X-RAY OBSERVATIONS OF HELIUM-LIKE SCANDIUM FROM THE ALCATOR C-MOD TOKAMAK

Scandium (Z = 21) has been injected into the Alcator C-Mod tokamak. X-ray wavelengths of Delta n = 1 transitions to the ground state in Sc19+ and Sc18+ have been accurately determined. Time-resolved spectra have been obtained from different radial locations following the injections and have been compared with favourable agreement to the results of collisional radiative-transport code modelling. X-rays emitted from the outermost regions of the plasma (r/a(L) greater than or equal to 2/3) appear in time after x-ray emission from the plasma centre. These x-rays arise from upper levels in Sc19+ populated by radiative recombination of Sc20+, which can only exist in the outer regions after the scandium has penetrated to the plasma centre, become ionized to Sc20+, and diffused back out before recombining. The density of scandium in the plasma has been determined for a variety of operating conditions. Only about 10% of the injected scandium penetrates to the plasma centre, and the scandium density in the plasma decreases with increasing electron density.