NERSC Initiative for Scientific Exploration (NISE) 2009 Awards

Collisionless Dissipation in Turbulent Plasmas

Michael Shay, University of Delaware

Sponsoring NERSC Project: Kinetic Physics of Magnetic Reconnection (m733), Principal Investigator: Michael Shay, University of Delaware

The dissipation of energy in turbulence plasmas plays a fundamental role in a wide range of systems, one of which is heating the outer portions of the sun and the supersonic solar wind, which is a stream of fast-moving particles that slams into the Earth at high speeds. This energy dissipation also plays a critical role in laboratory plasmas, such as fusion plasmas which could provide clean and sustainable energy in the future. However, exactly how this energy is dissipated into heat is not well understood. We hope to make progress on this critical problem of dissipation in turbulence by simulating turbulence in these plasmas using models which include the physical basis for this heating.

Description of Proposed Research: The dissipation of energy in turbulent collisionless plasmas plays a fundamental role in a wide range of systems, and a breakthrough in our understanding would lead to great progress. But the question remains: how do turbulent collisionless plasmas dissipate their energy? In turbulent plasmas, energy cascades from larger to smaller scales and eventually dissipates into heat. Exactly how this occurs, however, in a plasma with no collisions is not well understood. Answering this question will provide significant insight into the dynamics of the solar corona, the solar wind, astrophysical plasmas such as accretion disks and tokamaks. In the solar wind and corona, a key clue to this damping is the often found preferential heating perpendicular to the mean magnetic field. Large-scale PIC hybrid simulations using kinetic ions and fluid electrons will be used to simulate turbulent plasmas, and the kinetic ion heating will be carefully studied. The hybrid version of P3D allows simulations of a significant part of the inertial range while still allowing fast timescale damping effects such as ion cyclotron resonances to occur.

We have previously run modest-sized two-dimensional simulations of an Orszag-Tang vortex, a two-dimensional equilibrium that exhibits a fast transition to a nonlinear cascade and turbulence. Consistent with observations of turbulence in space plasmas, we found heating of ions perpendicular to the mean magnetic field (Parashar et al., Physics of Plasmas, 16, 032310, 2009). However, the mechanism responsible for this heating is currently unknown. To study this dissipation mechanism responsible for the heating, it will be necessary to run larger systems with different mass ratios and larger inertial ranges. This will require larger 2D Orszag-Tang simulations. In addition, we will also simulate some 2D turbulence systems with forcing initial conditions, which allows the dissipation to reach steady state, although the simulations are more computationally expensive. Finally, we will run one large-scale three-dimensional turbulence simulation, which is critical to determine if any critical effects are missing in the 2D systems.