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 RI1b: Space and Astrophysical Plasmas II |
Hide Abstracts |
Chair: Annick Pouquet, National Center for Atmospheric Research Room: Adam's Mark Hotel Plaza Ballroom ABC |
Thursday, October 27, 2005 3:00PM - 3:30PM |
RI1b.00001: Turbulent magnetic dynamo excitation at low magnetic Prandtl number Invited Speaker: Planetary dynamos likely result from turbulent motions in magnetofluids with kinematic viscosities that are small compared to their magnetic diffusivities. Laboratory experiments are in progress to produce similar dynamos in liquid metals. We present computations of thresholds in critical magnetic Reynolds number above which dynamo amplification can be expected for mechanically-forced turbulence (helical and non-helical, short wavelength and long wavelength) as a function of the magnetic Prandtl number [1]. Complications result from the fact that the kinetic turbulent spectrum is much broader in wavenumber space than the magnetic spectrum because of the high mechanical Reynolds number. The numerical difficulties are bridged by a combination of overlapping direct numerical simulations and subgrid models of MHD turbulence. Typically, the critical magnetic Reynolds number increases steeply as the magnetic Prandtl number decreases, and then reaches an asymptotic plateau at values of at most a very few hundred. In the turbulent regime and for magnetic Reynolds numbers large enough, both small and large scale magnetic fields are excited. The interaction between different scales in the flow will be also briefly discussed [2]. \newline \newline [1] P.D. Mininni, Y. Ponty, D.C. Montgomery, J.-F. Pinton, H. Politano, and A. Pouquet, ApJ 626, 853 (2005); Y. Ponty et al, PRL 94, 164502 (2005); P.D Mininni and D. Montgomery, arXiv:physics/0505192.\newline [2] A. Alexakis, P.D. Mininni, and A. Pouquet, arXiv:physics/0505183; arXiv:physics/0505189. [Preview Abstract] |
Thursday, October 27, 2005 3:30PM - 4:00PM |
RI1b.00002: Turbulence and Plasma Physics in Clusters of Galaxies Invited Speaker: The intracluster medium appears to be in a turbulent state. It is also threaded by randomly tangled magnetic fields. In the past few years there has been a dramatic increase in the quantity and quality of observational data on cluster turbulence and magnetic fields. The observed magnetic fields are strong enough to be dynamically important. The turbulence and magnetic field regulate the viscous heating and heat transport that determine the thermal structure of clusters. A coherent theory of magnetized cluster turbulence is necessary for understanding cluster behaviour on both large and small scales. The strength and certainly the structure of the cluster fields are determined by their interaction with the turbulence. This talk will first describe the fundamental properties of the turbulent generation of magnetic fields: (1) what type of field structure can be produced and maintained; (2) how a dynamical saturated state is achieved; (3) what are the observable signatures of the field structure in clusters. The field structure in no small measure depends on the nature of the viscous and magnetic cutoffs. These are determined by the plasma physics of the intracluster medium, which has very low collisionality. It will be shown that, under very general conditions, cluster plasmas threaded by weak magnetic fields are subject to firehose and mirror instabilities. These are driven by the anisotropies of the plasma pressure (viscous stress) that naturally arise in any weakly magnetized plasma that has low collisionality and is subject to stirring. The effect is captured by the extended MHD model with Braginskii viscosity, but, as the instability growth rates are proportional to the wavenumber down to the ion gyroscale, MHD equations with Braginskii viscosity are not well posed and a fully kinetic description is necessary. The instabilities may lead to the amplification of magnetic fields in clusters to the observed strength of $\sim10\mu$G on cosmologically trivial time scales ($\sim10^8$~yr). The saturation of the instabilities controls the effective transport properties of the cluster plasmas. [Preview Abstract] |
Thursday, October 27, 2005 4:00PM - 4:30PM |
RI1b.00003: Laboratory study of dense planetary interiors and giant impacts using laser-driven shock waves Invited Speaker: The behavior of matter at Megabar pressures, a few times solid density, and eV temperatures presents a fundamental challenge, one that is critical to our understanding of dense planetary interiors, planetary evolution models, and giant impacts. Under these conditions bulk matter is strongly coupled, with temperatures approaching the Fermi energy and electron wavelengths comparable to the interatomic spacing - a quantum-classical ``transition'' regime not amenable to many of the traditional theoretical approaches used in condensed matter or plasma physics. The laser-driven shock wave has matured into a powerful tool for accessing and probing these conditions with several new techniques having been developed recently. Measurements of the equation-of-state and transport properties of important planetary materials including silica ( SiO$_{2}$ ) and hydrogen have been performed. In particular, silica - the major constituent of terrestrial planets - has been shown to undergo an insulator-to-conductor transition above melting at conditions similar to those in giant impacts (such as the one believed to have created the Moon) and at the earth's core-mantle boundary. This continuous transformation, occurring at pressures between 1 to $\sim $4 Mbar, is accompanied by an anomalously high specific heat that returns to the Dulong-Petit value at completion of the transformation. This is suggestive of a ``bond-breaking'' process in the condensed system - analogous to dissociation in a gas - as the fluid transforms from liquid to dense plasma. Work performed in collaboration with T. R. Boehly, P. M. Celliers, J. H. Eggert, J. E. Miller, D. D. Meyerhofer, and G. W. Collins under the auspices of the US DOE by LLNL under Contract No. W-7405-ENG-48 and by the U. Rochester under Cooperative Agreement No. DE-FC03-92SF19460. [Preview Abstract] |
Thursday, October 27, 2005 4:30PM - 5:00PM |
RI1b.00004: Radiative shocks: an opportunity to study Laboratory Astrophysics. Invited Speaker: A shock becomes radiative when it produces a significant upstream ionizing photons. This phenomenon occurs for shock velocities exceeding a given threshold which depend strongly on the medium. These velocities are typically or the order of 100 km/s and more, common value in astrophysics. Here we shall present a serie of experiments performed at LULI laboratory using the old 6 beams and the new LULI2000 facility. Scaling laws and hydrodynamic simulations allowed to design the target characteristics according to the available laser energy. A strong shock was driven in a layered solid target (CH-Ti-CH) which then accelerates into a gas cell ($\approx $60km/s) filled with Xenon at low pressure (0.1-0.3bar) producing a radiative supercritical shock. A laser beam (8ns-532nm) probes the Xenon gas in the transverse direction and was injected into either a Mach-Zenhder or a VISAR interferometer. In this last case two additional optical framing cameras was used. On the rear side, self-emission and VISAR diagnostics were utilized. All these diagnostics allow to determine many relevant parameters linked to the shock or the radiative precursor. Indeed we shall present experimental data for the shock temperature and velocities, the precursor 2D time evolution, its electron density, density gradient and temperature. Data were obtained for different laser intensities and gas pressures. Comparisons with 1D (MULTI) and 2D (DUED) radiative hydrodynamic codes will be presented for all measured parameters (shock velocity, shape, radial expansion, and temperature as well as precursor velocity and precursor electron density). [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