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

Radially peaked density profiles were found in discharges heated purely wit h 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 conve ction is confirmed with two independent transient transport studies. Flatne ss 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 peak ing parameter was found to decrease with increasing heating power. No clear density dependence was found. Strong gas puffing, or the corresponding hig h edge density prevents the formation of the peaked profiles.