Scaling of plasma transport in Ohmically heated tokamaks

Abstract

Detailed numerical simulations of the radial temperature and density profiles and energy confinement times measured in Pulsator (2 × 1013 < ⟨ne⟩ < 7 × 1013 cm−3), and in the high-density regime of Alcator C (1014 <⟨ne⟩ < 4.5 × 1014 cm−3) and FT yield a uniform scaling of the transport in Ohmically heated tokamak discharges. The anomalous electron thermal conductivity can be described by a modified Coppi-Mazzucato law, χe [cm2·s−1] = 4.5 × 1010 (Bt [G], ne [cm−3], Te [ke V], Ai ion atomic mass), where the l/(Te q) dependence cannot be distinguished from a law. The particle diffusion is given by the neoclassical term and an anomalous contribution of (0.1–0.3) χe. The transport due to sawtooth oscillations is treated by an enhanced, Bohm-like diffusion within the q = 1 surface. In this model, the apparent saturation of the energy confinement time with density observed especially in Alcator C can be explained without anomalous enhancement of the neoclassical ion heat conductivity. An explanation of the saturation under the usual Alcator-scaling χe ∼ l/ne would require a five-times enhancement above the neoclassical value. The increase in ion heat conductivity due to the toroidal-field ripple is found to be negligible in the regimes considered.