The current column contraction and ion charge increase induced by the current build-up in a pulsed vacuum discharge

It is shown that three current build-up regimes may be realized in a pulsed vacuum discharge, depending on the relationship between the pulse duration τ and the characteristic times of the plasma flow (τ_L) and the cathode spot spreading (τ_S). For a rapid build-up (τ<τ_L), the cathode plasma jet exhibits the neck formation with high values of the plasma temperature and density. At a slow current increase (τ_L<τ<τ_S), the current column is subject to a uniform magnetic contraction, while at a very slow current growth rate (τ>τ_S), the contraction becomes insignificant. The first two cases give rise to additional nonequilibrium ionization in the current column, which leads to the ion charge increasing with the current. The third case is characterized by a “frozen” ion composition corresponding to the plasma parameters in the near-cathode region.