Disruption mitigation studies in DIII-D

Data on the discharge behavior, thermal loads, halo currents, and runaway e lectrons have been obtained in disruptions on the DIII-D tokamak [J. L. Lux on and L. G. Davis, Fusion Technol. 8, 2A 441 (1985)]. These experiments ha ve also evaluated techniques to mitigate the disruptions while minimizing r unaway electron production. Experiments injecting cryogenic impurity "kille r'' pellets of neon and argon and massive amounts of helium gas have succes sfully reduced these disruption effects. The halo current generation, scali ng, and mitigation are understood and are in good agreement with prediction s of a semianalytic model. Results from "killer'' pellet injection have bee n used to benchmark theoretical models of the pellet ablation and energy lo ss. Runaway electrons are often generated by the pellets and new runaway ge neration mechanisms, modifications of the standard Dreicer process, have be en found to explain the runaways. Experiments with the massive helium gas p uff have also effectively mitigated disruptions without the formation of ru naway electrons that can occur with "killer'' pellets. (C) 1999 American In stitute of Physics. [S1070-664X(99)92005-0].