Dependence on ion temperature of shallowangle magnetic presheaths with adiabatic electrons
Abstract
The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the angle between the wall and the magnetic field B is oblique. Here, we consider the fusionrelevant case of a shallowangle, α ≪ 1, electronrepelling sheath, with the electron density given by a Boltzmann distribution, valid for α/√(τ+1) ≫ √(m_{e}/m_{i}), where m_{e} is the electron mass, m_{i} is the ion mass, τ = T_{i}/ZT_{e}, T_{e} is the electron temperature, T_{i} is the ion temperature and Z is the ionic charge state. The thickness of the magnetic presheath is of the order of a few ion sound Larmor radii ρ_{s} = √{m_{i}(ZT_{e} + T_{i})}/Z_{e}B, where e is the proton charge and B = B is the magnitude of the magnetic field. We study the dependence on of the electrostatic potential and ion distribution function in the magnetic presheath by using a set of prescribed ion distribution functions at the magnetic presheath entrance, parameterized by τ. The kinetic model is shown to be asymptotically equivalent to Chodura's fluid model at small ion temperature, τ ≪ 1, for lnα > 3lnτ ≫ 1. In this limit, despite the fact that fluid equations give a reasonable approximation to the potential, ion gyroorbits acquire a spatial extent that occupies a large portion of the magnetic presheath. At large ion temperature, τ ≫ 1, relevant because T_{i} is measured to be a few times larger than T_{e} near divertor targets of fusion devices, ions reach the Debye sheath entrance (and subsequently the wall) at a shallow angle whose size is given by √(α) or 1/√(τ), depending on which is largest.
 Publication:

Journal of Plasma Physics
 Pub Date:
 December 2019
 DOI:
 10.1017/S0022377819000771
 arXiv:
 arXiv:1907.09421
 Bibcode:
 2019JPlPh..85f7901G
 Keywords:

 fusion plasma;
 plasma sheaths;
 Physics  Plasma Physics
 EPrint:
 44 pages, 8 figures