Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session C16: DPP Contributed Orals I |
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Sponsoring Units: DPP Chair: Benjamin Chandran, University of New Hampshire Room: Hyatt Regency Dallas Landmark D |
Saturday, April 22, 2006 1:30PM - 1:42PM |
C16.00001: Weak compressible MHD turbulence in the solar corona Benjamin Chandran This presentation describes a weak-turbulence calculation for fast waves and Alfven waves in low-beta plasmas. Numerical and analytic results are presented for the anisotropic power spectra and nonlinear time scales for energy transfer to small scales and high frequencies. It is argued that high-frequency waves produced by MHD turbulence in the solar corona are a promising mechanism for explaining the observed anisotropic heating of minor ions in the corona. [Preview Abstract] |
Saturday, April 22, 2006 1:42PM - 1:54PM |
C16.00002: Spectrum of MHD Turbulence Stanislav Boldyrev We propose a phenomenological theory of strong incompressible magnetohydrodynamic turbulence in the presence of a large-scale external magnetic field. We argue that in the inertial range of scales, magnetic-field and velocity-field fluctuations tend to align the directions of their polarizations. However, in driven turbulence the perfect alignment cannot be reached, it is precluded by the presence of a constant energy flux over scales. As a consequence, the directions of fluid and magnetic-field fluctuations at each scale~$\lambda$ become aligned within the angle~$\phi_{\lambda}\propto \lambda^{1/4}$, which leads to scale-dependent depletion of nonlinear interaction and to the field-perpendicular energy spectrum~$E(k_{\perp})\propto k_{\perp}^{-3/2}$. Our results may be universal, i.e., independent of the external magnetic field, since small-scale fluctuations locally experience a strong field produced by large-scale eddies. The application of the results to diffractive interstellar scintillation is discussed. [Preview Abstract] |
Saturday, April 22, 2006 1:54PM - 2:06PM |
C16.00003: Charged Grains and the Dynamics of Saturn's F Ring Lorin Matthews, Truell Hyde The dynamics of Saturn's F Ring have been a matter of curiosity ever since the probes Pioneer 11 and Voyagers 1 and 2 sent back data revealing the ring's unusual features. Once again this dynamic ring is in the spotlight with Cassini now at Saturn sending back high-resolution images of its rapidly evolving features. The F Ring displays clumps, kinks, waves and braids, structures which evolve on the time scale of months, and has recently been described as a spiral ring. Several models have been proposed to explain these features; most of these invoke perturbations caused by the shepherding moons or km sized moonlets imbedded in the ring and are purely gravitational in nature. These models assume that both the plasma densities at the F Ring and the charges on the grains are small enough that electromagnetic forces can be ignored. Most models also assume that the effects of solar radiation pressure and solar gravity act on a timescale of thousands of years and thus don't contribute to the evolving features. This study shows that electromagnetic and radiation forces can lead to significant perturbations of the orbits of micron sized grains, even for very small grain charges, and may have implications for the steep size-distribution seen in the F Ring. [Preview Abstract] |
Saturday, April 22, 2006 2:06PM - 2:18PM |
C16.00004: Structural Phases in Complex Plasmas Truell Hyde, Ke Qiao, Jie Kong, Lorin Matthews, Jorge Carmona, Jerry Reay, Mike Cook, Jimmy Schmoke Dust particles imbedded within plasma acquire a charge from collisions with free electrons and ions in the plasma. If the ratio of the inter-particle potential energy to the average kinetic energy is sufficient, the particles form a ``liquid'' structure with short range ordering or a crystalline structure with longer range ordering. When the dust particles form a crystalline structure and reside within two-dimensionally extended lattice planes, different stable crystalline phases have been observed experimentally. Recently, it has been found that energetically favored structures can be formed employing various external DC biases. Additionally, dust free region formation employing this form of external DC bias will also be discussed. Experimental results will be shown to be in good agreement with current theoretical predictions for a strongly coupled complex plasma system. [Preview Abstract] |
Saturday, April 22, 2006 2:18PM - 2:30PM |
C16.00005: MHD Turbulence in the Madison Dynamo Experiment Mark Nornberg, Cary Forest, Roch Kendrick, Erik Spence, Adam Bayliss, Craig Jacobson, Carlos Parada Initial results from the Madison Dynamo Experiment provide details of the inductive response of a turbulent flow of liquid sodium to an applied magnetic field. The magnetic field structure is reconstructed from both internal and external measurements. A mean toroidal magnetic field is induced by the flow when an axial field is applied, thereby demonstrating the Omega Effect. Poloidal magnetic flux is expelled from the fluid by the poloidal flow. Small-scale magnetic field structures are generated by turbulence in the flow. The resulting magnetic power spectrum exhibits a power-law scaling consistent with the equipartition of the magnetic field with a turbulent velocity field. The magnetic power spectrum has an apparent knee at the resistive dissipation scale. Large-scale eddies in the flow cause significant changes to the instantaneous flow profile resulting in intermittent bursts of non-axisymmetric magnetic fields, demonstrating that the transition to a dynamo is not smooth for a turbulent flow. [Preview Abstract] |
Saturday, April 22, 2006 2:30PM - 2:42PM |
C16.00006: Measurements of the two-point correlation function in the ion phase-space Fred Skiff, Ahmed Diallo, Ilker Uzun We report measurements of the ion two-point correlation function in a magnetized plasma cylinder. Using a single frequency laser and two movable light-collection systems we measure the space, time, and particle velocity dependence of the two-point correlation function on the axis of a weakly- collisional magnetized plasma in a plasma cylinder where the main source of fluctuations is dissipative drift instability. Due to convection of waves out of the plasma volume, the collective fluctuations remain at low amplitude. In addition to the well known drift instability, we observe other collective effects in the two-point correlation that are anomalous. A kinetic component to the fluctuations, which exhibits a particle velocity dependent phase velocity, suggests plasma nonlinearity at low amplitude. Very quiet plasma discharges are made using a customized inductively coupled plasma source that operates continuously at low power (5W). Singly ionized Argon plasmas are produced with Te$\approx$2eV and Ti$\approx$0.1eV at a density near $10^{9}\: cm^{-3} $. Each particle species exhibits a nearly Maxwellian velocity distribution function and the drift instabilities are at a relative density fluctuation of near $10\%$. The plasma discharge is stabilized and large data sets are taken to obtain 58 db of dynamic range on the correlation measurement. [Preview Abstract] |
Saturday, April 22, 2006 2:42PM - 2:54PM |
C16.00007: Search for Zonal Flow Associated with ITG Modes in CLM Amiya Sen, Vladimir Sokolov, Xiao Wei The basic physics transport experiments in the Columbia Linear Machine (CLM) have indicated inverse gyro-Bohm dependence of the ion thermal conductivity on the isotope mass. This paradox was explained by an unique 3-wave coupling of two ITG modes and an ion acoustic wave [1]. However, the competitive process of transport regulation via zonal flow [2] was not investigated. A series of basic experiments to detect any generation of zonal flow is now reported. Towards this end we have carefully examined the fine structure of the power spectrum and found two side bands with a shifts of about 3 kHz. An interpretation of this in terms of frequency modulation (FM) of ITG mode by a zonal flow appears plausible. Detailed cross correlation studies of two axially separated probes (one capacitive probe, the other ring probe) reveal two distinct modes (both m=0): one with $k_{//}\ne $ 0, $\omega $/2$\pi $ $\sim $ 2 to 10 kHz ion acoustic wave, and the other with $k_{//}$ = 0, $\omega $/2$\pi \quad \le $ 5 kHz, possibly a zonal flow. A theory using reductive perturbation method has been developed for self coupling of ITG mode in conjunction with the 3- wave coupling of two ITG radial harmonics and an ion acoustic mode. The results agree with the experimental values within a factor of 2. \newline [1] V. Sokolov and A. K. Sen, Phys. Rev. Lett, \textbf{92}, 165002 (2004). \newline [2] P.H.Dimond, S.Champeaux, M.Malkov et al., Nucl.Fusion \textbf{41}, 1067 (2001). [Preview Abstract] |
Saturday, April 22, 2006 2:54PM - 3:06PM |
C16.00008: Linear Stability Analysis of Self-Similar Ablation Flows of ICF Serge Gauthier, Jean-Marie Clarisse, Carine Boudesocque-Dubois, Virginie Lombard The stability of an ablative flow is of crucial importance in inertial confinement fusion (ICF). In particular, laser imprinting on accelerated target determines the seeds for the acceleration phase Rayleigh-Taylor instability. We present the first analysis of the imprinting problem using a dynamic solution. We first exhibit a family of exact self-similar solutions of the gas dynamics equations with nonlinear heat conduction. Such self-similar solutions are representative of the early stage of the pellet ablation by a laser. A wide variety of ablation configurations may thus be obtained. Linear stability analyses of such time-dependent solutions are then performed by solving an initial and boundary value problem. These similar solutions and their 3D perturbations are computed using a highly accurate numerical method, namely an auto-adaptive multidomain Chebyshev spectral method.Various solutions may be studied, depending on the choice of initial conditions. Generally and by contrast with steady low Mach number models for ablation front, we obtain that: (a) maximum perturbation amplitudes in the thin ablation layer are reached for transverse wavenumber k=0; (b) the growth and damping of ablation front perturbations are clearly related to thermal diffusion; (c) ablation front perturbations seem to persist although the transverse wave number increases. [Preview Abstract] |
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