48 research outputs found
Fast magnetic reconnection in laser-produced plasma bubbles
Recent experiments have observed magnetic reconnection in
high-energy-density, laser-produced plasma bubbles, with reconnection rates
observed to be much higher than can be explained by classical theory. Based on
fully kinetic particle simulations we find that fast reconnection in these
strongly driven systems can be explained by magnetic flux pile-up at the
shoulder of the current sheet and subsequent fast reconnection via two-fluid,
collisionless mechanisms. In the strong drive regime with two-fluid effects, we
find that the ultimate reconnection time is insensitive to the nominal system
Alfven time.Comment: 5 pages, 4 figures, accepted by Phys. Rev. Let
High-Resolution Particle-In-Cell Simulations of Two-Dimensional Bernstein-Greene-Kruskal Modes
We present two dimensional (2D) particle-in-cell (PIC) simulations of 2D
Bernstein-Greene-Kruskal (BGK) modes, which are exact nonlinear steady-state
solutions of the Vlasov-Poisson equations, on a 2D plane perpendicular to a
background magnetic field, with a cylindrically symmetric electric potential
localized on the plane. PIC simulations are initialized using analytic electron
distributions and electric potentials from the theory. We confirm the validity
of such solutions using high-resolutions up to a 2048^2 grid. We show that the
solutions are dynamically stable for a stronger background magnetic field,
while keeping other parameters of the model fixed, but become unstable when the
field strength is weaker than a certain value. When a mode becomes unstable, we
observe that the instability begins with the excitation of azimuthal
electrostatic waves that ends with a spiral pattern
Model for Incomplete Reconnection in Sawtooth Crashes
A model for incomplete reconnection in sawtooth crashes is presented. The
reconnection inflow during the crash phase of sawteeth self-consistently
convects the high pressure core toward the reconnection site, raising the
pressure gradient there. Reconnection shuts off if the diamagnetic drift speed
at the reconnection site exceeds a threshold, which may explain incomplete
reconnection. The relaxation of magnetic shear after reconnection stops may
explain the destabilization of ideal interchange instabilities reported
previously. Proof-of-principle two-fluid simulations confirm this basic
picture. Predictions of the model compare favorably to data from the Mega
Ampere Spherical Tokamak. Applications to transport modeling of sawteeth are
discussed. The results should apply across tokamaks, including ITER.Comment: 5 pages, 3 figures, accepted for publication in PR
A comparison of spectral element and finite difference methods using statically refined nonconforming grids for the MHD island coalescence instability problem
A recently developed spectral-element adaptive refinement incompressible
magnetohydrodynamic (MHD) code [Rosenberg, Fournier, Fischer, Pouquet, J. Comp.
Phys. 215, 59-80 (2006)] is applied to simulate the problem of MHD island
coalescence instability (MICI) in two dimensions. MICI is a fundamental MHD
process that can produce sharp current layers and subsequent reconnection and
heating in a high-Lundquist number plasma such as the solar corona [Ng and
Bhattacharjee, Phys. Plasmas, 5, 4028 (1998)]. Due to the formation of thin
current layers, it is highly desirable to use adaptively or statically refined
grids to resolve them, and to maintain accuracy at the same time. The output of
the spectral-element static adaptive refinement simulations are compared with
simulations using a finite difference method on the same refinement grids, and
both methods are compared to pseudo-spectral simulations with uniform grids as
baselines. It is shown that with the statically refined grids roughly scaling
linearly with effective resolution, spectral element runs can maintain accuracy
significantly higher than that of the finite difference runs, in some cases
achieving close to full spectral accuracy.Comment: 19 pages, 17 figures, submitted to Astrophys. J. Supp
Synthesis of 3-D coronal-solar wind energetic particle acceleration modules
1. Introduction Acute space radiation hazards pose one of the most serious risks to future human and robotic exploration. Large solar energetic particle (SEP) events are dangerous to astronauts and equipment. The ability to predict when and where large SEPs will occur is necessary in order to mitigate their hazards. The Coronal-Solar Wind Energetic Particle Acceleration (C-SWEPA) modeling effort in the NASA/NSF Space Weather Modeling Collaborative [Schunk, 2014] combines two successful Living With a Star (LWS) (http://lws. gsfc.nasa.gov/) strategic capabilities: the Earth-Moon-Mars Radiation Environment Modules (EMMREM) [Schwadron et al., 2010] that describe energetic particles and their effects, with the Next Generation Model for the Corona and Solar Wind developed by the Predictive Science, Inc. (PSI) group. The goal of the C-SWEPA effort is to develop a coupled model that describes the conditions of the corona, solar wind, coronal mass ejections (CMEs) and associated shocks, particle acceleration, and propagation via physics-based modules. Assessing the threat of SEPs is a difficult problem. The largest SEPs typically arise in conjunction with X class flares and very fast (\u3e1000 km/s) CMEs. These events are usually associated with complex sunspot groups (also known as active regions) that harbor strong, stressed magnetic fields. Highly energetic protons generated in these events travel near the speed of light and can arrive at Earth minutes after the eruptive event. The generation of these particles is, in turn, believed to be primarily associated with the shock wave formed very low in the corona by the passage of the CME (injection of particles from the flare site may also play a role). Whether these particles actually reach Earth (or any other point) depends on their transport in the interplanetary magnetic field and their magnetic connection to the shock