Overview of physics results from MAST

Abstract

Substantial advances have been made on the Mega Ampère Spherical Tokamak (MAST). The parameter range of the MAST confinement database has been extended and it now also includes pellet-fuelled discharges. Good pellet retention has been observed in H-mode discharges without triggering an ELM or an H/L transition during peripheral ablation of low speed pellets. Co-ordinated studies on MAST and DIII-D demonstrate a strong link between the aspect ratio and the beta scaling of H-mode energy confinement, consistent with that obtained when MAST data were merged with a subset of the ITPA database. Electron and ion ITBs are readily formed and their evolution has been investigated. Electron and ion thermal diffusivities have been reduced to values close to the ion neoclassical level. Error field correction coils have been used to determine the locked mode threshold scaling which is comparable to that in conventional aspect ratio tokamaks. The impact of plasma rotation on sawteeth has been investigated and the results have been well-modelled using the MISHKA-F code. Alfvén cascades have been observed in discharges with reversed magnetic shear. Measurements during off-axis NBCD and heating are consistent with classical fast ion modelling and indicate efficient heating and significant driven current. Central electron Bernstein wave heating has been observed via the O–X–B mode conversion process in special magnetically compressed plasmas. Plasmas with low pedestal collisionality have been established and further insight has been gained into the characteristics of filamentary structures at the plasma edge. Complex behaviour of the divertor power loading during plasma disruptions has been revealed by high resolution infra-red measurements.

Non-linear modulation of short wavelength compressional Alfvén eigenmodes

“…1(c)). Similar modes, from TAE up to the co-propagating hfCAE, have also been seen in the beam heated, low aspect ratio MAST [10][11][12][13] and the first observations of the broad spectrum of Alfv enic activity were made on START. 13,14 Experimental and theoretical efforts are underway to characterize these various modes, understand the mechanisms for excitation of the modes, how apparently disparate modes interact, and how these modes affect the fast ion population.…”

The effect of resonant magnetic perturbations on the divertor heat and particle fluxes in MAST

“…Several means of ELM control exist, including, vertical kicks [3], pellet pacing [4] and the application of external resonant magnetic perturbations (RMPs) [5]. An investigation of the effect of RMPs on the divertor heat loads and particle loads are reported in this paper using data from the Mega Amp Spherical Tokamak (MAST) [6]. MAST is equipped with a set of 18 RMP coils, which allow perturbations with a range of toroidal mode numbers to be applied to the plasma.…”

Time–frequency analysis of fusion plasma signals beyond the short-time Fourier transform paradigm: An overview

“…The need for time-frequency (t-f) analysis [1,2] in nuclear fusion research, when processing data coming from dedicated high-temperature plasma diagnostics, has always been strongly felt [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Indeed, fusion plasmas are an incredibly rich source of physical phenomenology whose spectral content changes in time, as often seen in plasma turbulence and MHD instabilities.…”

“…This gives rise to signals exhibiting, in general, time-varying spectra that can range from a few to hundreds of kHz, and can change as fast as in some hundredths of ms. The basic, almost universal approach to process and analyse such signals has been the short-time Fourier transform (STFT), from which the spectrogram (SPEC) has emerged as the ancestor of modern t-f distributions [1][2][3][4][5][6][7][8][9][10][11]13,[16][17][18][19][20]. Sometimes, more sophisticated versions of time-local Fourier spectra, like wavelets [12,17], or alternative yet equivalent techniques, such as complex demodulation [14], have been used, but always within a framework that can be called the STFT paradigm.…”

“…The authors' basic motivation has been the divulgation of these less-known t-f tools to as broader a nuclear fusion audience as possible, so fellow researchers can get well acquainted with and take full advantage of them. The worthiness of such a task can be easily justified by the significant fraction of reports monitoring the recent progress on controlled thermonuclear research in which t-f analysis is actually present, although always within the realm of the STFT paradigm [6][7][8][9][10][11][12].…”