphere. [Preview Abstract] |
Monday, April 24, 2006 5:10PM - 5:30PM |
S16.00006: Nonlinear interactions between shear Alfv\'{e}n waves in a laboratory plasma Invited Speaker: Electromagnetic turbulence is thought to play an important role in plasmas in astrophysical settings (e.g. the interstellar medium, accretion disks) and in the laboratory (e.g. transport in magnetic fusion devices). From a weak turbulence point of view, nonlinear interactions between shear Alfv\'{e}n waves are fundamental to the turbulent energy cascade in magnetic turbulence. An experimental investigation of nonlinear interactions between shear Alfv\'{e}n waves in the Large Plasma Device (LAPD) will be presented. Two Alfv\'{e}n waves, generated by a resonant cavity, are observed to beat together, driving a low frequency perturbation at the beat frequency. The low frequency perturbation then scatters the Alfv\'{e}n waves, generating a series of sidebands. The observed interaction is very strong, with the normalized amplitude of the driven low frequency mode comparable to the normalized magnetic field amplitude ($\delta B/B$) of the interacting Alfv\'{e}n waves. Experimental details of this interaction will be presented along with other phenomena associated with large amplitude Alfv\'{e}n waves in LAPD, including electron heating and background density modification. Initial results from counter-propagating wave interaction experiments will also be discussed. \newline \newline In collaboration with Brian Brugman, Dept. of Physics and Astronomy and Center for Multiscale Plasma Dynamics, UCLA. [Preview Abstract] |
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