Edge plasma control by local island divertor in LHD

In the Large Helical Device (LHD) program, one of the key research issues is to enhance helical plasma performance through the edge plasma control. For the first time in the LHD program, the edge plasma control was performed with a local island divertor (LID) that is a closed divertor, utilizing an m/n=1/1 island generated externally by 20 small perturbation coils, and fundamental LID functions were demonstrated experimentally. It was found that the outward heat and particle fluxes crossing the island separatrix flow along the field lines to the backside of the divertor head, where carbon plates are placed to receive the heat and particle loads. Accordingly high efficient pumping was demonstrated, which is considered to be the key in realizing high temperature divertor operation, resulting in an improvement of energy confinement. In the present experiment, relatively good energy confinement is achieved in the high density regime at a magnetic axis position, Rax, of 3.75 m. Results of edge modelling are also presented by using the EMC3-EIRENE code. (author)