Ultra fast Langmuir probe measurements in MAST-U edge plasma (plasma strand project)

Supervisor/s – Dr Paul Bryant (University of Liverpool) & Dr Peter Ryan (CCFE)

Langmuir probes are commonly used to measure plasma parameters such as electron temperature, ion saturation current and floating potential in fusion and technological plasmas. Conventional Langmuir probe systems, however, are not well suited for plasma turbulence investigations due to either a spatial or temporal resolution limitation. Experiments have shown that the edge region in a fusion plasma is very turbulent. For example, fast transient filamentary structures transport plasma particles and heat radially outward across the open magnetic field lines (scrape-off-layer). The midplane reciprocator, fitted with several electrodes at the probe head (see image), is a key diagnostic for measuring filament properties in the scrape-off-layer.

A novel ultra fast Langmuir probe system, based on a field programmable gate array (FPGA), is currently being developed through a collaboration between UKAEA, University of Liverpool and MIT and is due to end in March 2023. This is expected to provide real-time outputs (~1 MHz temporal resolution) of all three plasma parameters using a single electrode. This proposed, primarily experimental, project will continue the development of this system and will use the diagnostic to investigate fast transport phenomena in the edge plasma region of the MAST-U spherical tokamak.

The ultra fast Langmuir probe system will be implemented on the midplane reciprocator to temporally resolve the plasma parameters of individual filaments and the broadband background turbulence. These detailed measurements will enable a better understanding of the role of turbulent processes for heat and particle transport across the scrape-off-layer. In turn several important phenomena depend on the heat and particle transport in the edge region. These include the power exhaust channel width in the scrape-off-layer; the level of interaction between the plasma and first wall; the formation of an edge transport barrier during the L- to H-mode transition; and the evolution of edge localised (ELMs) and coherent modes. In addition, turbulence in the divertor and its impact on detachment will be investigated by implementing the ultra fast Langmuir probe system on divertor target embedded electrodes and on the divertor chamber reciprocator. Real-time control of divertor conditions using the plasma parameter outputs in a feedback loop could be explored.

The proposed project provides opportunities to develop analytical and technical skills from a range of diagnostics and plasma sources, FPGA programming and practical electronics work. Experience of at least one programming language (e.g. Python, Matlab) and prior experience of some electronics would be an advantage. The project will develop technical writing skills through annual reports and publications, and presentation skills at national and international conferences. The student would be based at CCFE with regular visits to University of Liverpool for testing the ultra fast Langmuir probe system on a low temperature rf magnetised plasma source.

The project will mainly be based at the Culham Centre for Fusion Energy (CCFE) with visits to the University of Liverpool for week long stays during prototype testing. There will be opportunities to travel to national and international conferences and collaboration with international research institutions.

This project is offered by The University of Liverpool. For further information please contact: Dr Paul Bryant (P.M.Bryant@liverpool.ac.uk) or Dr Peter Ryan (peter.ryan@ukaea.uk).

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