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

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 lambdasub(n) approx. 0.5 cm, in agreement with a simple model. The influence of the plasma current Isub(p) and line averaged density n-bar on the scrape-off conditions has been studied. The scrape-off density nsub(e) increases with n-bar at a fixed value Isub(p), but is substantially reduced when Isub(p) is raised, at fixed n-bar. Neither Isub(p) nor n-bar seem to alter the scrape-off temperature Tsub(e) appreciably, at least in the region accessible to the probes. (author)