Current quenches caused by density limit disruptions have been investi
gated in JT-60U divertor plasmas in order to develop general methods o
f reducing the current decay rate and of suppressing the generation of
runaway electrons. The reduction of the impurity influxes at an energ
y quench and the direct neutral beam (NB) heating of the plasma core d
uring a current quench were beneficial for reducing the speed of the c
urrent quench. The low stored energy just before the energy quench and
the high safety factor at the plasma edge had the advantage of decrea
sing the impurity influxes at the energy quench. The detached plasma s
tate was useful for degrading the energy confinement for both joule an
d 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) measu
red just before the energy quench. Fast controlled plasma shutdown wit
h energy quenches but without a current quench was demonstrated succes
sfully from 2 MA to zero, with a value of dI(p)/dt of -6 MA/s. This sh
utdown 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.