Scaling of plasma transport in Ohmically heated tokamaks

Detailed numerical simulations of the radial temperature and density profiles and energy confinement times measured in Pulsator (2x1013<<7x1013 cm-3), and in the high-density regime of Alcator C (1014<<4.5x1014 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, chisub(e) [cm2.s-1]=4.5x1010 (Bsub(t) a/R)/(nsub(e)sup(0.8)Tsub(e)qsqrt Asub(i))(Bsub(t)[G], nsub(e)[cm-3], Tsub(e)[keV],Asub(i) ion atomic mass), where the 1/(Tsub(e)q) dependence cannot be distinguished from a 1/Tsub(e)sup(0.6) law. The particle diffusion is given by the neoclassical term and an anomalous contribution of (0.1-0.3) chisub(e). 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 chisub(e) approx.= 1/nsub(e) 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. (author)