Using mixed gases for massive gas injection disruption mitigation on Alcator C-Mod

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 (similar to 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.