The first experimental results of self-regulation between the plasma relative electron density fluctuations, , and the perpendicular velocity, , 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 and a narrowing of the probability density functions (PDFs) due to plasma turbulence suppression. Sharp decreases in the standard deviation and skewness routinely coincide with the start of the LCO in a narrow region corresponding to the steepest pedestal pressure and gradients. The decrease in skewness during the LCOs in the regions closer to the SOL, separatrix, and core plasma follows 2–5 ms later. The reduction in 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 PDFs are in the region of the maximum negative . Further evidence of the spatially localised nature of self-regulation between and PDFs is provided from information geometry analysis for the formation and collapse of the ETB.
This paper presents the first experimental results on the self-regulation between plasma density fluctuations and perpendicular velocity over limit cycle oscillations in the edge and scrape-off layer regions of the MAST-U spherical tokamak. It shows how turbulence suppression during the L-H-mode-like transitions leads to changes in the probability density functions of these parameters.