Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session BI2: Basic Plasma Physics I |
Hide Abstracts |
Chair: Scott Hsu, Los Alamos National Laboratory Room: Adam's Mark Hotel Plaza Ballroom EF |
Monday, October 24, 2005 9:30AM - 10:00AM |
BI2.00001: Initial results from the Madison Dynamo Experiment Invited Speaker: The Madison dynamo experiment is designed to self-generate magnetic fields from flows of conducting metal in a simply connected spherical geometry. This talk will present initial results from the experiment since commencement of operations in August of 2004. Thus far, the experiment has been operated at 60\% of its design specifications achieving magnetic Reynolds numbers of 130, based on propellor tip speed. The technical operation of the experiment has been demonstrated (i.e., transfers of liquid sodium, rotating seals, etc). The experimental approach to understanding the electromagnetic properties of the sodium involves comparisons between experimental measurements of the magnetic field in the sodium experiment, measurements of the velocity field in a dimensionally identical water experiment, and predictive MHD codes that model the currents induced in the turbulent flows by externally applied fields. Initial results include: direct observation of an $\omega$ effect, the production of a toroidal magnetic field from a poloidal magnetic field; the expulsion of poloidal flux by vortical fluid motion; measurement of gain for the expected dynamo eignenmode; and measurement of the turbulent shredding of a large scale magnetic field by small scale turbulence, as determined from a spatial array of magnetic probes generating mode number spectra. As background, the theoretical basis for the experiment and hydromagnetic modeling results will be reviewed, including results from recent 3D MHD computation of the backreaction and the role of turbulence on self-excitation. An interesting implication of the simulations is that one role of the turbulence is to increase the critical magnetic Reynolds number for self-excitation, a result consistent with an increased resistivity due to the turbulence, as in the beta effect. Future plans will be discussed, including the strategy observing self-excitation. [Preview Abstract] |
Monday, October 24, 2005 10:00AM - 10:30AM |
BI2.00002: Novel vorticity probe and its use for the study of strong cross-field sheared flow in the Large Plasma Device Invited Speaker: We report evidence for the existence of coherent structures created by the Kelvin-Helmholtz instability in steady-state, shear-flow driven plasmas in the LArge Plasma Device (LAPD) facility at UCLA. The measurements are performed with the Vorticity Probe, a newly designed probe that can directly measure the plasma vorticity associated with the $\vec E\times\vec B$ shear flow by means of a method that is both simpler and more accurate than the methods used in neutral fluids. Because the rate of change of vorticity is a key quantity in nonlinear models, like in Hasegawa-Mima equation, for interchange modes in plasmas, its direct measurement is critical for verification purposes. The physical origin of the rate of change of plasma vorticity from $\vec E\times\vec B$ flow is the divergence of the ion polarization current. Vortex coherent structures occur when the vorticity is a nonlinear function of the stream function (which for magnetized plasmas is the electric potential divided by the magnetic field strength). A strong-shear-flow regime in the LAPD was used to create the Kelvin-Helmholtz instability. Comparisons of the measured vortex characteristics with the results from nonlinear simulations of the systems will be described. We also carry out nonlinear simulations of density and magnetic flux as passively advected scalars in the fluctuating potential and study the mixing of particles and magnetic flux across the shear layer, for which the Kelvin-Helmholtz instability is believed to be the underlying mechanism in both space and laboratory plasmas. [Preview Abstract] |
Monday, October 24, 2005 10:30AM - 11:00AM |
BI2.00003: Stereoscopic particle image velocimetry studies of transport in dusty plasmas Invited Speaker: Over the past six years, the Auburn Plasma Sciences Laboratory (PSL) has been applying particle image velocimetry (PIV) techniques to the study of microparticle transport in dusty plasmas. PIV is a powerful experimental tool - originally developed in the fluid physics community - in which displacement of micron-sized particles is determined by two successive illuminations by a planar laser sheet. Since the time interval between the illuminations is a known quantity (i.e., set by the experimenter), the velocity of the particles can be computed. In a fluid, this measurement gives an indication of the collective motion of the medium. By contrast, in a dusty plasma, the PIV technique gives a direct measurement of the microparticle transport in the plasma. Furthermore, because the PIV technique is a measurement over a region, not just a point measurement, it can provide a global perspective on transport over the entire dusty plasma. In this manner, PIV provides a tool to study global plasma transport phenomena at a kinetic level. This presentation will briefly compare stereo-PIV to two-dimensional PIV and other optical dusty plasma diagnostics. The presentation will then highlight three unique capabilities of the stereo-PIV diagnostic: measuring three-dimensional particle transport, obtaining information on the three- dimensional velocity space distribution function, and reconstructing particle motion near dust acoustic waves. [Preview Abstract] |
Monday, October 24, 2005 11:00AM - 11:30AM |
BI2.00004: Measurements of Ion Density Fluctuations in Phase-Space Invited Speaker: Resolving the ion density fluctuations in phase-space is a natural extension of the large number of density fluctuation measurements performed using electric probes, and opens a new window into the kinetic nature of density fluctuations in magnetized plasma. Using Laser Induced Fluorescence (LIF) and a two-point correlation function implementation, we resolve ion density fluctuations in space, time, and ion velocity. By computing the cross-power spectra between two spatially resolved LIF signals, the ion density fluctuations reveal two components with distinct correlation lengths. The largest component has a three-meter wavelength and is explained by fluid theory. In addition, a short wavelength component, with a short correlation length that is consistent with the ion mean free path, is detected. This latter component propagates at a phase velocity close to the ion parallel velocities. We refer to this newly-identified ion velocity-dependent component as the ``kinetic component,'' which is associated with a spectral feature near $\omega $* characterized by $\delta \omega $/$\omega \quad \sim $ 0.1. As the ion-neutral collision frequency is increased in the discharge, we observe a threshold that is marked by a narrowing of the drift wave spectrum. At this threshold, the kinetic component vanishes. Through correlations between Langmuir probe and LIF, we measure the phase shift between the fluctuating potential and each velocity stream. We observe a velocity dependent fluctuation induced transport rate near the drift frequency, which significantly depart from the classical prediction of the drift wave transport rate. The basic issues in resolving the ion density fluctuations in phase-space, as well as the key physics components that we observe will be presented. [Preview Abstract] |
Monday, October 24, 2005 11:30AM - 12:00PM |
BI2.00005: Study of Two-Fluid MHD Physics and the Effects of Boundary Conditions on Magnetic Reconnection in MRX Invited Speaker: Magnetic reconnection is an important physical process which leads to topology change and magnetic field evolution in many laboratory and astrophysical plasmas. In general, the reconnection process is determined by local plasma dynamics in the diffusion region as well as by global boundary conditions. In this talk we report recent experimental results on both aspects of reconnection physics from the Magnetic Reconnection eXperiment (MRX) [1]. Globally, the effects of boundary conditions have been studied by varying the distance between the two ``flux-cores'' which are used to drive reconnection in MRX. It is found that, despite large changes in both the current sheet length and the outflow speed, the observed reconnection rate can be understood in the framework of a generalized Sweet-Parker using the local plasma parameters and an effective local resistivity [2]. There are two leading theories to explain the local physics of fast reconnection: one involves anomalous resistivity due to wave-particle interactions, and the second is based on two-fluid MHD effects arising from the decoupling of electron and ion motions in the diffusion region [3]. The hallmark of the latter is a quadrupole out-of-plane magnetic field, which has never previously been observed in a laboratory experiment. Using an array of magnetic pickup coils with a spatial resolution comparable to the electron skin depth, we have successfully detected the quadrupole out-of-plane field [4], strikingly similar to that predicted by theory. Detailed physical analysis based on these new experimental results, including comparisons with the space and astrophysical observations [5], will also be presented. This work is supported by DOE, NASA and NSF. In collaboration with Y. Ren, S. Gerhardt, H. Ji, R. Kulsrud, and M. Yamada. \newline \newline [1] M.Yamada et al., Phys. Plasmas, 7, 1781 (2000) \newline [2] A. Kuritsyn et al., to be submitted (2005) \newline [3] e.g. J. Birn, J.F. Drake, M.A. Shay et al., 106, 3715 (2001) \newline [4] Y. Ren et al., to appear in Phys. Rev. Lett. (August 2005) \newline [5] e.g. F.S. Mozer et al., PRL 89, 015002 (2002) [Preview Abstract] |
Monday, October 24, 2005 12:00PM - 12:30PM |
BI2.00006: Electrostatic turbulence and transport in a simple magnetized plasma Invited Speaker: Magnetically confined plasmas are subject to gradient driven drift instabilities, causing anomalous transport. The conditions under which these instabilities grow and develop nonlinearly are investigated on the TORPEX toroidal device (R=1m, a=0.2m), taking advantage of its diagnostic accessibility and of the possibility of controlling key quantities such as the magnetic field pitch angle and the density profile. Plasmas are produced by low field side injection of microwaves (P$\le $20kW) with f=2.45GHz, in the EC frequency range. Typical density and temperature are n$_{e}\le $10$^{17}$m$^{-3}$ and T$_{e}\le $5eV. The magnetic field is mainly toroidal ($\le $0.1T), with a small vertical component ($\le $2mT). In addition, an ohmic transformer can be used to drive plasma current and investigate the changes in the character of fluctuations as the magnetic flux surfaces are progressively closed. A quasi-static confinement model, verified experimentally, highlights the role of parallel flows in short circuiting drift-induced charge separation. The related confinement time and the fluctuation properties depend strongly on the vertical field, which can thus be used as a control parameter. Among the diagnostics dedicated to turbulence studies, a movable 4-tip probe measures the local dispersion relation and the profile of the fluctuation-induced flux. This flux is compared with the transport parameters inferred from the plasma response to microwave power modulation. An 86-tip probe reconstructs the temporal evolution of structures in ion saturation current or in floating potential across the plasma cross-section. The local statistical description of the turbulence can therefore be combined with 2D spatio-temporal turbulence imaging. Methods to characterize statistically the measured structures and construct quantitative observables will be discussed, along with initial comparisons with fluid codes. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700