Improved Confinement in High-Density Ohmic Discharges in ASDEX

The unsaturated linear rise of the energy confinement time with density, ${\ensuremath{\tau}}_{E}\ensuremath{\sim}{\overline{n}}_{e}$, up to the density limit is recovered in Ohmically heated discharges in ASDEX. Improvement of a factor of 2 is reached with ${\ensuremath{\tau}}_{E}\ensuremath{\approx}150$ ms at ${\overline{n}}_{e}=5\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ The improved state is characterized by peaked density profiles. The ion heat diffusivity decreases to the neoclassical value. The parameter ${\ensuremath{\eta}}_{i}=\frac{{L}_{{n}_{e}}}{{L}_{{T}_{i}}}$ governing the onset of ion-temperature-gradient modes, falls to the stability threshold. The improvement in confinement might therefore be attributed to the stabilization of ${\ensuremath{\eta}}_{i}$ modes.