Quiescence near the X-point of MAST measured by high speed visible imaging

TL;DR

This study uses high-speed imaging to identify a quiescent X-point region in MAST, which is consistent across various operational conditions and significantly influences heat flux distribution and filamentary activity in the divertor.

Contribution

It provides the first detailed characterization of the quiescent X-point region in MAST, linking it to plasma conditions and divertor filament dynamics.

Findings

QXR spans from the separatrix to 1.02 flux surface

Contains about 60% of the heat flux to the divertor target

Associated with higher frequency, short-lived fluctuations

Abstract

Using high speed imaging of the divertor volume, the region close to the X-point in MAST is shown to be quiescent. This is confirmed by three different analysis techniques and the quiescent X-point region (QXR) spans from the separatrix to the 1.02 flux surface. Local reductions to the atomic density and effects associated with the camera viewing geometry are ruled out as causes of the QXR, leaving quiescence in the local plasma conditions as being the most likely cause. The QXR is found to be ubiquitous across a significant operational space in MAST including L-mode and H-mode discharges across wide ranges of line averaged density, plasma current and NBI power. When mapped to the divertor target the QXR occupies approximately an e-folding length of the heat-flux profile, containing approximately 60% of the total heat flux to the target, and also shows a tendency towards higher frequency shorter lived fluctuations in the ion-saturation current. This is consistent with short- lived divertor localised filamentary structures observed further down the outer divertor leg in the camera images, and suggests a complex multi-region picture of filamentary transport in the divertor.

Full text

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