Physics of Intrinsic Plasma Rotation Explained for First Time

If humans could harness nuclear fusion, the process that powers stars like our sun, the world could have an inexhaustible, clean energy source. Scientists have taken another step towards that goal with research that uncovers why the hot, gaseous stews used in fusion reactions sometimes spontaneously rotate in their donut-shaped containment "pots," called tokamaks.

In theory, scientists could produce a steady stream of fusion energy on earth by heating up two types of hydrogen atoms, deuterium and tritium, to more than 100 million degrees centigrade. At these temperatures, the gas turns into a stew of electrically charged particles, called plasma. Powerful magnets confine and compress the plasma particles in a tokamak until they fuse together, releasing energy in the process.

However, researchers have yet to produce a fusion reaction of sufficient quality that it produces more energy than it consumes. That quality is determined in great part by how well plasma is confined at its edges, a process that's not well understood due to the complicated interactions between multiscale physics in the superheated gas. The SciDAC Center for Edge Physics Simulation (EPSI) uses large-scale simulation to understand the edge physics from first-principles equations, and ultimately to provide predictions of fusion performance.

This work is relevant to existing magnetic fusion experiments and essential for next-generation burning plasma experiments such as ITER.

The plasma edge presents a set of multi-physics, multi-scale problems involving complex 3D magnetic geometry. Perhaps the greatest computational challenge is the overlap of multiple temporal and spatial scales, which prevents the problem from being broken up into smaller units; a full kinetic simulation requires the use of at least 50,000 computing cores, and sometimes more.

One key factor in plasma performance is the stabilizing influence of toroidal rotation. In today’s tokamaks, rotation is driven mainly by external beams, but beam drive will be less effective in larger future devices such as ITER. On the other hand, self-acceleration provides an intrinsic rotation—a spontaneous rotation without external momentum input. This phenomenon has been observed in many experiments, but its origin has not been well understood until now.

The SciDAC-developed XGC1 code is the world’s first and only gyrokinetic code able to simulate the multiscale turbulence and background physics in realistic edge geometries including the magnetic separatrix and the material wall.

The EPSI team used XGC1 to model relevant multi-scale physics over the entire plasma volume of the DIII-D fusion reactor and explain the intrinsic rotation phenomenon. The simulation demonstrated for the first time that the toroidal momentum is generated at the edge, propagates inward as the edge pedestal is formed, and is redistributed by turbulence in the plasma core.

A useful analogy for this phenomenon is a heat engine, which converts some of the energy in a temperature difference into mechanical work. In fusion reactors that generate intrinsic rotation, the energy from hot spots at the plasma edge is converted to kinetic energy, which causes the plasma to rotate.

This is a key finding for modeling future reactors such as the international fusion experiment called ITER, and it is currently being validated by experiments in current reactors such as the NSTX. Multi-physics simulations such as this one are possible only on petascale computers, but modeling larger devices like ITER will require exascale systems.

S. Ku, J. Abiteboul, P.H. Diamond, G. Dif-Pradalier, J.M. Kwon, Y. Sarazin, T.S. Hahm, X. Garbet, C.S. Chang, G. Latu, E.S. Yoon, Ph. Ghendrih, S. Yi, A. Strugarek, W. Solomon, and V. Grandgirard, “Physics of intrinsic rotation in flux-driven ITG turbulence,” Nuclear Fusion 52, 063013 (2012), doi:10.1088/0029-5515/52/6/063013.