Impurity studies in the advanced toroidal facility

Impurities have played an important role in the initial stages of operation of the Advanced Toroidal Facility (ATF). Cleanup practices have been adequate enough that plasma heated by ECH only can be operated in a quasi-steady state; however, neutral-beam injected plasmas always collapse to a low temperature. It is not clear whether impurity radiation is actually responsible for initiating the collapse, but at the time the stored energy reaches a maximum, there are indications of poloidal asymmetries in radiation from low ionization stages, such as observed in marfes, which could play a dominant role in the plasma evolution. (author) 3 refs., 5 figs