A COMPARATIVE-STUDY OF TRANSPORT IN STELLARATORS AND TOKAMAKS

Experimental results on transport in stellarators and tokamaks are rev iewed in a comparative sense. The objective is to learn about the impo rtance of plasma current and magnetic shear for anomalous transport in high-temperature plasmas. On the basis of scaling expressions, the ab solute values and parameter dependences of the confinement time are si milar if the plasma current is expressed in terms of magnetic field pa rameters. 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 si milar features as in ohmically heated tokamak discharges. A linear and a saturated regime can be distinguished. The critical density, at whi ch saturation sets in, has a similar value and it seems to decrease wi th increasing machine size. Power degradation, transient transport, pr ofile consistency and non-local transport are treated as related probl ems, which are connected to the question of the temperature dependence of the thermal diffusivity. Results from the various experiments cann ot yet be described with a consistent physical picture. However, the i mportance of non-local effects is established in stellarators and toka maks, 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 respon sible for anomalous transport. In the core, density fluctuation amplit udes increase together with the diffusivity when the density is increa sed but the two parameters show opposite trends with increasing heatin g power. It turns out that transport in the two classes of devices is more alike than it has previously appeared. This indicates that the st rong toroidal plasma current, major rational values of the rotational transform inside the plasma or strong magnetic shear are not the centr al elements of a theoretical model for anomalous transport in fusion p lasmas.