Study of magnetic presheath stability to parallel drifts

The proposed project focuses on the kinetic study of sheaths at the interface between plasma in the scrape-off layer (SOL) andwall structures, for oblique angle between the magnetic field and the surface. 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 2D spectral (x, ky), hybrid code, with fluid electrons and a continuum, kinetic ion representation to efficiently 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 VFP equation across the entire sheath, encompassing the collisional presheath (up to several mfp thick), magnetic presheath and Debye sheath, together with adiabatic electrons (i.e. Boltzmann relation between ne and electric potential omm). Collisions - Will be simplified as appropriate for a study focussed on the sheath, but superior to the BGK approximation usually employed. Ion-ion Coulomb & charge-exchange ion-neutral collisions will be included. Neutrals will be a prescribed, passive species for ions to collide against. A spherical-harmonic expansion in velocity space, as used for electrons in SOL-KiT, will be used.

This code will be used to perform a fundamental physics study of the MPS stability to sheared flows. It will describe a periodic system (in 'y', i.e., along the wall) efficiently, owing its spectral nature. With few Fourier modes, the linear growth rate and thresholds will be investigated. The background on which the instability grows will be self-consistently kinetically obtained. The effect of collisions on the thresholds in different SOL regimes (sheath-, conduction-limited and detached) can be qualitatively explored. With an expanded range of Fourier modes, instability saturation can be tackled.