Publications associated with Theoretical Astrophysics & Plasma Physics

Dependence on ion temperature of shallow-angle magnetic presheaths with adiabatic electrons

Journal of Plasma Physics Cambridge University Press 85 (2019) 795850601

A Geraldini, FI Parra, F Militello

The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the angle α between the wall and the magnetic field B is oblique. Here, we consider the fusion-relevant case of a shallow-angle, α 1, electron-repelling sheath, with the electron density given by a Boltzmann distribution, valid for α/√τ + 1 √me/mi, where me is the electron mass, mi is the ion mass, τ = Ti/ZTe,Te is the electron temperature, Ti 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 ρs = √mi(ZTe + Ti)/ZeB, where e is the proton charge and B = |B| is the magnitude of the magnetic field. We study the dependence on τ 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 |ln α| > 3|ln τ | 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 Ti is measured to be a few times larger than Te 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.

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