Observation of the bulk ion density peaking in a discharge with an impurity hole in the LHD

Abstract

Radial profiles of bulk ion and impurity ions density are simultaneously measured and quantified using charge exchange spectroscopy and peaking parameter, Cv. The peaking parameter is positive for inward and negative for outward convection in a plasma with an impurity hole in the Large Helical Device. Following the formation of an impurity hole associated with the transition from L-mode to ion ITB plasma, the bulk ion becomes peaked by the inward convection (Cv>0), while impurities (helium and carbon) become hollow due to the outward convection (Cv<0). In contrast to the ion ITB plasma, in the L-mode plasma the impurity profiles are peaked (Cv>0), where the bulk ion density profile remains flat (Cv∼0). Understanding of the impurity behavior in the ion ITB plasma could lead to a self cleaning plasma reactor concept, with an efficient impurity exhaust.