Physics of the density limit in the W7-AS stellarator

Density-limit discharges in the W7-AS stellarator, with constant line-integ rated density and a duration of up to 2 s, were studied at three values of the toroidal magnetic field (B = 0.8, 1.25 and 2.5 T). The central factor g overning the physics of the density limit in stellarators was demonstrated to be the decreasing net power to the plasma when the centrally peaked radi ated power density profile exceeds that of the deposited power density. The process was further accelerated by the peaking of electron density under t hese conditions. In discharges with B = 2.5 T, simulations of the centrally peaked radiation power density profiles could be shown to be due to peaked impurity density profiles. Laser blow off measurements clearly inferred an inward pinch of the injected aluminium. These discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS. The aim of producing steady-state discharges at the highest possible densit y in stellarators is naturally of special interest for reactor operation. S uch a scenario has been best achieved in H-mode discharges, in which ELMs r estricted the impurity influx to the plasma and an equilibrium in the plasm a parameters with suitably low radiation power levels was possible. A densi ty scan in ECRH discharges highlights the need to control impurity sources and choose electron densities well below the density limit in order that st eady-state operation can be attempted in discharges without ELMs. A simple model of bulk radiation predicted that the limiting density should depend on the square root of heating power and this was experimentally con firmed. The magnetic field scaling of the limiting density found experiment ally in this simple model will partly depend on the term concerning the rad ial profile of the impurity density, which in turn is a function of the dif fusion coefficient and inward pinch of the impurity ions. Theoretical studi es have shown that an assumption about the B dependence of the thermal cond uctivity leads to density limit scaling laws with an explicit B dependence.