Pressure gradients and flow patterns in the direct current positive column

A model is presented for the axial neutral particle pressure gradients and flow patterns in the positive column of a monatomic dc gas discharge. Taking into account both the charge imbalance volume force and charged-particle momentum divergence force components, an analytical expression is derived for the driving volume force acting on the neutral particles. This expression is used to numerically solve for the pressure gradients and flow patterns for both closed and bypass discharge tube configurations. For discharges with sufficiently small current densities (e.g., a 0.2 mA, 1.5 mm radius, 5 Torr He discharge), this model yields a reversal in the pressure gradient from that predicted by previous studies where the charge imbalance volume force had been neglected. For bypass tube configurations, this pressure reversal can result in an on-axis cathode directed neutral flow, as opposed to the previously predicted on-axis anode directed flow.