Formation of Edge Transport Barriers by L-H Transition and Large Reversed Plasma Current on LHD

<正> On the Large Helical Device(LHD)where nested magnetic surfaces are surroundedby the ergodic field layer,edge transport barrier(ETB)was produced in neutral-beam-injection(NBI)heated plasmas through transition and non-transition processes.The former case is the ETBformation by L-H transition,where characteristics of L-H transition observed in a tokamak plasmaare clearly recognized.The confinement improvement is the modest(～10%),compared with theISS95 international stellarator scaling.The threshold power for the transition is comparable orslightly lower than the ITER scaling law established by tokamaks and compact tori.The ETBis formed inside the ergodic field layer of the vacuum field.The ETB formation destabilizes edgecoherent modes such as m/n=1/1,2/3 and 1/2,of which rational surfaces are in the magnetichill.The formed ETB is partially and transiently destroyed by these coherent edge MHD modesand edge localized modes(ELMs)typically observed in Hα signals.The latter ETB is observed ina plasma with large reversed NBI-driven current more than 100 kA at Bt = 1 T.In these plasmas,the edge magnetic shear is enhanced by the current and the rotational transform in the core regionis expected to be appreciably reduced.Thus reduced rotational transform in the plasma centralregion will enhance outward heat and particle fluxes toward ergodic edge layer.The ETB withsteep electron temperature gradient up to～5 keV/m is formed by blocking enhanced outwardheat flux.