Presheath in fully ionized collisional plasma in a magnetic field

The quasineutral presheath layer at the boundary of fully ionized, collisional, and magnetized plasma with an ambipolar flow to an adjacent absorbing wall was analyzed using a two fluid magneto-hydrodynamic model. The plasma is magnetized by a uniform magnetic field B, imposed parallel to the wall. The analysis did not assume that the dependence of the particle density on the electric potential in the presheath is according to the Boltzmann equilibrium, and the dependence of the mean collision time r on the varying plasma density within the presheath was not neglected. Based on the model equations, algebraic expressions were derived for the dependence of the plasma density, electron and ion velocities, and the electrostatic potential on the position within the presheath. The solutions of the model equations depended on two parameters: Hall parameter (β), and the ratio (γ), where γ = ZT _e(ZT _e + T _i), and T _e, T _i and Z are the electron and ion temperatures and ionicity, respectively. The characteristic scale of the presheath extension is several times r _i/β, where r _i is the ion radius at the ion sound velocity. The electric potential could have a non monotonic distribution in the presheath. The ions are accelerated to the Bohm velocity (sound velocity) in the presheath mainly near the presheath-sheath boundary, in a layer of thickness ∼ r _i/β. The electric field accelerates the ions in the whole presheath if their velocity in the wall direction exceeds their thermal velocity.