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

摘要

The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the angle alpha between the wall and the magnetic field B is oblique. Here, we consider the fusion-relevant case of a shallow-angle, alpha 1, electron-repelling sheath, with the electron density given by a Boltzmann distribution, valid for alpha/root tau + 0 root m(e)/m(i), where m(e) is the electron mass, m(i) is the ion mass, tau = 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 rho(s) = root m(i)(ZT(e) + T-i)/ZeB, where m(e) is the proton charge and B = vertical bar B vertical bar is the magnitude of the magnetic field. We study the dependence on tau 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 tau. The kinetic model is shown to be asymptotically equivalent to Chodura's fluid model at small ion temperature, tau 1, for vertical bar ln alpha vertical bar > 3 vertical bar ln tau vertical bar 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, tau 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 root alpha or 1/root tau, depending on which is largest.