Spatio-temporal turbulence dynamics over the MAST-U edge transport barrier evolution

The first experimental results of self-regulation between the plasma relative electron density fluctuations, n ̃e, and the perpendicular velocity, v ̃⊥, over limit cycle oscillations (LCOs) across the edge and near scrape-off layer (SOL) regions of the MAST-U spherical tokamak are presented. The dithering phase is preceded by the localisation of the emissivity at the magnetic null point. The L–H-mode-like transitions of the LCOs are characterised by a decrease in the mean n ̃e and a narrowing of the probability density functions (PDFs) due to plasma turbulence suppression. Sharp decreases in the n ̃e standard deviation and skewness routinely coincide with the start of the LCO in a narrow region corresponding to the steepest pedestal pressure and v ̃⊥ gradients. The decrease in n ̃e skewness during the LCOs in the regions closer to the SOL, separatrix, and core plasma follows 2–5 ms later. The reduction in n ̃e is always reflected in changes in the PDF structure; the mean perpendicular velocity consistently shifts from positive to negative values following the L–H-mode-like transitions. The most significant changes in the mean v ̃⊥ PDFs are in the region of the maximum negative v ̃⊥. Further evidence of the spatially localised nature of self-regulation between n ̃e and v ̃⊥ PDFs is provided from information geometry analysis for the formation and collapse of the ETB.