Achievement of regimes with high density, low qa and good confinement on ASDEX

The density limit in ASDEX decreases for qa < 2.7. However, in gettered discharges at high current (500 kA), the density limit shows again a steep increase to a higher second maximum (M=n-bareR/Bt=7.2x1019m-2·T-1) at qa=2.05. The lowest qa-value achieved so far in Ohmic and beam heated plasmas is 1.9 (MOH=6.6x1019). With continuous pellet injection, peaked density profiles with very high central values (2.2x1020m-3) were obtained in Ohmic and weakly beam heated discharges. In pellet refuelled discharges, τE-values of 160 ms are achieved with n-circumflexeτE=3.5x1019sm-3. The confinement in the L-regime seem to be governed by the same principles as in the OH-saturation regime. τE can be increased by deuterium operation or by ICRF instead of NI heating. A recipe for achieving the H-mode has been developed which was successfully applied to the cases of qa ≤ 2, with ICRH or at high density with pellet refuelling. (author)