Intermittent fluctuations in the boundary of magnetically confined fusion plasmas

Abstract

Fluctuation-induced transport in the form of hot and dense blob-like filaments is a profound concern for the successful operation of commercially-viable fusion reactors. These turbulent phenomena are inherent in the scrape-off layer (SOL), a region of magnetic field lines intersecting with the material surfaces. These filaments propagate radially outward leading to damaging plasma-wall interactions.

Filaments are notably diagnosed in Langmuir probes and gas puff imaging (GPI). Their measurements are known to display skewed and flattened probability density functions. Moreover, their frequency power spectra manifest a characteristic Lorentzian shape. These statistical properties remain consistent across different fusion devices, confinement modes and plasma parameters. A well-known stochastic model, called the filtered Poisson process (FPP), can reproduce these salient statistical features.

This thesis presents an in-depth study into far SOL fluctuations, from low-confinement mode plasmas. Under various plasma and machine parameters, the Richardson-Lucy deconvolution method is employed for amplitude and waiting-time extraction, revealing how these change with the aforementioned parameters. The strengths and weaknesses of this approach are reported.

A detailed analysis of far SOL fluctuations in Alcator C-Mod was performed, encompassing a broader range of densities that have not been studied before using the FPP. Trends in the FPP model parameters are observed, estimated from Langmuir probes and GPI. The mean amplitudes and fluxes increase with density thereby amplifying plasma--wall interactions. Far SOL fluctuations across six distinct devices: Alcator C-Mod, TCV, DIII-D, KSTAR, MAST, and MAST-U are assessed. Across all of these devices, the amplitudes increase linearly with the density. However, the study shows no universal relationship between Greenwald fraction and the FPP model parameters. The disparities in the behaviour of the far SOL fluctuations are highlighted between conventional and spherical tokamaks. The study underlines how plasma and machine parameters effect far SOL fluctuation statistics, offering insight for next-step fusion devices.