MEASUREMENTS OF NEUTRAL HYDROGEN PROFILES ON THE EXTRAP-T2 REVERSED-FIELD PINCH FROM TIME-RESOLVED C5+ LINE EMISSION

The investigations of the radial distributions of C5+ emission from th e EXTRAP-T2 reversed-field pinch (RFP) plasma show that the emission p rofile varies a lot, even during one plasma discharge. At central elec tron temperatures of about 150 eV it was expected that the C5+ emissio n should emerge from the plasma centre. In comparison, C4+ is always o bserved to radiate from the centre. Our measurements of C5+ emission h ave, however, shown that this is not always the case, the emission oft en comes from the plasma edge. The analysis of the measurements has le d us to conclude that the edge emission comes from charge-exchange rec ombination with neutral hydrogen near the carbon first wall, These obs ervations provide a way to estimate the change in neutral hydrogen den sity during local plasma-wall interaction.