Neutral density measurements at the plasma edge of the Tore Supra tokamak with carbon charge exchange lines

Abstract Direct measurements of the neutral (hydrogen) density n 0 in tokamak plasmas are almost completely lacking. Thus, indirect information via charge exchange spectroscopy (CX) of sensible intensity ratios of impurity (carbon) ions are used. Two methods are used for the evaluation of the n 0 . The first method is based on ground state transitions of CV and the second on CX-recombination of completely ionized carbon (C 6+ ). CX-recombination occurs at the interface between the hot plasma and the cold periphery. Consequently, plasma situations with a cold and dense edge where n 0 is naturally high are analyzed. These cold edge plasmas are characterized by the appearance of a highly radiating region at the inner wall of the tokamak (Marfe). The inferred Marfe neutral density is between 10 17 and 10 18 m −3 . In addition, the changes of this radiating zone induced by the ergodic divertor (ED) of Tore Supra are studied: an n 0 of 10 16 m −3 is deduced inside the ergodized region where the CV line is emitted.