Spatio-Temporal Turbulence Dynamics over the MAST-U Edge Transport Barrier Evolution

Detailed results are presented from the first experimental study of self-regulation between plasma relative density fluctuations, ñₑ, and perpendicular velocity, ṽ⊥, over limit cycle oscillations (LCO) across the edge and near scrape-off layer (SOL) regions of the MAST-U spherical tokamak. The start of 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 ñₑ and a narrowing of the probability density functions (PDF) due to plasma turbulence suppression. Sharp decreases in the ñₑ, standard deviation, σñₑ and skewness, S_ñₑ, are observed to routinely coincide with the start of the LCO in a narrow region of (0.970≤ψN≤ 0.978), corresponding to the location of the steepest pedestal pressure and ṽ⊥ gradients. The decrease in S_ñₑ during the LCOs in the regions closer to the SOL, separatrix, and core plasma follows 2 to 5 ms later. The reduction in ñₑ is always reflected in changes in the PDF structure; the mean perpendicular velocity, ṽ⊥, consistently shifts from positive to negative values following the L-H-mode-like transitions. The most significant changes in ṽ⊥ PDF structure are observed in the region of location of the maximum negative ṽ⊥. Further evidence of the spatially localised nature of self-regulation between the turbulence level and ṽ⊥ is provided from information geometry analysis for the formation and collapse of the ETB.