OBSERVATION OF SELF-MITIGATION OF A DENSITY LIMIT DISRUPTION IN DIII-D

OAK-B135 Density limit disruptions set an upper bound on the electron density in tokamaks and are important for future reactor-size tokamaks, which will typically need to operate at high densities to achieve ignition. In the standard picture of disruptions, a large MHD mode, or combination of MHD modes, causes a mixing of previously nested magnetic flux surfaces across much of the profile. Rapid heat and particle transport across the separatrix result, and the thermal energy of the discharge is lost along open field lines into the divertor on a millisecond time scale or faster. In this work, a density limit disruption is initiated by ramping up the density in a lower single-null discharge in the DIII-D tokamak. As in most disruptions, a large MHD precursor is observed. However, in contrast with the disruption scenario described above, it is found that the plasma thermal energy, rather than being conducted into the divertor, is dominantly lost by radiation to the main chamber walls. This has been referred to as self-mitigation of the disruption, in comparison to the intentional mitigation of localized heat loads in disruptions by the introduction of pellets or liquid or gas jets to enhance radiation. The self-mitigation effect appears to result from a release of neutrals (deuterium and carbon) from the graphite vacuum vessel walls. These results could have favorable implications for the severity of divertor heat loads during density limit disruptions in future large tokamaks