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

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Abstract
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.
Funding Information
  • USDOE (DE-SC0019005)
  • EPSRC Energy Programme (EP/W006839/1)

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