The scrape-off layer characteristics as a function of the plasma current and density in the FT Tokamak discharges

Abstract

The temperature and the density of the plasma at the limiter shadow of FT (Frascati Tokamak) during ohmic operation and their radial variation have been measured by means of single Langmuir probes. No clear dependence on the radius for the temperature has been found, while the density decays towards the wall exponentially with an e-folding length lambda _n approximately 0.5 cm, in agreement with a simple model. The influence of the plasma current I_p and line averaged density n on the scrape-off conditions has been studied. The scrape-off density n_e increases with n at a fixed value I_p, but is substantially reduced when Ip is raised, at fixed n. Neither I_p nor n seem to alter the scrape-off temperature T_e appreciably, at least in the region accessible to the probes.