Physics of the density limit in the W7-AS stellarator

Density limit discharges in the W7-AS stellarator, with constant line integrated density and a duration of up to 2 seconds, were studied at three values of the toroidal magnetic field (B = 0.8, 1.25 and 2.5 T). The central factor governing the physics of the density limit in stellarators was demonstrated to be the decreasing net power to the plasma when the centrally peaked radiated power density profile exceeds that of the deposited power density. The process was further accelerated by the peaking of electron density under these 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 density in stellarators is naturally of special interest for reactor operation. Such a scenario has been best achieved in H-mode discharges, in which ELM's restricted the impurity influx to the plasma and an equilibrium in the plasma parameters with suitably low radiation power levels was possible. A density scan in ECRH discharges highlights the need to control impurity sources and choose electron densities well below the density limit in order that steady state operation can be attempted in discharges without ELM's. A simple model of bulk radiation predicted that the limiting density should depend on the square root of heating power and this was experimentally confirmed. The magnetic field scaling of the limiting density found experimentally is attributed in this simple model to the term concerning the radial profile of the impurity density, which in turn is a function of the diffusion coefficient and inward pinch of the impurity ions. It is also expected that the dependence of the ion temperature on magnetic field must also be influential and the consequences of this dependence for neoclassical impurity ion transport requires further analysis. (orig.)