Theoretical Progress on Runaway Electron Suppression by Massive Gas Injection

Development of techniques to mitigate the severity of emergency plasma termination/plasma disruptions is deemed one of the highest priorities for ITER. The current method of mitigation by massive gas injection (MGI) is not fully understood; whether MGI can achieve sufficient density to avoid avalanche runaway electron formation in the high toroidal electric field E$_{\phi }$ is presently uncertain. It will be shown why direct penetration of broad gas jets cannot happen: ablation pressure drag (or magnetic pressure imbalance) exerted over the frontal surface of the jet is too strong for usual jets. Evidence on DIII-D is that MHD processes, occurring predominately during the short thermal quench TQ phase, cause inward diffusion of gas jet ions ``stuck'' at the plasma edge. To explore this process we have developed a 1-D large-aspect-ratio circular flux surface code for the evolution of E$_{\phi }$ with radiation and transport cooling. We use resistive wall boundary conditions, and a 2D axisymmetric CFD code describes the heavily-fueled vacuum region and plasma boundary conditions.