Experimental measurement of turbulence in the divertor volume of MAST-U

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

Turbulence in the scrape-off layer is complex with multiple regions displaying differing characteristics, yet understanding turbulent transport in the scrape-off layer remains a critical step towards a predictive capability for future fusion devices. Understanding cross-field transport inside the divertor volume has been historically neglected, however it strongly impacts conditions at the divertor surface which is a critical interface for the operation of a fusion reactor. This project will provide a deep and systematic experimental analysis of cross-field transport in the divertor volume of MAST-U using a specialised probe diagnostic.
MAST-Upgrade will be installed with a new divertor reciprocating probe in 2019/2020 and comparisons of turbulence measurements made on this probe with upstream will provide unique insight into turbulence in the super-X chamber. Recording power spectra, correlation lengths, fluctuation amplitudes and other such measurements at high time resolution will provide detailed characterization of the turbulence in the divertor of MAST-U. The current set of probe heads available to MAST-Upgrade are not optimized for turbulent measurements beyond single point time-series. Whilst each probe can make measurements of the ion-saturation current, only the Mach probe can be used for measurements of density and temperature (via its triple probe arrangements) at high time resolution, but these measurements are made at discrete length scales that exceed turbulent correlation lengths. The Ball-pen probe can measure the plasma potential directly, but again the arrangement of the probe tips on the probe head prevents a more detailed measurement of the internal structure of fluctuations. This project will develop a new probe head for the MAST-U RP systems, interchangeable between midplane and divertor. The probe head will be designed using a synthetic diagnostic to optimize the positioning and type of probes used, and will use both Langmuir and Ball-pen probes to provide measurements of electron density and temperature, alongside detailed measurements of turbulent correlation length scales and internal fluctuation structure at high time resolution. The synthetic diagnostic will be applied to STORM simulations to test novel measurement techniques exploiting the new probe head, in preparation of analysis of data from MAST-Upgrade. The new probe head will be installed on both the midplane and divertor RP systems to provide a comprehensive analysis of the characteristics of turbulence both upstream and downstream. This analysis will provide deep insight into the level of turbulent transport present in the divertor both in absolute terms, and relative to the upstream SOL, radially across the SOL and PFR region.
The aim is to run the experiments using newly designed Langmuir probe in the second MAST-U campaign, late 2020

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509796/1 01/10/2016 30/09/2021
2370242 Studentship EP/N509796/1 30/09/2019 31/03/2023 William Fuller