Transport in tokamaks – a review of experiment

Abstract

The main experimental results on confinement and transport processes in tokamaks are summarised. It is difficult to draw definite conclusions from measurements of the global energy containment time even in the quasi-steady phase of those ohmically heated discharges, which do not exhibit gross instability or appreciable effects due to runaway electrons. However, comparison of particle and energy confinement suggests that the loss process is mainly conduction rather than convection. In order to separate the various energy input and loss mechanisms the assumptions of classical ohmic heating and equipartition are made. The experimental evidence for these assumptions is presented. The dominant energy transport process is electron thermal conduction, for which a formalism is suggested for comparing experimental data with various theoretical models. Particle and iimpurity transport appears to require the neoclassical pinch tenn in addition to anomalous particle diffusion at a significantly lower rate than electron conduction. The contribution of the sawtooth instability to transport can be measured in principle and is only important in certain conditions. 1-D models of the discharge are quite successful provided empirical forms of the electron conduction and particle diffusion are used. Partial dat a exists on various related topics: plasma rotation, radial electri c fields, the boundary layer, the effects of toroidal field ripple and low frequency instabilities. This material is summarised.