A comparative study of transport in stellarators and tokamaks

Abstract

Experimental results on transport in stellarators and tokamaks are reviewed in a comparative sense. The objective is to learn about the importance of plasma current and magnetic shear for anomalous transport in high-temperature plasmas. On the basis of scaling expressions, the absolute values and parameter dependences of the confinement time are similar if the plasma current is expressed in terms of magnetic field parameters. The degradation of confinement with heating power is of the same order in both devices and it is difficult to explain it in terms of a diffusivity which depends on temperature or temperature gradient. The density dependence of confinement observed in stellarators has similar features as in ohmically heated tokamak discharges. A linear and a saturated regime can be distinguished. The critical density, at which saturation sets in, has a similar value and it seems to decrease with increasing machine size. Power degradation, transient transport, profile consistency and non-local transport are treated as related problems, which are connected to the question of the temperature dependence of the thermal diffusivity. Results from the various experiments cannot yet be described with a consistent physical picture. However, the importance of non-local effects is established in stellarators and tokamaks, although observed in different types of perturbation experiments. In stellarators and tokamaks, fluctuation measurements in the scrape-off layer are consistent with drift-wave-like turbulence being responsible for anomalous transport. In the core, density fluctuation amplitudes increase together with the diffusivity when the density is increased but the two parameters show opposite trends with increasing heating power. It turns out that transport in the two classes of devices is more alike than it has previously appeared. This indicates that the strong toroidal plasma current, major rational values of the rotational transform inside the plasma or strong magnetic shear are not the central elements of a theoretical model for anomalous transport in fusion plasmas.