Physics Highlights, 2009
Experiments for studying the dynamics of solar flux loops
MRX vacuum vessel for the initial experiments. A plasma arc (dark red) is initiated from and maintained by two electrodes which are connected to a pulsed power supply (not shown). Various coils provide the necessary vacuum magnetic field for the experiments: current flowing along the 8 toroidal field coils (green) provides the toroidal magnetic field, Bt, which is aligned parallel to the axis of the plasma arc; one pair of DC coils external to the vacuum chamber (big gray circles) provides a steady component of the equilibrium field (BE). The current flowing in the plasma arc provides the poloidal field component (Bp), twisting the field lines. Also shown is the 2-D magnetic probe array.
Initial experiments have been carried out in Argon, Helium and Hydrogen plasmas in Magnetic Reconnection Experiment (MRX) to study the detailed dynamic evolution of flux ropes relevant to the solar atmosphere. The figure below shows the experimental setup. A plasma arc is initiated and maintained by two electrodes. The electrodes are connected to a pulsed power supply. The plasma arc can be maintained at the desired height for much longer time than the Alfven time (0.5-1 millisecond >100 τA), as is the case for a solar flare. Internal probe arrays are used to measure the magnetic profile of the flare. The q value, which describes the rotational transform of field lines, is the key for characterizing the global stability. When the external toroidal ?eld is high (with q > 1) the flux loop immediately reaches a stable equilibrium and does not move around. When the toroidal field is low (with q < 1) the flux loops becomes unstable and moves around violently. The initial data have shown that line-tying plays an important role in the cathode side of the plasma arc as well as in the discharge evolution. The results from these experiments will be digested in the context of Coronal Mass Ejection (CME) dynamics, which we believe will make a valuable contribution to understanding eruptions on the Sun; this study will contribute to the interpretation of data from Hinode, STEREO, and the upcoming SDO mission by providing ''ground truth'' for the many theoretical and modeling groups working on simulations of the twisted flux rope model.
Experimental and Theoretical Results on Magnetorotational Instability (MRI)
The MRI is thought to be one of the primary mechanisms by which angular momentum is transported in astrophysical accretion disks. It may also act as a dynamo, and may play a role in galaxy disks and in stars. An attempt to study its properties in the laboratory is now underway. After successfully transitioning to the liquid metal phase at the Princeton MRI experiment, we began MHD experiments by imposing an axial magnetic field on hydrodynamically unstable flows with the outer cylinder at rest after allowing about one minute for the system to reach a statistically steady state. After about 15 seconds, coherent MHD structures emerge with azimuthal mode number m=1. By spatial Fourier decomposition, we found there are actually two rotating modes: one rotates faster than the other. Both of these speeds increase with the strength of the imposed magnetic field. These two modes exhibit the predicted dependence of rotating speeds on magnetic field by fast and slow magnetocoriolis waves as shown in the figure A. The observation is significant in that the slow magnetocoriolis wave goes to zero frequency and becomes unstable giving rise to the MRI with sufficient rotation and shear. The observation of these driven damped waves gives us a means of diagnosing the stability threshold for the MRI.
Numerical simulations play a key role in interpreting the results of the experiment. We have conducted a series of preliminary 3D MHD numerical simulations of the nonlinear development of MRI for low magnetic Prandtl numbers relevant to the experiment. The figure B shows a volume rendering of axial magnetic field in the experimental geometry for one of the cases run in Rayleigh-unstable regime at Re=3000 and Rm=1.
Mass Dependence of Ion Heating Associated with Magnetic Reconnection in MST
on heating associated with magnetic reconnection is observed in laboratory experiments, the magnetosphere, and in solar flares. In the MST reversed field pinch laboratory plasma, the ions can attain temperatures >1 keV during rapid magnetic reconnection events, well in excess of the electron temperature. This is a clear indication of non-collisional ion heating. The energy reservoir for heating is the mean magnetic field that confines the plasma, but the process by which the energy conversion occurs is not known. New measurements have identified that the energy conversion efficiency depends on the mass of the ions. The fraction of the energy loss in the mean magnetic field that appears as ion thermal energy scales as ΔEtherm / ΔEmag ~ Mi^0.5, shown in the figure below. Similar mass dependence is observed in the solar corona and wind. This is an important new clue that should help discriminate possible mechanisms, including viscous dissipation of plasma flows, resonant cyclotron heating in a nonlinear turbulent cascade, and stochastic heating in a turbulent electric field. This work was presented as an invited talk at the 2009 APS April Meeting, and will also be an invited talk at the 2009 APS Division of Plasma Physics Meeting.
Fast Magnetohydrodynamic (MHD) Reconnection
Understanding how magnetic reconnection can be fast even when the Ohmic diffusion time is many orders of magnitude longer than the dynamical time is a critical issue in reconnection research. Although much work on fast reconnection has focused on collisionless or kinetic effects, a number of recent theoretical and computational studies have shown that even MHD, or fluid theory, admits fast reconnection. We identified a secondary tearing instability that causes an initially uniform and thin current sheet to break up and reconnect more rapidly, at a rate that appears insensitive to the Ohmic diffusion rate. The time progression of the instability is shown below in the figure below.
Particle Transport in a Stochastic Magnetic Field
Plasma particle transport in a stochastic magnetic field is a key process in many environments ranging from laboratory plasmas, where it can destroy confinement, to clusters of galaxies, where it determines heat transport. Particle transport associated with magnetic tearing has now been measured in the reversed field pinch laboratory plasma MST. Multiple tearing modes appear, causing the magnetic field to become stochastic. Advanced laser interferometry was used to measure the correlations between density fluctuations and magnetic field fluctuations to determine the radial particle flux. This fluctuation induced particle transport is sufficient to account for the observed particle flux, as shown in the figure below. The dashed line is the measured particle flux; the solid line is the flux computed from a theory of transport induced by fluctuations. The two agree quite well. Surprisingly, the flux is larger than expected for well-established test particle expectations, suggesting that a more complete theory is needed.