DREAM: a fluid-kinetic framework for tokamak disruption runaway electron simulations

Runaway electrons generated during a tokamak disruption pose a severe threat to future reactor-scale devices. Due to the exponential sensitivity of the runaway generation rate to the plasma current, robust avoidance and mitigation schemes cannot be fully validated in today's medium-size tokamaks. Comprehensive and validated runaway electron generation models are thus essential for the development of such schemes. In this contribution we present the Disruption Runaway Electron Analysis Model (DREAM), a new simulation tool specifically designed to study the generation of runaway electrons during tokamak disruptions. The tool combines 1D fluid models for the background plasma (electric field, temperature, poloidal flux, ion charge states) with either fluid or kinetic models for the electrons in tokamak geometry. To enable accurate and efficient simulations of the whole disruption, electrons are separated into three sub-populations based on their energies, allowing different models to be used for thermal, superthermal, and relativistic electrons simultaneously. Notably, the thermal and runaway electrons can be treated using conventional fluid models, while the superthermal electrons are evolved using a reduced kinetic equation, providing precise accounting of the transient---and thus inherently kinetic---hot-tail runaway generation mechanism. In addition to the novel treatment of electrons, DREAM incorporates a number of physical mechanisms which have never before been brought together in a complete, self-consistent disruption simulation, including radial transport of heat and electrons, dynamic evolution of ion charge states, collisions with partially ionized atoms, the effect of passive conducting structures on the electric field, and hyperresistivity. The first studies  conducted with DREAM indicate that fast electron radial transport may provide a path to effective runaway electron avoidance in ITER.