High-velocity liquid jet injection into tokamak plasmas for disruption mitigation

Proposed is a new concept for disruption mitigation and fast shutdown in to kamaks: the injection of hydrogen or helium liquid jets. Liquid jets can ra pidly cool the plasma to reduce divertor heat loads and large halo current forces while simultaneously raising the density sufficiently to prevent run away electron generation. Massive similar to 40- to 100-fold density increa ses equivalent to similar to 50 g of deuterium are necessary for this purpo se in the International Thermonuclear Experimental Reactor (ITER). It is sh own that only two or three simultaneously injected high-velocity (800 to 12 00 m/s) jets can easily deliver this amount of fuel within a period of simi lar to 20 ms and thus avoid runaway electron buildup during the 50- to 500- ms current quench phase. Optimum jet parameters, such as radius, velocity, driving pressure, and injection time, predicted from a jet ablation/penetra tion model, lead to an innovative pulsed injector design concept. The desig n concept is also based on a thermodynamic process path that allows the low est possible temperature at the nozzle orifice, given the constraint of a h igh, similar to 700-atm driving pressure. By having a cold jet exit the noz zle orifice, the potential problem of rapid boiling (flashover) during jet propagation across vacuum space between the nozzle orifice and the tokamak plasma can be overcome. A one-dimensional fluid-dynamic calculation, includ ing finite compressibility, shows that a specially designed liquid Laval no zzle is needed for liquid helium injection because the jet velocity is supe rsonic (Mach number similar to 4). This injector concept is being considere d for a proposed disruption mitigation experiment on DIII-D.