L-H transition studies on the ST40 tokamak

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The plasma transition from the low to high (L-H) confinement regime in tokamaks, one of the most remarkable discoveries in fusion history, refers to the sudden improvement of confinement when input power is increased above a critical value. The L-H transition is accompanied by the formation of a pedestal at confined plasma edges, a relatively narrow edge plasma region with significantly enhanced pressure profile gradients and reduced transport. This in turn boosts the temperature of the core plasma thus improving the fusion performance compared to L-mode. This phenomenon has been reliably reproduced in different fusion devices since its first discovery in 1980s; the study of the L-H transition, the backwards H-L transition and the pedestal, has remained one of the most important research topics in Magnetically Confined Fusion ever since.For ITER, questions on how to enter and exit the H-mode with the available heating power remain a crucial part of the practical machine operation planning. Recently, new tokamak concepts wandering further away from the conventional design of ITER, JET, etc., have been proposed, which require understanding of the access to H-mode in new regimes. The topic of L-H transitions is thus rapidly evolving, with a growing number of experimental studies planned on existing machines. These studies involve upgraded suites of edge, core and divertor plasma diagnostics and utilise advanced analysis techniques.This project will make a unique and important contribution to ongoing world-wide H-mode access studies through the analysis of experimental results on the new ST40 super-conducting spherical tokamak at Tokamak Energy Ltd. To facilitate this, a novel interpretative model will be developed. ST40 plasmas will occupy new domains of low and high confinement operation with this spherical tokamak's exceptional combination of high magnetic field, low aspect ratio and augmented heating power. The validity of traditional analysis methodology, using mean values or variance across the instantaneous transitions into and out of H-mode, has limitations due to large, time-dependent fluctuations. Thus, the particular emphasis of this project will be on the investigation of the time-evolution of the L-H/H-L transitions, the edge pedestal whereavailable and the development of a novel probability distribution function (PDF) statistical method.The experimental aspect of the project will be to map out the operational space for L-H (and H-L) transitions over a series of available parameter scans, such as density, magnetic field, Ip, torque or additional heating scheme. These results are expected to make a significant contribution to ongoing cross-spherical tokamak H-mode access studies on MAST, MAST-U and NSTX. On the theoretical and computational side, the project will employ an existing simulation code to solve the 3D Fokker-Planck equation and use it to develop and tune the lighter PDF statistical model. The code will need to be parallelised to speed up computations and upgraded to include a wider range of noise sources.