Initial results of local island divertor experiments in large helical device

A local island divertor (LID) experiment has begun in the Large Helical Device (LHD) to demonstrate improved plasma confinement. It was clearly shown that the particle flow is controlled by adding a resonant perturbation field to the LHD magnetic configuration, and guided to the backside of an m/n=1/1 island which is created by the perturbation field. The particles recycled there, on carbon plates of a divertor head, were pumped out, so that the line-averaged core plasma density was reduced by a factor of ∝2 at the same gas puff rate, compared with non-LID discharges. Accordingly, some of fundamental LID functions were demonstrated in the sixth experimental campaign in 2002-2003. Clear-cut improvement of the plasma confinement is, however, on the way, because the discharge could not be optimized, due to a large amount of outgas from the divertor head to the core plasma. The size of the divertor head was found to be larger than the optimum one, and hence, the core plasma struck the divertor head a little. (orig.)