Physics of enhanced confinement with peaked and broad density profiles

There exist two classes of discharges on ASDEX which overcome the severe confinement deterioration at high densities or heating power. The first class is characterized by unusually peaked density profiles and the second one by exceptionally broad ones. These profiles and the concomitant confinement enhancement imply a change in both the particle and heat transport. Whereas the changes in particle transport are not fully understood, transport analyses point out that the improved heat transport in the first profile class can be explained by reduced ion contribution losses coming close to the neoclassical ones. The different results for the ion transport with flat and peaked density profiles are qualitatively consistent with that expected from eta _i-modes. This conclusion is supported by momentum transport studies. The analyses, however, cannot yet explain the electron heat transport which is anomalously high in all regimes.