Improved Confinement in High-Density Ohmic Discharges in ASDEX

The unsaturated linear rise of the energy confinement time with density, ${\tau }_E\sim {\overline{n}}_e$, up to the density limit is recovered in Ohmically heated discharges in ASDEX. Improvement of a factor of 2 is reached with ${\tau }_E\approx 150$ ms at ${\overline{n}}_e=5\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$ The improved state is characterized by peaked density profiles. The ion heat diffusivity decreases to the neoclassical value. The parameter ${\eta }_i=\frac{L_{n_e}}{L_{T_i}}$ 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 ${\eta }_i$ modes.