Transport and confinement in the Mega Ampère Spherical Tokamak (MAST) plasma

Abstract

A combination of recently installed state-of-the-art imaging and profile diagnostics, together with established plasma simulation codes, are providing for the first time on Mega Ampère Spherical Tokamak (MAST) the tools required for studying confinement and transport, from the core through to the plasma edge and scrape-off-layer (SOL). The H-mode edge transport barrier is now routinely turned on and off using a combination of poloidally localized fuelling and fine balancing of the X-points. Theory, supported by experiment, indicates that the edge radial electric field and toroidal flow velocity (thought to play an important role in H-mode access) are largest if gas fuelling is concentrated at the inboard side. H-mode plasmas show predominantly type III ELM characteristics, with confinement H H factor (w.r.t. scaling law IPB98[y, 2]) around ∼1.0. Combining MAST H-mode data with the International Tokamak Physics Activities (ITPA) analyses, results in an L-H power threshold scaling proportional to plasma surface area (rather than P LH ∼ R 2 ). In addition, MAST favours an inverse aspect ratio

Investigation Of A Transient Energetic Charge Exchange Fux Enhancement ('spike-on-tail') Observed In Neutral-beam-heated H-mode Discharges In The National Spherical Torus Experiment

“…Resonant interaction of high-energy particles with magnetic perturbations in toroidal devices can produce large-scale modification of the particle distribution, sometimes leading to particle loss. Spherical tokamaks, including NSTX [20 -23], START [24] and MAST [25,26], are particularly susceptible to fast ion driven instabilities due primarily to their relatively low toroidal field but also in some instances due to the direct effect of the low aspect ratio.…”

Edge transport barriers in magnetic fusion plasmas

“…The determination of critical edge parameters is further complicated by the fact that the ETB power threshold can be strongly dependent on machine parameters, such as magnetic field and plasma size and shape [98] and also on the plasma or divertor configuration and the direction of the ion ∇B drift with respect to the divertor X-point position [99]. For example, the threshold power in MAST is reduced for the doublenull configuration [100] whereas, in NSTX, a lower threshold power is obtained with single-null operation [101]. Variations in the threshold power (by a factor of two) have been observed in DIII-D depending on the direction of the ion ∇B drift relative to the X-point [99].…”

“…Also, the location of the neutral particle input can affect the threshold. For example, the threshold power for the ETB is reduced when gas is injected from the high field side instead of the low field side in MAST [100] and NSTX [101].…”

“…This model aims to explain the nonlinear evolution and rapid growth of the ELM. Experimental observations of such tube-like structures have been made on MAST [100,116]. Fig.…”

Transient Enhancement ('Spike-on-Tail') Observed on Neutral-Beam-Injected Energetic Ion Spectra Using the E||B Neutral Particle Analyzer in the National Spherical Torus Experiment

“…Spherical tokamaks, including NSTX [20 -23], START [24] and MAST [25,26], are particularly susceptible to fast ion driven instabilities due primarily to their relatively low toroidal field but also in some instances due to the direct effect of the low aspect ratio. The loss of energetic ions on NSTX has been confirmed by direct measurement of ion losses with a scintillator Fast-Loss Ion Probe (sFLIP) [27].…”