Instability of the boundary layer between a streaming plasma and a vacuum magnetic field

Analysis is carried out for the long-wavelength stability of the boundary layer formed when a plasma or neutralized ion beam is incident upon, and reflected by, an applied magnetic field. This equilibrium is similar to the Ferraro–Rosenbluth sheath which forms the basis for many models in magnetic fusion and magnetospheric physics. For wavelengths much greater than the sheath thickness ah, the equilibrium is unstable with growth rate (in the regime kah≪m/M ) proportional to (kg)1/2≡(kahΩiΩe) 1/2. The nonlinear evolution of the instability describes plasma propagation across the magnetic field. This instability is similar to the classical flute instability except that polarization along the boundary is due to electron, and not ion, motion.