THE SOFTENING OF CURRENT QUENCHES IN JT-60U

Current quenches caused by density limit disruptions have been investigated in JT-60U divertor plasmas in order to develop general methods of reducing the current decay rate and of suppressing the generation of runaway electrons. The reduction of the impurity influxes at an energy quench and the direct neutral beam (NB) heating of the plasma core during a current quench were beneficial for reducing the speed of the current quench. The low stored energy just before the energy quench and the high safety factor at the plasma edge had the advantage of decreasing the impurity influxes at the energy quench. The detached plasma state was useful for degrading the energy confinement for both joule and NB heated plasmas within a short time period. Runaway electrons were not generated at plasma densities of more than 5 x 10(19) m(-3) measured just before the energy quench. Fast controlled plasma shutdown with energy quenches but without a current quench was demonstrated successfully from 2 MA to zero, with a value of dI(p)/dt of -6 MA/s. This shutdown was obtained by lowering the stored energy within a short time period of 20 ms, actively using the detached plasma state produced by intense helium gas puffing.