Description
A quantitative understanding of the impact of magnetic islands (MIs) on transport and their possible role in enhanced confinement is important for the design and optimization of future stellarator devices. The Wendelstein 7-X stellarator allows for magnetic configurations where static 5/5 MIs form near the separatrix inside the confined region. These configurations have been found to provide improved confinement compared to equivalent configurations where MIs are placed in the scrape-off layer. Confinement transitions associated with MI growth beyond a critical width, $w>w_c$ with $w$ the MI width, can be triggered in turbulence-dominated scenarios by scanning $w$ with external control coils. In turbulence-suppressed regimes, where ITG turbulence is suppressed with density gradients, these transitions are observed spontaneously. Upon these transitions, the plasma diamagnetic energy decreases by ~30-40 %, associated with a flattening of both ion ($T_i$) and electron ($T_e$) temperature profiles. The Coherence Imaging Charge Exchange Recombination Spectroscopy (CICERS) diagnostic is employed to characterize the impact of these transitions within one MI on $T_i$, carbon impurity density ($n_C$) and flow velocity ($v_C$), delivering poloidally resolved 2D images of such parameters. While turbulence-dominated regimes show only minor changes in $n_C$ and $v_C$, associated with the flattening of the profiles, the turbulence-suppressed scenarios display a strong $n_C$ accumulation centered around the O-point of the MI, with $n_C$ increasing up to a factor of 4. Additionally, an island inboard-outboard flow asymmetry is observed, potentially linked to the development of $E\times B$ flows inside the MI. Maintaining $w$ below $w_c$ via strong coil-driven island reduction prevents the spontaneous temperature flattening and associated confinement degradation observed in these configurations in high-performance turbulence-suppressed operation; under these conditions, the current record of Stellarator triple product is obtained.