Results of a 3D particle simulation of magnetic reconnection using 670 million particles. The top panel shows the strong electron current (in the out-of-plane direction) generated during reconnection. The bright region cuts across the magnetic x-line. The bottom panel shows the intense electric fields self-generated by the plasma in a plane perpendicular to that in the top figure and cutting through the region of strongest current. The adjacent regions of positive and negative polarity of the electric field are the signature of double layers, which are localized regions of intense electric field which scatter and heat electrons. Such layers are expected to be a prolific source of energetic electrons during magnetic reconnection in fusion and astrophysical plasmas.

Parvez Guzdar, Bill Dorland, James Drake, Adil Hassam, Robert Kleva, and Sergei Novakovskii, University of Maryland

Research Objectives
The Maryland Theory and Computational Physics Magnetic Fusion Energy Program focuses on (1) 3D simulation of particle, ion, and electron energy transport in the core and edge region of tokamak plasmas using the two-fluid Braginskii code Edge3D and the electromagnetic gyrokinetic code GS2; (2) 3D simulation of high-b disruptions and sawtooth crashes for tokamak plasmas using the toroidal resistive magnetohydrodynamics (MHD) code TORMHD; (3) 2D and 3D simulations of novel centrifugal confinement devices using MHD codes; and (4) 2D and 3D full-particle, hybrid, and two-fluid simulations of magnetic reconnection.

Computational Approach
The GS2 code is based on a continuum treatment of the gyrokinetic equations. The second-order accurate algorithm is comprised of an implicit treatment of the linear dynamics, an explicit, pseudo-spectral treatment of the nonlinear terms, and an Adams-Bashforth integrator in time. The gyrokinetic problem involves the usual 3D spatial grid, as well as a 2D velocity space grid, for a total of five dimensions. The Edge3D code is suitable for exploring transport in the colder edge regions of fusion plasmas. It is based on a fourth-order finite difference scheme with a trapezoidal leapfrog scheme for time stepping.

The TORMHD code solves the MHD equations on a toroidal grid. The basic computational scheme is the same as in Edge3D. The P3d code has been developed to explore magnetic reconnection or other nonlinear plasma phenomena. P3d can be run as a two-fluid, hybrid (particle ions and fluid, finite-mass electrons) or a full (particle electrons and ions) model. The full-particle version has been run with up to 1 billion particles to explore 3D collisionless magnetic reconnection.

Accomplishments
The nonlinear gyrokinetic finite-b studies of electron temperature gradient driven instabilities have established the existence of long radial "streamers" which strongly enhance the transport from these short-wavelength instabilities over what had been previously predicted. Simulations of tokamaks with toroidal flows have demonstrated that these flows have a stabilizing influence on sawteeth.

The release of magnetic energy during magnetic reconnection in nature and also in some laboratory experiments (sawteeth in tokamaks) is typically much faster than can be explained by resistive MHD models. We have shown that at the small spatial scales where magnetic reconnection occurs, the MHD model breaks down. At these scales, whistler and kinetic Alfvén waves dominate the dynamics. The dispersive property of these waves causes reconnection to remain fast, consistent with observations, even when the out-of-plane magnetic field is large and/or the system size is very large. The new model resolves the longstanding discrepancy in the energy release time between magnetic reconnection models and observations.

Significance
The goal of building an efficient fusion reactor is best served by understanding what processes control confinement in present-day devices and then proposing techniques for improving confinement properties. Work on magnetic reconnection and anomalous transport has spin-off applications in space and astrophysical plasma applications.

Publications
R. G. Kleva and P. N. Guzdar, "Fast disruptions by ballooning mode ridges and fingers in high temperature, low resistivity toroidal plasmas," Phys. Plasmas 8, 103 (2001).

M. Shay, J. Drake, B. Rogers, and R. Denton, "Alfvénic collisionless, magnetic reconnection and the Hall term," J. Geophys. Res. 106, 3751 (2001).

J. Birn, R. E. Denton, J. F. Drake, B. N. Rogers, M. A. Shay, M. Hesse, M. Kuznetsova, Z. W. Ma, and A. Bhattachargee, "GEM magnetic reconnection challenge," J. Geophys. Res. 106, 3715 (2001).

http://www.ireap.umd.edu/Theory/research.htm