  Current-free double layers of the type reported in plasmas in the presence of
an expanding magnetic field [C. Charles and R. W. Boswell, Appl. Phys. Lett.
82, 1356 (2003)] are modeled theoretically and with particle-in-cell/Monte
Carlo simulations. Emphasis is placed on determining what mechanisms affect the
electron velocity distribution function (EVDF) and how the EVDF influences the
double layer. A theoretical model is developed based on depletion of electrons
in certain velocity intervals due to wall losses and repletion of these
intervals due to ionization and elastic electron scattering. This model is used
to predict the range of neutral pressures over which a double layer can form
and the electrostatic potential drop of the double layer. These predictions are
shown to compare well with simulation results.
