PhD opportunities

Study of magnetic presheath stability to parallel drifts

Supervisor: Dr Robert Kingham   

Type: Computational & Theoretical

Funding: EPSRC DTA  or  Imperial President’s PhD Scholarship

The proposed project focusses on kinetic study of sheaths at the interface between plasma in the scrape-off layer (SOL) and wall structures, for oblique angle between the magnetic field and the surface.  The sheath is crucial as it sets the boundary condition for the plasma exhaust streaming to the diverter in tokamaks.  In order to manage the power flux to the diverter, modern tokamaks operate with the magnetic field impinging on the diverter at a very shallow angle, to spread the power over as large an area as possible and thereby mitigate material damage.  In this geometry, the sheath separates into three regions; the thin Debye sheath adjacent to the solid, followed the magnetic pre-sheath (thickness on the order ion Larmor radius) and then the collisional pre-sheath.

There are large ExB drifts in the magnetic presheath (MPS) due to the sheath E-field (normal to the wall, and strongly varying with distance) and the component of B-field parallel to the wall. These ExB flows will be sheared and potentially prone to Kelvin-Helmholtz-like instabilities. This has been observed in a limited number of collisionless, PIC simulations [Theilhaber and Birdsall (1989), which Parker et al (1992)], which were of limited applicability to divertor conditions in modern tokamaks and also poorly resolved. Yet such instabilities have the potential to change the sheath characteristics (e.g. transmitted particle and energy fluxes) and concentrate fluxes on small areas of the surface, which could be detrimental to the survivability of plasma facing components to exhaust in tokamaks. Understanding this instability, and how to mitigate it is therefore of importance.

The idea is to develop a kinetic code and use it to study sheared flows in the MPS for planar, electron-repelling sheaths, including warm ion effects (needed for Ti>Te found near the divertor) and ion collisions. The code will focus on solving the ion Vlasov-Fokker-Planck equation across the entire sheath, encompassing the collisional presheath (up to several collisional mfp thick), magnetic presheath and Debye sheath, together with adiabatic electrons.

This project is part of a collaboration with CCFE.  The PhD student would be primarily based at Imperial, with regular visits to CCFE. There will be a CCFE co-supervisor.

Background reading

P. C. Stangeby, "The Plasma Boundary of Magnetic Fusion Devices”, IoP Press (2000)

R. Chodura,  Phys. Fluids 25, 1628 (1982);   [ https://doi.org/10.1063/1.863955 ]

K. Theilhaber and C.K. Birdsall, Phys. Fluids B 1, 2244 (1989); [ https://doi.org/10.1063/1.859041 ]

A. Geraldini, F.I. Parra, F. Militello, Plasma Phys. Control. Fusion 60, 125002 (2018);  [ https://doi.org/10.1088/1361-6587/aae29f ]