MHD simulations of rapid shutdown scenarios by massive particle injection in DIII-D, Alcator C-Mod and ITER are performed in order to study runaway electron transport during mitigated disruptions. The simulations include a runaway electron (RE) confinement model using drift-orbit calculations for test particles. A comparison of limited and diverted plasma shapes is studied in DIII-D simulations, and improved confinement in the limited shape is found due to both spatial localization and reduced toroidal spectrum in the nonlinear MHD activity. C-Mod simulations compare shutdown scenarios in which impurity (Ar) fueling is concentrated in the edge versus the core, and find the confinement of REs in the core is maintained until the onset of the m = 1 n = 1 mode, which is delayed in the case of edge deposition, relative to core deposition. But, the overall RE loss fraction is 100% regardless of Ar fueling profile. A comparison of simulations across the three devices points to a trend of increased RE confinement with increasing device size, wherein all REs are lost in C-Mod, all are confined in ITER, and a partial loss is observed in DIII-D. This trend is related to a reduction in the fluctuating field amplitude near the plasma edge during the thermal-quench-induced MHD activity. The result bodes poorly for RE mitigation strategies in ITER that rely on MHD deconfinement of runaway electrons.

The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only highpower radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing componentsapproaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated a) Paper AR1 1, Bull. Am. Phys. Soc. 58, 21 (2013). b) Invited speaker.

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Abstract. Mixed gases are used for massive gas injection disruption mitigation on Alcator C-Mod in order to optimize radiation efficiency, halo current reduction, and response time. Gas mixtures of helium and argon (argon fraction 0-50%) are investigated in detail, as well as mixtures of deuterium, argon, krypton, and helium. Experiments show that injecting He/Ar mixtures leads to faster thermal and current quenches than with pure helium or argon injection, thus improving the time response of the disruption mitigation system and reducing the halo current. Small fractions of argon (∼5-10%) in helium also lead to optimized radiation fractions with large electron density increases in the core plasma. These results are consistent with the expectation that small fractions of argon will be entrained with the faster helium in the early phases of gas flow. The gas mixing allows one to simultaneously exploit the fast particle delivery rate of light helium gas and the large radiation capability of argon.

Massive gas injection rapid shutdown experiments have been conducted on the Alcator C-Mod tokamak using two toroidally separated gas injectors, in order to investigate the effect of multiple gas injection locations on the toroidal asymmetry in the radiated power. Toroidal radiation asymmetry is diagnosed by an array of six single-channel photodiodes mounted on the vessel wall. The presence of magnetohydrodynamic (MHD) activity is diagnosed using an array of magnetic pickup (Mirnov) coils, mounted on stalks on the vessel wall. Scans were conducted of the relative timing between the two jets, of the 95th percentile safety factor, and of the plasma elongation. It is observed that firing the two gas jets so that the injected impurities arrive at the plasma at nearly the same time produced an increase in the toroidal radiation asymmetry. In addition, the radiation asymmetry in the thermal quench phase correlates with the growth rate of low toroidal mode number MHD modes, indicating that these mode(s) are playing a role in setting the radiation asymmetry.

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