A fast shutdown technique for large tokamaks

A practical method is proposed for the fast shutdown of a large ignited tok amak. The method consists of injecting a rapid series of 30-45 (6 mm) deute rium pellets doped with a small (0.0005%) concentration of krypton impurity , and simultaneously ramping the plasma current and shaping fields down ove r a period of several seconds using the poloidal field system. Detailed mod elling with the Tokamak Simulation Code using a newly developed pellet mass deposition model shows that this method should terminate the discharge in a controlled and stable way without producing significant numbers of runawa y electrons. A partial prototyping of this technique was accomplished in TF TR.