Radiation power profiles and density limit with a divertor in the W7-AS stellarator

The introduction of a divertor into the W7-AS stellarator has allowed access to a high density regime where the radiation profiles reach a steady state and the plasma no longer suffers a radiative collapse as in previous limiter discharges. In contrast to earlier experiments where the impurity confinement time measured by Al laser blow-off increases with increasing line integrated density, the divertor discharges above a density threshold decreases with increasing line integrated density. The observation that the divertor plasma radiates mainly at the plasma edge rather than the plasma centre is a further indication that changes to the impurity transport coefficients at these high densities are the basis for the achievement of steady state discharges in the divertor configuration of W7-AS. Copyright (2002) Australian National University- Research School of Physical Sciences and Engineering