Observations of centrally peaked impurity profiles following pellet injection in the Alcator-C tokamak

Differently filtered x-ray arrays have been used to instantaneously measure central carbon and molybdenum profiles, Alcator’s dominant light and heavy impurities, following pellet injection. Approximately 40 ms after the pellet, the width of the carbon profile was close to the neoclassical prediction of a source-free equilibrium state. (Carbon is a plateau impurity.) The experimental molybdenum profile, which is more uncertain, was between a factor of 1.3 and 3.0 that of the asymptotic prediction of neoclassical theory. Carbon, the dominant nonhydrogenic contributor to Zeff, was found to dramatically affect sawtooth dynamics by altering the central resistivity. Specifically, following injection of the pellet the carbon profile became progressively more peaked, the effect of which was to lengthen the sawtooth period. Approximately 40 ms after the pellet, an internal disruption occurred which reduced on-axis carbon and molybdenum by a factor of 3. (In comparison, the corresponding temperature drop was only about 10%.) After this internal disruption, the sawtooth period shortened as a consequence of carbon not dramatically repeaking. The instrumentation, as well as the plasma conditions needed to make such measurements, will be described. Details of this work are contained in the report PFC/JA-85-41. This work was supported in part by the U.S. DOE Contract No. DE-AC02-78ET51013.