A new plasma condensation phenomenon in the W7-AS island divertor

In the W7-AS stellerator with island divertor very high densities can be achieved in the high-density H-mode. As a consequence, the plasma in the divertor detaches as previously shown by the reduction of the particle flux at some strike point positions and an upward movement of the ionisation front. Our spectroscopic measurements prove the simultaneous formation of a high-density (8×1020 m−3), low temperature (0.2 eV) region at the detachment transition in the divertor on top. This plasma is strongly recombining, as a result a strong up/down pressure asymmetry develops. Since flux amplification at the strike points was not found in the island divertor, we suppose that a nonlinear thermal condensation instability leads to the observed high-density region.