THE PRESENT STATUS OF IMPURITY TRANSPORT INVESTIGATIONS

The present status of impurity transport studies in tokamaks is reviewed. Ohmically heated and neutral-beam heated L-mode discharges exhibit anomalous impurity transport. The values of particle diffusivity range from 0.5 to 2 m2/s. With improved energy confinement such as in the H-mode the impurity transport is suppressed, which results in a radiation collapse of the H-mode state in the absence of the edge-localized mode (ELM). With high power neutral-beam heating, ELMs frequently expel particles and heat out of the separatrix surface, which results in long (approximately 5 seconds) quasi-stationary H-mode operation.