BIASED LIMITER EXPERIMENTS ON THE ADVANCED TOROIDAL FACILITY (ATF) TORSATRON

The Advanced Toroidal Facility (ATF) torsatron incorporates two rail l imiters that can be positioned by external controls. The influence on the plasma parameters of biasing these limiters both positively and ne gatively with respect to the walls has been investigated. Experiments have been carried out in the electron cyclotron heated plasmas at 200 kW with a typical density of 5 X 10(12) cm-3 and a central electron te mperature of approximately 900 eV. Negative biasing produces only smal l changes in the plasma parameters, but positive biasing increases the particle confinement by about a factor of 5, although the plasma stor ed energy does fall at the higher voltages. In addition, positive bias ing produces the following effects compared with floating limiter disc harges: the core density profiles become peaked rather than hollow, th e electric field at the edge becomes more negative (pointing radially inward), the magnitudes of the edge fluctuations and the fluctuation i nduced transport are reduced, the fluctuation wavelengths become longe r and their propagation direction reverses from the electron to the io n diamagnetic direction. Neither polarity of biasing appears to affect the impurity content or transport.