Simulation of L-mode to H-mode Plasma Transition
Written by Tim Lash, Focus Fusion Society Contributor. Edited by Ignas Galvelis, Supervising Director.
After 6.3 sextillion (6.3×10^21) CPU cycles of the Titan super computer running at the Oak Ridge Leadership Computing Facility (OLCF), a team of researchers has successfully simulated the spontaneous transition of turbulence at the edge of a fusion plasma from low confinement mode (L-mode) magnetic containment to high confinement mode (H-mode). It took three days for Titan to run this simulation. The simulation itself was modeling a mere 270 microseconds of real time plasma behavior.
U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) physicists utilized 90 percent of Titan’s capacity during that three-day time-slot. The Titan machine is the nation’s most powerful supercomputer for open science. The simulation utilized the extreme-scale plasma turbulence code XGC developed at PPPL in collaboration with a nationwide team. The simulation’s lead author C. S. Chang of Princeton Plasma Physics Laboratory represented a team of nine researchers from PPPL, UC San Diego and MIT.
L-mode containment describes confined plasma that maintains and exhibits turbulent behavior. Turbulent plasmas have difficulty generating fusion reactions. H-mode plasma displays more stability, increased confinement times and reduced heat transport from high temperature plasmas. All three effects enhance the chance of fusion reactions in a plasma. In the presence of strong enough magnetic fields, a barrier is formed between H-mode and L-mode plasma that preserves heat in the plasma core. Understanding the transition between these modes, and the forces that support H-mode, should provide better insight into designing plasma containment systems whose aim is to generate fusion reactions.
The original paper, published in Physical Review Letters volume 118, issue 17, 175001 – 25 April 2017, highlights two contributing forces at play during the formation of this transport barrier. Prior work attempting to describe this process focused on understanding turbulent Reynolds-stress driven sheared flows. However, other experimental observations revealed that the Reynolds-stresses alone were not strong enough to explain the L-H bifurcation. This simulation revealed that these Reynolds-stresses were augmented by neoclassical ion orbit losses. These two effects acting simultaneously quenched the turbulent transport and together help form a transport barrier just inside the last closed magnetic flux surface.
This simulation is the first driven from first order physics principles. Previous models relied on assumptions regarding instability mechanisms, ignored possible important kinetic effects, or were not carried out in a realistic geometry. This simulation used the known geometry of MIT’s Alcator C-Mod tokamak.
Simulations such as this help efforts whose aim is to better contain the inherent instabilities within high temperature plasma. Alternately, the Focus Fusion-1 reactor is designed to leverage plasma instability and has a heating advantage when generating its record setting plasma temperatures.