### Session GI2: Space and Astrophysical Plasmas

Chair: Peter Gary, Los Alamos National Laboratory
Room: Landmark B

 Tuesday, November 18, 2008 9:45AM - 10:15AM GI2.00001: Katherine E. Weimer Award Talk: Instabilities and magnetic reconnection in space plasma and the physics of laser-plasma interactions Invited Speaker: Lin Yin First-principles, kinetic simulations are invaluable for understanding nonlinear plasma physics in a diverse range of complex systems ranging from magnetic reconnection in space and astrophysics to laser plasma interactions in both long- and short-pulse regimes. Employing the state-of-the-art particle-in-cell (PIC) simulation code VPIC, with and without Coulomb collisions, to conduct three-dimensional simulations at unprecedented scales on the world's most powerful supercomputers, we address three outstanding problems. First, magnetic reconnection in electron-positron plasmas involves a complex interaction of tearing and kink modes. The diffusion region tends to elongate in the outflow direction and become unstable to secondary kinking and formation of plasmoid-rope'' structures. These secondary instabilities determine the time-dependent nature of reconnection. Second, backward stimulated Raman scattering (SRS) in a laser speckle involves two intricate electron plasma wave behaviors, wavefront bending and self-focusing, caused by trapped electron nonlinear frequency shift and amplitude-dependent damping. Self-focusing exhibits loss of angular coherence by formation of a filament necklace and leads to a reduction of electron plasma wave coherence. The latter reduces source coherence for backscattered light and increases damping, which fundamentally limits how much backscatter can occur from a laser speckle. Finally, we show how GeV ion beams can be generated via the interaction of ultra-intense short-pulse lasers with solid-density targets. This Break-out Afterburner'' mechanism promises to enable development of ultra-compact GeV ion beam sources. (Collaborators: B. J. Albright, B. Bergen, K. J. Bowers, W. Daughton, J. C. Fernandez. K. A. Flippo, B. M. Hegelich, J. Kline, T. Kwan, D. Montgomery, H. Rose, and D. Winske) Tuesday, November 18, 2008 10:15AM - 10:45AM GI2.00002: Experimental Verification of the Stationary Inertial Alfven Wave and its Relevance to Auroral Plasma Physics Invited Speaker: Mark Koepke A small, off-axis mesh anode electrode at one plasma-column end is used to create a paraxial channel of both electron current and depleted density in the Large Plasma Device Upgrade (LAPD-U) at UCLA. It is shown that the on-axis, larger, surrounding-plasma column rotates about its cylindrical axis because a radial electric field is imposed by a multiple-segmented-disk termination electrode on the same end as the mesh-anode electrode. The radial profile of azimuthal velocity is shown to be consistent with rigid-body rotation. Launched inertial Alfven waves are shown to concentrate in the off-axis channel of electron current and depleted plasma density. In the absence of launched waves, time varying boundary conditions, or spatially structured boundary conditions, we demonstrate that a non-fluctuating, non-traveling pattern in the plasma density arises spontaneously in the channel, but only in the combined presence of electron current, density depletion, and cross-field convection (i.e., rotation). The experimental verification of stationary inertial Alfven waves is based on these results and the predictions from a model of finite-collisionality, finite-pressure stationary Alfven waves that links laboratory and auroral plasma regimes. Ground-based optical observations will be shown that indicate the need for a quasi- static theory of structured electron acceleration within auroral arcs. The properties of the stationary inertial Alfven wave suggest it as promising candidate. Tuesday, November 18, 2008 10:45AM - 11:15AM GI2.00003: First MHD Results of the Princeton MRI Experiment Invited Speaker: M.D. Nornberg The accretion of gas, dust, and plasma orbiting a strong gravitational source is responsible for the observed luminosity of systems such as binary stars and active galactic nuclei. Accretion disk dynamics also set the timescale for star and planet formation in protostellar disks. The accretion rate is governed by how quickly angular momentum can be transported through the disk. Inferred accretion rates suggest that viscous transport is insufficient to explain observations. The proposed mechanism for turbulent transport in these disks is the magnetorotational instability (MRI), a linear instability caused by the Maxwell stress introduced by an ambient magnetic field coupled to the Keplerian sheared flow. The MRI is sufficiently generic that it should be observable in any hydrodynamically stable rotating shear flow with a radially-decreasing angular velocity for sufficiently high magnetic Reynolds number with an applied axial magnetic field. The Princeton MRI Experiment was designed to study the stability of rotating shear flow in a magnetized conducting fluid. The unique design of this experiment allows the generation of quiescent shear flow at high Reynolds number. The experiment has been filled with a gallium eutectic alloy and operated with an applied axial magnetic field of up to 5~kG. The most recent measurements show the emergence of nonaxisymmetric MHD modes from magnetized turbulent shear flow using an array of radially-aligned induction coils. The modes precess toroidally and have a frequency splitting proportional to the rotation speed, a characteristic of magnetocoreolis (MC) waves. The relationship between the MRI and MC waves and a method of identifying the MRI through observation of excited MC waves is illustrated though analytical analysis, experimental data, and comparison with simulations. Tuesday, November 18, 2008 11:15AM - 11:45AM GI2.00004: Merger of super-Alfvenic current filaments during collisionless Weibel instability of relativistic electron beams Invited Speaker: Gennady Shvets The Weibel instability (WI) is one of the most basic and long-studied collective plasma processes. The dynamics and energetics of its nonlinear saturation is important for both laboratory and astrophysical plasmas. The WI is likely to play an important role in the Fast Ignitor scenario because it can result in the collective energy loss of a relativistic electron beam in both coronal and core plasma regions. Collisionless WI has been suggested as an important mechanism for relativistic collisionless shock formation in gamma ray bursts. This talk will focus on the strongly nonlinear long-term stage of the instability, during which the beam density of filaments is compressed to the background plasma density, and the ambient plasma is fully evacuated. Analytic and numerical results demonstrate that the beam filaments can carry super-Alfvenic currents by assuming current and density profiles similar to the Hammer-Rostoker equilibrium. This has profound implications for the long-term evolution of the magnetic field and beam current and explains the long-standing puzzle: why magnetic field energy initially increases, but eventually decreases during the collisionless WI. Novel numerical and analytic tools will be described that enable computationally efficient modeling of collective beam filamentation in both collisionless and collisional ambient plasmas. [1] O. Polomarov, A. Sefkow, I. Kaganovich, and G. Shvets, Phys. Plasmas 14, 043103 (2007); O. Polomarov, I. Kaganovich, and G. Shvets, Merger of super-Alfvenic current filaments during collisionless Weibel instability of relativistic electron beams,'' submitted to PRL (2008). Tuesday, November 18, 2008 11:45AM - 12:15PM GI2.00005: Phase-space turbulent cascade of entropy in magnetized weakly collisional plasmas Invited Speaker: Tomoya Tatsuno Plasma turbulence is often observed in collisionless or weakly collisional environments, both in astrophysical objects and in fusion devices. The turbulence in these systems is damped at small spatial scales (high $k$), where the distribution functions $f_k(v)$ contain fine-scale structures which arise from phase mixing. Irreversibility comes from collisions, which tend to smooth the fine velocity-space structure, even when the collisionality parameter is small. In this kind of system, the turbulent cascade takes place in phase space (i.e., in velocity space as well as in real, or wavenumber, space). Here, we consider phase mixing that arises from nonlinear interactions. This phase mixing corresponds to an entropy cascade''. We present the first numerical simulations of this phase-space cascade in magnetized weakly collisional plasmas, using the electrostatic gyrokinetic model. Assuming the homogeneity along the field line, we have focused on a decaying turbulence problem using AstroGK [1] in a simplified four-dimensional phase space (2D in real space perpendicular to the field line and 2D in velocity space). Since the gyro-averaged ExB drift introduces a nonlinear phase mixing, we obtain velocity-space structures perpendicular to the ambient field [2], rather than parallel to the magnetic field, such as would be produced by conventional Landau damping. Kolmogorov-like dimensional arguments [3] predict that the perpendicular velocity structures are finer for the higher wave numbers, with specific spectral indices. Our simulations confirm these arguments. The cascade finally terminates at the dissipation scale derived from the collision operator in the velocity space, where the actual irreversibility is realized. All of these properties are realized in the gyrokinetic simulations. [1] http://astro.berkeley.edu/\~{}ghowes/astrogk [2] W. Dorland and G. Hammett, Phys. Fluids B {\bf 5}, 812 (1993). [3] A. A. Schekochihin et al., to be published in Plasma Phys. Control. Fusion (2008); e-print arXiv:0806.1069. Tuesday, November 18, 2008 12:15PM - 12:45PM GI2.00006: Spontaneous generation of self-organized zonal flows in turbulent plasma Invited Speaker: Raoul Trines Drift wave turbulence is ubiquitous in magnetised plasma, in particular on density gradients that can be found in plasma edge configurations. Such configurations arise in both laboratory and space environments, while appropriate scaling the equations governing the drift waves allows them to be applied over a wide range of length and time scales. Therefore, the study of drift wave dynamics has applications ranging from the magnetosphere boundary to small laboratory plasma devices such as CSDX at UCSD [G.R. Tynan et al., J. Vac. Sci. Tech-A {\bf 15}, 2885 (1997)]. Recently, it was found that the interaction between drift modes and zonal flows at a plasma edge leads to self-organisation of the drift waves and the formation of solitary zonal flow structures [R. Trines \emph{et al.}, Phys. Rev. Lett. {\bf 94}, 165002 (2005)]. The interaction between broadband drift mode turbulence and zonal flows has been studied in numerical simulations based on the wave-kinetic approach. In these simulations, a particle-in-cell representation is used for the quasi-particles, while a fluid model is employed for the plasma. Simulation results show the development of self-organised zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyro-radius wide, drifting towards steeper relative density gradients. These results will be compared to observations made at the magnetopause by the Cluster satellites [R. Trines \emph{et al.}, Phys. Rev. Lett. {\bf 99}, 205006 (2007)] and to measurements performed on CSDX. This work is supported by the STFC Accelerator Science and Technology Centre and the STFC Centre for Fundamental Physics.