Runaway Electron Confinement Modeling

Key Challenges: Understand scenarios in which one might need to quickly shut down a fusion reactor plasma in case of disruptions that could seriously damage reactor components. The primary concern is runaway electron (RE) current, which has been observed in two smaller fusion devices. However, the threat posed by RE to ITER is difficult to assess experimentally. Simulation is therefore required but for ITER is computationally more intensive than other tokamaks (such as Alcator C-Mod and DIII-D) both because of its larger size (requiring more grid points) and its longer current quench time (requiring more time steps).

Why it Matters: Electrons with energies larger than some critical threshold value in a tokamak plasma will be continuously accelerated by the tokomak's electric field and this "runaway electron" problem can generate energies that can cause serious damage to the confining structures. The RE population can grow exponentially and runaway gain in ITER is believed to be much larger than in existing tokamaks. One possible loss term is radial transport of REs out of the plasma due to MHD perturbations. The question is, will disruption-induced MHD provide a large enough loss term in ITER?

Accomplishments: Results were obtained via a NERSC "NISE" allocation in 2010. The NIMROD extended MHD code was used to simulate a fast shutdown of an ITER plasma with a focus on RE confinement. A comparison of the ITER simulation with those of the two smaller tokamaks also performed and the cross-device comparison yielded key insight into the transport effects.  There is an apparent strong trend in which RE confinement increases with increasing device size.  There are important stochastic plasma effects, as well, and stochasticity extends to the separatrix in the smaller devices, but not in ITER (see figure at below).  RE losses due to MHD decrease as device size increases, too.  The results suggest that disruption-induced MHD is not likely to deconfine REs in ITER. 

Investigators: Valerie Izzo (General Atomics)

More Information: These results were reported in a talk at the 23rd IAEA Fusion Energy Conference and have been submitted as a paper to Nuclear Fusion.  See the General Atomics web site for a preprint.