Massive gas injection plans for disruption mitigation studies in NSTX-U

Predicting and controlling disruptions is an important and urgent issue for ITER and impacts the designs for reactors based on the ST and Tokamak concepts. Disruptions have been such a ubiquitous feature of tokamak operations for decades that, while work is in progress to develop reliable methods to avoid disruptions, some may be unavoidable. For these cases, a fast discharge termination method is needed to minimize the deleterious effects of the disruption, particularly the generation of large populations of runaway electrons. Massive Gas Injection (MGI) is one approach to addressing this difficult issue for ITER. NSTX-U plans to compare MGI from different poloidal locations to assess the gas penetration efficiency. We are starting to model gas penetration using DEGAS-2 and it appears that the scrape-off-layer plasma may place limits on the achievable gas penetration fraction to the separatrix.