Characteristics of monotonic sheaths near a wall with grazing magnetic incidence

Abstract

We consider a magnetised plasma in contact with an absorbing planar wall, where the angle $\alpha$ between the magnetic field and the wall is small, $\alpha \ll 1$ (in radians) and the system is symmetric tangential to the wall. The finite ratio $\gamma$ of the characteristic electron gyroradius $\rho_{\rm e}$ to the Debye length $\lambda_{\rm D}$, $\gamma = \rho_{\rm e} / \lambda_{\rm D}$, is retained via a grazing-incidence ($\alpha \ll 1$) gyrokinetic treatment [1,2]. Building on a previously developed iterative scheme [2,3] to solve for the steady-state electrostatic potential in the quasineutral magnetic presheath of width $\sim \rho_{\rm S}$, we developed a scheme that simultaneously solves for both the presheath and the non-neutral Debye sheath of width $\sim \lambda_{\rm D}$ in the limit $\lambda_{\rm D} / \rho_{\rm S} \rightarrow 0$. The code, called GYRAZE, thus provides the energy-angle distribution of ions at the wall and the velocity distributions of electrons reflected by the wall for different values of wall potential. A monotonic electrostatic potential profile, assumed in this work, can only exist for magnetic field angles larger than a critical value [3]. While the critical angle is shown here to significantly increase with $\gamma$, it is still typically smaller than the magnetic field angle at divertor targets of a fusion device.