Development of an Edge Transport Barrier at the H-Mode Transition of ASDEX

The thermal wave of a minor disruption can initiate the H phase of a neutral-beam-heated divertor tokamak discharge. Its propagation is used to probe the plasma edge conditions at the H transition. The results show the existence of a transport barrier which forms at the plasma edge and impedes the flow of particles and energy across the plasma surface, giving rise to improved confinement properties. Location and extension of the barrier coincide with the edge zone of increased shear specific to the divertor configuration.