The role of plasma-molecule reactions in influencing detachment (plasma strand project)

Supervisor: Mark Bowden (University of Liverpool)

Tokamak exhaust, the flow of energy and particles from the core plasma to the walls, is one of the central challenges of the ITER device and any future fusion reactor. High-energy species from the core plasma flow onto wall surfaces in the divertor, the region in which the wall tiles are specially designed to handle the high heat load. One aspect of the solution to handling the high-energy plasma flow is to operate the tokamak in a discharge mode called detachment [2]. Detachment is known to be accompanied by low-temperature, low-density plasma conditions at the divertor tiles. The plasma in contact with the tile surfaces will be characterised by processes that include the flow of atoms and ions onto the surface, the subsequent surface-moderated reactions that lead to recycling of those species into atoms and molecules that leave the surface and fuel the divertor plasma, the flow of energy onto the surfaces and its subsequent re-radiation, the sputtering of surface materials, and the generation of hybrid molecules composed of different hydrogen isotopes and possibly even surface species. The surface itself is likely to be micro-structured, with the effect of the surface structure on plasma properties yet to be clarified. All of these features are recognised as being relevant to divertor operation, but even with the diagnostic access to the divertor region on machines such as ASDEX-Upgrade and MAST-U, there is only limited information available by which their relative importance can be judged.

This project will focus on the role of plasma-molecule interactions in the MAST-U divertor, with the aim of determining their influence on detachment.  The question about which process is resulting in momentum loss during detachment is heavily debated and experimentally unknown. Simulations seem to give conflicting results. This primarily experimental project will focus on measurements of atomic and molecular emission, using simulation codes to help interpret measurements, and relate the results to detachment. Several different aspects of detachment physics could be investigated.

Elastic collisions between the plasma and the molecules would result in momentum loss. These interactions also result in a kinetic energy conversion to the molecules, heating up the molecules, exciting the molecules and thus providing Fulcher band emission. The kinetic energy conversion to the molecules may influence detachment strongly. Quantifying this and comparing this to any radiative losses would be helpful for understanding the role plasma-molecule interactions play during detachment.

Interpretation of atomic and molecular emission will be done using a combination of plasma simulations (SOLPS, SD1D) and collisional-radiative modelling based on the work of IPP Garching [3,4]. In addition, the project will interact with current plasma-surface modelling being carried out within the CDT.

  1. K Nordlund et al, 2014, “Multiscale modelling of plasma–wall interactions in fusion reactor conditions”, J. Phys. D: Appl. Phys., 47, 224018 (21pp)
  2. Krasheninnikov, S.I. et al, 2016, “Divertor plasma detachment”, Physics of Plasmas, vol. 23, no. 5.
  3. Wünderlich, S. Dietrich and U. Fantz, Application of a collisional radiative model to atomic hydrogen for diagnostic purposes, J. Quant. Spectrosc. Radiat. Transfer 110 (2009)
  4. yacora.de

The project will be based at Culham with regular interactions with the diagnostic group at Liverpool.

The project will provide the opportunity for a student to develop skills in experimental spectroscopy. While this is a purely experimental project, the student will require understanding of background theory and modelling required to interpret experimental observations.

This project is offered by University of Liverpool. For further information please contact Dr Mark Bowden (mark.bowden@liverpool.ac.uk)