Turbulence in L-H transitions on MAST and MAST-U*

As several studies have shown, there exists a critical power threshold P_LH beyond which tokamak plasmas transition to a state of reduced turbulence and improved confinement, known as the H mode. There does not yet exist a quantitative model for P_LH and attempts at finding an empirical scaling law have not yet produced one which captures all the parameter dependencies. A scoping study on MAST data (expanded from [L. Howlett et al. EPS Plasma Phys. conf. proc. P3.1073 (2021)]) will be presented which reveals MAST H mode behaviour with density, with different types of transitions and boundary behaviours present in different locations of the parameter space. Studies on C-Mod [Y. Ma et al. Plasma Phys. Control. Fusion 54 (2012)] have shown a dependence of P_LH on divertor geometry parameters. The nature of this dependence will be explored through experiments comparing L-H transitions with conventional and Super-X divertor configurations on MAST-U. Initial results of these experiments will be presented, including a study of edge turbulence dynamics using data from the beam emission spectroscopy (BES) diagnostic.

Some recent studies (e.g. [I. Cziegler et al., Nucl. Fusion 55 (2015)]) have shown that L-H transitions occur at a critical energy transfer from turbulence to zonal flows, both in favourable and unfavourable ion grad-B drift configurations [I. Cziegler et al., Phys. Rev. Lett. 118 (2017)]. The generality of this result as well as the link between this and divertor configurations will be explored through the experiments on MAST-U by mapping out the parameter space of P_LH while studying the turbulence dynamics at the edge. In preparation of the experiments, selected MAST transitions have been analysed by employing velocimetry techniques and bispectral methods on 2D BES data to investigate wave coupling and spectral energy transfer.