Experimental characterization of transport effects of internal magnetic islands in Wendelstein 7-X

Magnetic Islands (MIs) are often linked to degraded plasma confinement or even disruption, however they can also behave as internal transport barriers improving performance. The flexibility of the Wendelstein 7-X (W7-X) stellarator allows for a magnetic configuration featuring a 5/5 static island chain within the separatrix, which shows enhanced confinement with respect to other configurations due to an electron transport barrier. In this work, the CICERS diagnostic is employed to investigate the impact of MIs on ion temperature (Ti), carbon density (nC) and flow velocity (vC) experimentally deriving 2D maps of their poloidally resolved spatial structure. Two experimental scenarios are found where it is believed that the island grows over a certain threshold size by external control coils in turbulence-dominated plasmas or due to beta effects in turbulence-suppressed discharges, as predicted by HINT modeling. Above the threshold, a transition to a lower confinement state is found (with changes in diamagnetic energy of ∼ 30 %). Accordingly, flattening of Ti, accumulation of nC inside the MIs and a modification of vC potentially linked to the development of ExB vortex flows around the MI are observed. Significant differences regarding the shape of vC as well as the level of nC accumulation are found in both scenarios, hinting towards additional mechanisms impacting these processes.