Overview of recent experimental results on MAST

Abstract

The low aspect ratio of the mega amp spherical tokamak (MAST) allows differentiation between different forms of the H-mode threshold scaling. With optimized fuelling using inboard puffing, and a connected double null divertor (DND) magnetic configuration, the H-mode power threshold data lie about 1.7 times higher than recent scaling laws. Slight magnetic configuration changes, of the order of the ion Larmor radius, around a connected DND significantly influence H-mode access. H-mode confinement in discharges with low frequency edge localized modes (ELMs) is generally consistent with international scaling laws, e.g. IPB98(y,2). Strong indications of both particle and energy internal transport barriers have been seen. Normalized beta values β N > 5 have been obtained, approaching the ideal n = 1 no wall external kink stability limit. Sawtooth triggered neo-classical tearing modes have been observed; numerical modelling of the island evolution reproduces mode behaviour well and confirms the significance of stabilizing field curvature effects. Divertor power loading studies, including transient effects due to ELMs, show a strong bias of power efflux to the outboard targets, where it is more easily handled. ELM energy losses, W ELM , are less than 4% of the stored energy in all regimes explored so far, but ELM effluxes extending 30 cm outside the outboard separatrix have been measured. Toroidally asymmetric divertor biasing resulted in significant broadening of the D α profile on the biased components and a reduction in the total power to the unbiased components. Halo current magnitudes and asymmetries are generally small compared with conventional tokamaks;

Beam ion driven instabilities in the National Spherical Tokamak Experiment

“…11 TAEs in low-aspect ratio tokamaks ͓or spherical tokamaks ͑STs͔͒ driven by beam ions have been observed in Small Tight Aspect Ratio Tokamak ͑START͒, [12][13][14] NSTX, 11 and Mega-Amp Spherical Tokamak ͑MAST͒ experiments 15 and have been studied theoretically in application for NSTX 16 -18 and START.…”

Evolution of Filament Structures during Edge-Localized Modes in the MAST Tokamak

“…These models range from those where the filaments are the main loss mechanism to those where the filaments exist only for a short time and serve only to slow the edge rotation and result in a collapse back to L mode. In this Letter, for the first time the 3D propagation of filaments through an ELM will be presented using visible imaging on the MAST [6] tokamak (at Culham Science Centre, U.K.) and the consequence for ELM energy loss models will be discussed.…”

Next-Step Spherical Torus Experiment and Spherical Torus Strategy in the Fusion Energy Development Path

“…In another experiment, the high normalized beta value of b N £ 6.5 (b N £ 10 li) was achieved where the so-called no-wall beta limit was significantly exceeded by about 35% [11]. In the area of plasma confinement, the observed global plasma energy confinement time in the NBI heated NSTX and MAST plasmas exceed the conventional confinement scalings both in H-mode and L-mode, diverted as well as inboard-limited plasmas [8,12]. In Moreover, due to the low loop voltage (~ 0.1 V compared to 0.5 V for typical NBI heated discharges), the high poloidal beta regime was maintained for over t-skin or about 5 t E [9].…”

“…to have an adequate experimental database as well as theory and modeling to be able to predict the plasma performance. Indeed, the progress in the ST plasma research being conducted worldwide has been quite rapid and encouraging [7,8]. In the area of high beta operations, the high average toroidal beta values of <b T > ~ 35% were achieved at high plasma current of Ip ~ 1.2 MA with E T ~ 200 kJ of stored energy on NSTX [9], which is a significant progress from the previous START high beta results obtained with Ip ~ 0.2 MA with E T ~ 20 kJ [10].…”