Key parameters for the confinement of neutral-beam-heated tokamaks

The parameters and processes governing the low and high (L and H) confinement in ASDEX are investigated by computer modelling. The L-transport is shown to be connected with the non-Ohmic electron heating. After neutral injection, the L-phase is maintained by the ion-electron energy transfer rate for the significant time span of about 100 ms. It is found that the profile parameters η e,i = d ln T e,i /d ln n discriminate between the L-transport and the H-transport. Small η-values around unity characterize the H-regime, while larger η-values correspond to the L-regime. The operational limits of the H-mode are connected with variations of η e and η i due to changes in the injection heating profile. These results suggest that drift instabilities such as dissipative trapped-electron modes are involved in the anomalous L- and H-transport.