LIQUID JETS FOR FAST PLASMA TERMINATION IN TOKAMAKS

Recent simulations by Putvinskij et al. (PSI Conference, 1996) have sh own that introducing impurities into the plasma in order to mitigate a dverse disruption effects in ITER may actually be deleterious because of a potentially unwelcome phenomenon: generation of multi-MeV runaway electrons by the collisional avalanche mechanism (Rosenbluth, M.N., e t al., in Fusion Energy 1996 (Proc. 16th Int. Conf. Montreal, 1996), V ol. 2, IAEA, Vienna (in press) Paper FP-26). The injection of a liquid hydrogen jet to deliver a massive density increase is proposed as a m eans of avoiding runaways, while providing the same beneficial effects as impurities. A discussion of many jet related topics, such as ablat ion/penetration, jet breakup time and stability, is presented. Owing t o an ablation pressure instability, it is predicted that the jet will quickly break up into a regular chain of droplets with dimensions of a pproximately the size of the jet radius. It is found that while deep p enetration in the plasma can easily be achieved, bubble growth and dis ruptive boiling (hashing) during the propagation in the vacuum gap bet ween the nozzle exit and the plasma are the main processes limiting th e jet survival time. Calculations indicate that for ITER reference par ameters, the jet can remain coherent in vacuum for a distance similar to 1 m before disintegrating. On the basis of this present understandi ng, the prospect for the safe termination of ITER discharges by high d ensity liquid jet injection appears promising.