Improved Confinement in High-Density Ohmic Discharges in ASDEX

The unsaturated linear rise of the energy confinement time with density, τE~n̄ₑ, up to the density limit is recovered in Ohmically heated discharges in ASDEX. Improvement of a factor of 2 is reached with τE≈150 ms at n̄ₑ=5×10¹³ cm^-3 The improved state is characterized by peaked density profiles. The ion heat diffusivity decreases to the neoclassical value. The parameter ηᵢ=L_nₑL_Tᵢ 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 ηᵢ modes.