The magnetic field in a plasma torus – inside the dough part of the doughnut shape – is always stronger near the tightly-spaced coils that pass through the doughnut hole than it is at the outer diameter, where the coils are spaced apart. As a result, charged particles diffuse outward. Eventually some escape, carrying away heat and damaging the inner walls of the containment vessel.

The complex dynamics of diffusion – and virtually every other aspect of plasma dynamics – are the focus of William Tang, a principal research physicist at the Princeton Plasma Physics Laboratory (PPPL) and Princeton University professor of astrophysical sciences. With collaborators at Columbia University, Lawrence Livermore National Laboratory, Oak Ridge National Laboratory (ORNL) and the Max Planck Institute for Plasma Physics, Tang is using Argonne National Laboratory’s Mira supercomputer, an IBM Blue Gene/Q, to simulate these energy losses. The group wants to balance the losses with self-heating from fusion reactions within the plasma.

The factors that determine whether a given particle will escape from a burning plasma are many and complex. Choong-Seock Chang, head of the SciDAC Center for Edge Physics Simulation for Plasma Fusion and a PPPL principal research physicist, and his collaborators at Brown University, ORNL and the University of Colorado are using the Oak Ridge Leadership Computing Facility’s Titan supercomputer to model events at the edge of the plasma in fine detail.

Venting heat and reaction products through a device called the divertor is vital to a tokamak reactor’s performance, especially as the plasma energy increases and the system moves spontaneously into a high-confinement mode, or H-mode. Engineers have proposed making the divertor from tungsten. Brian Wirth, professor and governor’s chair of computational nuclear engineering at the University of Tennessee, Knoxville, is using Mira and Titan to model how tungsten responds to this heat and helium plasma. The result will be parameters for optimal divertor design.