Spatial distribution of impurities and H neutrals in ELMO Bumpy Torus (EBT)

Absolute-brightness measurements and spatial scans in the vacuum ultraviolet have been used to investigate the presence and spatial density distribution of various charge states of carbon and aluminium (the major impurities) and neutral hydrogen in the ELMO Bumpy Torus. The impurity concentrations are quite low; the total fractional carbon concentration in the toroidal core (compared to the electron density) is less than 6 × 10 −5 . Sources for the C and Al impurities are examined. Experimental evidence and arguments based on the space potential and drift surfaces which are presented indicate that the surface blanket plasma plays a role analogous to a divertor, shielding the core plasma from impurities.