Particle-in-cell modeling of magnetized argon plasma flow through small mechanical apertures

Motivated by observations of supersonic argon-ion flow generated by linear helicon-heated plasma devices, a three-dimensional particle-in-cell (PIC) code is used to study whether stationary electrostatic layers form near mechanical apertures intersecting the flow of magnetized plasma. By self-consistently evaluating the temporal evolution of the plasma in the vicinity of the aperture, the PIC simulations characterize the roles of the imposed aperture and applied magnetic field on ion acceleration. The PIC model includes ionization of a background neutral-argon population by thermal and superthermal electrons, the latter found upstream of the aperture. Near the aperture, a transition from a collisional to a collisionless regime occurs. Perturbations of density and potential, with millimeter wavelengths and consistent with ion acoustic waves, propagate axially. An ion acceleration region of length ∼200 λD,e -300 λD,e forms at the location of the aperture and is found to be an electrostatic double layer, with axially separated regions of net positive and negative charge. Reducing the aperture diameter or increasing its length increases the double layer strength.