Seminars (GYP)

Electrostatic turbulence, related structures and their effect on particle, heat and momentum transport are investigated in TORPEX simple magnetized plasmas using high resolution diagnostics, control parameters, linear fluid models and nonlinear numerical simulations. The nature of the dominant instabilities is controlled by the ratio between vertical and toroidal magnetic field intensities. For Bv/BT>3%, only ideal interchange instabilities are observed. A critical pressure gradient to drive the interchange instability is experimentally identified. Interchange modes give rise to blobs, radially propagating filaments of enhanced plasma pressure. The measured values of blob velocities and sizes span a wide range and are described by a single analytical expression, from the small blob size regime in which the blob velocity is limited by cross-field ion polarization currents, to the large blob size regime in which the limitation to the blob velocity comes from parallel currents to the sheath. As a first attempt at controlling the blob dynamical properties, limiter configurations with varying angles between field lines and the conducting surface of the limiter are explored. Mach probe measurements clearly demonstrate a link between toroidal flows and blobs. To complement probe data, a fast framing camera and a movable gas puffing system are installed. Density and light fluctuations show similar signatures of interchange activity, but the fast camera images can be obtained with higher spatial resolution, proving data on small turbulence scales. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector. A theory validation project is conducted in conjunction with TORPEX experiments, based on quantitative comparisons of observables that are defined in the same way in the data and in 2D and 3D local and global simulations.

Wind and Cluster spacecraft observations of reconnecting current sheets in the Earth`s magnetotail show strong electron temperature anisotropy. This anisotropy is accounted for in a solution of the Vlasov equation that was recently derived for general reconnection geometries with magnetized electrons in the limit of fast transit time [1]. A necessary ingredient is a parallel electric field structure, which maintains quasi-neutrality by regulating the electron density, traps a large fraction of thermal electrons, and heats electrons in the parallel direction. Based on the expression for the electron phase space density, equations of state provide a fluid closure that relates the parallel and perpendicular pressures to the density and magnetic field strength [2]. This new fluid model agrees well with fully kinetic simulations of guide-field reconnection, where the parallel electron temperature becomes many times greater than the perpendicular temperature. In addition, the equations of state relate features of the electron diffusion region that develop during anti-parallel reconnection to the upstream electron beta. They impose strong constraints on the electron Hall currents and magnetic fields [3]. For plasmas with low electron beta gradients in the anisotropic pressure can support large parallel electric fields over extended regions. This is likely important for energization of super-thermal electrons in the Earth magnetotail [4] and perhaps also for fast electrons observed during reconnection events at the sun. [1] J. Egedal, N. Katz, et al., J. Geophys. Res. 113, A12207 (2008). [2] A. Le, J. Egedal, et al., Phys. Rev. Lett., 102, 085001 (2009). [3] A. Le, J. Egedal, et al., Geophys. Res. Lett. 37, L03106 (2010). [4] J. Egedal, A. Le, et al., Geophys. Res. Lett. In press (2010).

Discussion on the microtearing challenge using Colin Roach's (Culham) slides from a talk in Vienna on microtearing studies.

We investigate experimentally the motion and structure of isolated plasma filaments propagating through neutral gas. Plasma filaments, or "blobs," arise from turbulent fluctuations in a range of plasmas. Our experimental geometry is toroidally symmetric, and the blobs expand to a larger major radius under the influence of a vertical electric field. The electric field, which is caused by ÑB and curvature drifts in a 1/R magnetic field, is limited by collisional damping on the neutral gas. The blob's electrostatic potential structure and the resulting E×B flow field give rise to a vortex pair and a mushroom shape, which are consistent with nonlinear plasma simulations. We observe experimentally this characteristic mushroom shape. We also find that the blob propagation velocity is inversely proportional to the neutral density and decreases with time as the blob cools [1]. [1] Katz N, Egedal J, et al, (2008) Phys. Rev. Lett. 101, 015003.