Peaked density profiles: evidence of inward transport in the W7-AS stellarator

Radially peaked density profiles were found in discharges heated purely with ECH in the currentless W7-AS stellarator. Changes in the central particle source are not significant and do not explain the density peaking. This is evidence for inward convection in W7-AS. The existence of the inward convection is confirmed with two independent transient transport studies. Flatness of the temperature profiles is essential for the density peaking, and an anti-correlation was found between the density and temperature gradients. The density peaks as the ECH-deposition is moved outwards. The density peaking parameter was found to decrease with increasing heating power. No clear density dependence was found. Strong gas puffing, or the corresponding high edge density prevents the formation of the peaked profiles.