Massive Gas Injection System for Disruption Mitigation on the DIII-D Tokamak

Massive injection of deuterium or noble gases ($>$10$^{22}$ molecules) has been very effective at mitigating disruptions in \hbox{DIII-D} [1]. Both the divertor heat loads and the first wall forces were reduced by more than a factor of four. Total electron densities (free and bound) of $\sim$10$^{21}$ m$^{-3}$ have been achieved, close to that required to prevent avalanche multiplication of runaways. Two tested configurations are described. Both use a fast solenoid valve with an orifice diameter of 4~mm with a flow rate in helium of 5$\times$10$^4$~Pa~m$^3$/s at 7~MPa. A new valve with a 20~mm orifice will be tested on \hbox{DIII-D} in 2006. This valve is actually close to that required for ITER. Calculations show that a set of four such valves can reach the no-avalanche density in ITER in $\sim$0.25~$t_{co}$ where $t_{co}$ is the plasma current quench time. How the gas jet interacts and mixes with the plasma is under investigation.\par \vspace{0.5em} \noindent [1]~D.G.\ Whyte, et al., Phys.\ Rev.\ Lett.\ ${\bf 89}$, 55001 (2001).