Hydrogen recycling and impurities during isotopic exchange in ISX-B

Working gas composition and edge impurity levels have been monitored during H-D changeover for non-gettered, ohmic discharges in the ISX-B tokamak using optical spectroscopy, mass spectrometry, and surface probe analysis. The isotopic exchange in the plasma is ~80% complete within the first 15 ms of the first discharge in the new isotope, but requires ~10 discharges to reach 98% completion. Fe edge densities increase by a factor of ~3 for D as compared to H plasmas, whereas 0 levels remain comparable for D and H operation. The changeover results indicate that the plasma isotopic composition in ISX-B is dominated by the fill gas, and that most of the smaller contribution from isotopic exchange with exposed surfaces is completed early in the discharge. The impurity data suggest a hydrogen sputtering mechanism for heavy impurity introduction and a chemical process for 0 release.