Simulation Center for Runaway Electron Avoidance and Mitigation

Mark Adams,¹ Amitava Bhattacharjee,² Allen Boozer,³ Boris Breizman,⁴ Dylan Brennan,^5,a Luis Chacon,⁶ Diego Del-Castillo-Negrete,⁷ Irene M. Gamba,⁴ Valerie Izzo,⁸ Lang Lao,⁹ Xianzhu Tang,^6,b and Guannan Zhang⁷

¹Lawrence Berkeley National Laboratory

²Princeton Plasma Physics Laboratory

³Columbia University

⁴University of Texas, Austin

⁵Princeton University

⁶Los Alamos National Laboratory

⁷Oak Ridge National Laboratory

⁸University of California, San Diego

⁹General Atomics

Runaway electrons can severely damage the plasma facing components on ITER during a major disruption and pose a major risk for tokamak fusion.  It has been recognized that an adequate disruption mitigation system (DMS) is essential for the safe operation of ITER. To aid the DMS design, ITER's International Tokamak Physics Activity group on magnetohydrodynamics (MHD) has organized a world-wide joint experimental campaign on existing tokamak facilities to examine the current knowledge bases for runaway generation and mitigation. Significant discrepancies have been found between experimental observations and existing theories. Furthermore, initial assessments by theory and modeling suggest that the primary candidates for runaway mitigation: massive gas injection and shattered pellet injection, both carry significant risk of aggravating the runaway damage.  It is time critical for the U.S. fusion program to launch a comprehensive theory and simulation program that provides physics guidance in the avoidance and mitigation of runaway electrons, and in tandem with domestic and international experiments, helps establish the qualitative and quantitative bases for safe operational scenarios and viable mitigation techniques.

The newly established simulation center for runaway electron avoidance and mitigation (SCREAM) assembles a national team of leading experts in runaway electron physics, tokamak disruptions, MHD simulations, and advanced computing.  The team will combine theoretical analysis and advanced simulation of runaway electron physics to focus on the risk ITER and tokamak fusion have to face.  The effort is spearheaded by Fusion Energy Sciences with applied mathematics and computational solutions facilitated by Advanced Scientific Computing Research SciDAC institutes.  The specific research tasks will (1) establish the fundamental physics of runaway generation, saturation, and dynamical evolution in a tokamak; (2) examine the critical path toward runaway avoidance; and (3) investigate the viability and effectiveness of the leading candidate schemes for runaway mitigation.  In all three areas, members of the team have carried out scoping studies that establish the readiness for rapid and critical advances, especially in the deployment and further development of large-to extreme-scale simulation tools. Our multi-pronged computational approach will include (1) relativistic Fokker-Planck solutions in discretized phase space, (2) self-consistent particle-in-cell techniques, (3) particle-based Monte-Carlo solutions, and (4) MHD-particle hybrid simulations. Cross-check between these different methods will provide an additional means for verification and will further bolster the fidelity of our physics prediction.  Validation against experimental results will bring confidence to the predictive capability for ITER and likely lead to new ideas for understanding and mitigating the thermal quench driven runaway electron phenomenon.

^a Lead Principal Investigator, Universities

^b Lead Principal Investigator, National Laboratories