We overview and compare the energy confinement behaviour of tokamaks andstellarators at high heating powers. Tokamaks show in all parameter ranges ananomalous enhancement of electron energy losses. At high heating powers abifurcation is observed in the plasma energy vs. heating power diagram, withthe low energy (L-regime) branch showing a clear deterioration of confinementtime with power. Divergent results exist concerning the corresponding behaviorof the high energy (H-regime) branch. Experiments with ECR-heating ofnet-current-free stellarator plasmas can be to a large extent explained byneoclassical electron heat conduction losses, including ripple effects. Inthese experiments anomalous transport enhancement has been identified only inthe outer plasma layers and at low temperatures and densities, which in thisform should not affect reactor aspects. At a given density, neoclassicalelectron heat conduction losses scale however in a similar manner with heatingpower as the L-regime losses observed in tokamaks. For a better discrimination- important due to the differing variation with size expected for the twotypes of losses - density scans over a range not accessible with present dayECRH-systems are needed. Corresponding experiments with NBI-heating on thehigh shear Heliotron-E device indeed gave evidence for a confinement anomalyincreasing with heating power, although not well fitted by tokamak L-regimeexpressions. For low shear systems, conclusive evidence is expected fromNBI-heating experiments on W VII-AS.
Electron cyclotron resonance heating in the Wendelstein VII-A stellarator