Bohm criterion failure in nonstationary plasmas

The Bohm criterion, generally used as a boundary condition at the plasma-sheath edge, is not compatible with the time evolution of the voltage fall across the collisionless cathode sheath in a pulsed hollow-cathode discharge. Similarly this criterion is not valid for the interpretation of probe measurements of the plasma in this discharge. The discrepancy disappears only when the discharge reaches its steady state. The duration of this invalidity is much longer than the time needed to fill the device with plasma and than the time needed for the formation of the plasma sheath. The sheath evolution in nonstationary plasmas is described quantitatively by assuming at the sheath edge a temporary growth of the ion speed from an initial low value up to the ion sound velocity, i.e., up to the Bohm criterion requirement.