Radiation measurements and modeling of the density limit on the W7-AS stellarator

Density limit discharges in the W7-AS stellarator with a strong density ramp were compared to a series of discharges with constant line integrated density approaching the maximum value achieved in the density ramp. The physics of the density limit in stellarators was demonstrated to be consistent with the predictions of the two-point model, indicating that this model successfully describes the density limit process in both stellarators and tokamaks. The discharges with a strong density ramp were found to have broader density profiles than those discharges with constant line integrated density. The latter discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS. Modeling of the radiation profile, to simultaneously match the measured bolometer and soft X-ray radial profiles of radiated power, implies that impurity density profiles were peaked and continuously increased during the discharge. The increase in radiated power decreased the net deposited power to the plasma and the diamagnetic energy fell. The aim of producing steady-state discharges at the highest possible density is aided by the reduction of impurity sources by helium glow discharge cleaning.