Changes in impurity particle transport following the injection of fueling pellets into tokamak discharges

Improvements in the energy confinement properties of tokamaks have been achieved by means of central fueling with solid hydrogen pellets. Evidence from laser scattering experiments indicates that the reduced anomalous transport may be due to the suppression of ηi driven modes. These changes in energy transport, which appear to be primarily in the ion channel, are accompanied by changes in majority species and impurity transport. In particular, for discharges showing enhanced confinement, electron density profiles remain peaked for long periods following the injection of even a single pellet, and impurity accumulation on axis is observed. Other enhanced confinement modes also show similar changes in particle and impurity transport; similarities with, and differences from, the pellet induced changes are discussed. (author). 19 refs, 3 figs