Dust transport and mobilisation in Mast-U (plasma strand)

Supervisor: Dr Paul Bryant (University of Liverpool)

Dust can have a significant impact on reactor performance and efficiency [1].  Energy can be radiated from the superheated dust cooling the core as well as contamination from dust evaporation and melting. Plasma ignition due to dust contamination can be problematic with reactor down-time to remove toxic radioactive dust. These issues are likely to be exacerbated in the next generation of fusion reactors such as ITER and DEMO due to the longer pulse times and greater power generation. Solid particles can enter the plasma from the walls or divertor regions due to the high energy fluxes during transient events such as ELMs (Edge Localised Modes). Their material composition resembles the tokamak walls and their shape ranges from spheres to flakes with sizes from nm to mms. They can also be mobilised from within wall tile gaps and surfaces and migrate around the reactor [2]. Once in the plasma the dust acquires charge and transport is determined by ion and neutral drag forces as well as electric and magnetic fields (e.g. E x B drifts).  An improved understanding of the transport, mobilisation and fate of dust is therefore of particular importance. Central to predictive dust transport codes such as DTOKs (Dust in TOKamaks) [3] are the ion and neutral drag forces and dust heating models. Accurate and reliable models are key to successfully predicting dust transport and fate in tokamaks.

The aim of this project is to experimentally study dust transport and mobilisation to: i) verify the physical models used in DTOKs in a magnetised plasma; ii) study the mobilisation and transport of dust in MAST-U with different divertor (the tokamak’s exhaust) configurations. Experiments verifying existing ion drag models and studying mobilisation from surfaces will be performed at Liverpool on the magnetised RF plasma experiment. Advanced plasma diagnostics such as RFEA, Langmuir and emissive probes, imaging emission spectroscopy and fast visible cameras will be used. To study dust mobilisation and transport in MAST-U a recently developed modified DSF (Divertor Science Facility) probe head will be used to inject dust with a separate head for MAST-U and ITER relevant slots (representing typical tile gaps). Stereoscopic imaging, using fast visible and infra-red cameras and a machine learning dust tracking algorithm [4], along with other diagnostics (such as Langmuir probes) will be used to reconstruct the dust trajectory. Plasma background simulations along with DTOKs predictions will then be used to verify existing models and study dust transport from the reconstructed trajectories.

This project will provide opportunities to develop expertise in electrical, optical (emission spectroscopy and tomographaphy) and laser based diagnostics. There will be opportunities for diagnostic development and hardware design. In addition, the student will gain skills in simulations, particle tracking algorithms, data interpretation and analysis. The project will develop technical writing skills through annual reports and publications, and presentation skills at national and international conferences. This project may be compatible with part time working.

[1] Roth J, et al, J. Nucl. Materials, 390, 1 – 9 (2009).

[2] S. Ratynskaia, L. Vignitchouk, P. Tolias,  et al, Nucl. Fusion 53 (2013) 123002.

[3] Simons L, Cowley C, Fuller P et al, Plasma Phys. Control. Fusion, 63, 045002 (2021).

[4] C. Cowley, et al, Physical Review E, 102(4), Article number 043311 (2020).

The project will be primarily based at the University of Liverpool with stays, a few weeks at a time, at Culham during experimental campaigns. There will be opportunities to conduct experimental campaigns on MAGNUM-PSI (DIFFER Institute, Netherlands) as well as collaborations with internationally renowned dusty plasma groups in Imperial College, Germany and USA as well as other Tokamaks.

This project is offered by University of Liverpool. For further information please contact Dr Paul Bryant (P.M.Bryant@liverpool.ac.uk).

This project may be compatible with part time study, please contact the project supervisors if you are interested in exploring this.

Image above: Simulation of dust trajectories (DTOKs) in the plasma beam of MAGNUM-PSI for increasing magnetic field and two dust sizes.

Image below: SEM image of Tungsten (9 mm diameter) dust particles used in MAGNUM-PSI.