Formation and structure of transport barriers during confinement transitions in toroidal plasma

The spatiotemporal structure of the transport barrier across L-H bifurcations in the H-1 heliac was investigated. It was found that a density pedestal similar to those observed in tokamaks develops during the L-H transitions in the H-1 heliac. Poloidally extended potential structures or zonal flows played the major role both in the spatial structure and in the temporal evolution of the pedestal formation. The results show that the zonal flows transiently generate radially localized maxima in the radial electric field shear in L mode which coincides with the radial location of the pedestal in the H mode.