Transport in toroidal devices-experimentalist's view

The authors summarize the present status of tokamak confinement studies. Under quiescent conditions, ion and impurity transport can be close to the neo-classical level. Generally, however, radial transport is enhanced by instabilities. There is good experimental evidence that anomalous ion heat and momentum transport are caused by ion temperature gradient driven turbulence. The same level of understanding is not reached in electron heat and particle transport. Electron heat transport is characterised by strong non-linearities, and particle transport by strong off-diagonal contributions. Where possible, the tokamak results are compared with those from stellarators. This comparison is meaningful because stellarators and tokamaks share many transport aspects: In both cases, transport is anomalous, degrades with heating power and increases toward the edge. Bifurcations like H-mode-like transitions are observed in the W7-AS stellarator. Though anomalous too, the comparison reveals that transport in stellarators is less confused by additional complexities like large off-diagonal contributions, profile resilience, disparities between steady state and perturbatively determined transport coefficients, an isotopic mass but an obvious lack of magnetic field and minor radius scaling.