Study of the scrape-off plasma of the Frascati Tokamak in Ohmic discharges by means of Langmuir probes

Abstract

Langmuir probe data obtained from about 290 Ohmic discharges in the Frascati Tokamak (FT) have been analysed in the following ranges of macroscopic plasma parameters: line averaged plasma density n̄e = 2.5 × 1013 to 2.7 × 10l4 cm−3, plasma current Ip = 230–450 kA, toroidal magnetic field BT = 6 T and 8 T. The principal parameters of the scrape-off layer (SOL) plasma, i.e. the density and the temperature at the limiter radius, neL and TeL, respectively, and their e-folding lengths λn, and λT have been obtained and correlated with those of the main plasma. The scaling laws for neL and TeL as a function of the macroscopic plasma parameters are explained only partially with the existing simple diffusion models, which cannot account for the experimentally observed dependence of neL on the safety factor qL. The empirical laws are neL = 3.96 × 10−2 n̄e1.38 qL and TeL = 6.5 (Ip/qL)2/3 n̄e−0.75, where n̄e and neL are in units of 10l3 cm−3, Te is in eV and Ip in kA. For λn, the scaling λn, = 1.05 n̄e−0.25 cm holds, with the same units. No reliable law is derived for λT, but the approximate relationship λT ≈ 2.5–4λn always holds true. The fluctuations in the SOL and their role in determining the perpendicular particle diffusion and, hence, λT are discussed.