Boundary Transparency of a Discharge in the Knudsen Regime

An integral characteristic of the balance, namely, the boundary transparency defined as the ratio of a flux of neutrals via the side surface of the discharge to their flux from the cathode, is treated in application to vacuum discharges with gas-active cathodes. This quantity is determined with due regard for the "burn-out" and angular distribution of the flux on cathode in the Knudsen regime of the motion of neutrals. Singularities of the behavior of concentration and hydrodynamic flow velocity under these conditions are revealed.