Sheath and presheath in a collisionless plasma containing negative ions with an open magnetic field

The effects of an axisymmetric magnetic mirror field with diverging field lines on the plasma presheath and sheath are numerically investigated by solving the plasma–sheath equation for a collisionless plasma in the presence of negative ions. The positive and negative ions with a finite temperature are generated throughout the volume where the sources vary according to the ionization rate. Electrons are assumed to obey the Boltzmann relation. The sheath formation and Debye shielding, which depends considerably on the magnetic strength profile, decrease remarkably on applying an expanding magnetic field. The dependence of the particle densities and the electrostatic potential on the magnetic field profiles are explicitly investigated in this study. Negative ions with sufficient energy penetrate into the sheath following the field lines and eventually reflect back at a point where the mirror force and the electrostatic force balance each other. (author)