Abstract
The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era.
Highlights
A spherical torus or tokamak (ST) is a tokamak with an aspect-ratio (A 1⁄4 R/a) of less than 2, where R is the plasma major radius and a is the minor radius
The density gradient term alone, if large enough, could determine the electron temperature gradient (ETG) critical temperature gradient. This physics was investigated after the edge localized modes (ELMs) event where the density gradient term a/Lne in the pedestal region increased by a factor of 5 but a/LTe remained constant This resulted in ETG fluctuation reduction and a factor of 2 reduction in the transport coefficient in the density gradient region
The spherical tokamak (ST) is a class of tokamak confinement devices that has numerous features that make it unique for fundamental studies in plasma physics and plasma-surface interactions, and attractive as a fusion reactor concept
Summary
A spherical torus or tokamak (ST) is a tokamak with an aspect-ratio (A 1⁄4 R/a) of less than 2, where R is the plasma major radius and a is the minor radius. It should be noted that the present paper mainly utilizes material from peer-reviewed journals, but where appropriate, major conference-related publications are included. This holds for descriptions of the most recent results, since ST research is very much a rapidly evolving area in magnetic fusion. Since the present paper is a ST review article, the references are mainly given for ST research publications. The main purpose of this review paper is to give a comprehensive description of recent progress in ST research, rather than give a historical perspective on the research For those readers interested in the historical development of the ST, relevant references are usually given within the publications cited in this paper
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