Frozen state of ionisation in a cathodic plasma jet of a vacuum arc

The regions and conditions in which the Saha equation can be applied to the cathodic plasma of a vacuum arc are examined. Considering the relaxation times of ionisation and recombination, it is found that the plasma is in a frozen state of ionisation due to the rapid expansion for r>or approximately=(10-100) mu m, where r is the distance from the cathode spot. As a consequence the electron density and all ion densities should be proportional to r -2 . The parameters of the frozen state accessible to experimental investigations (r>100 mu m) result from the parameters of the strongly non-ideal plasma of the cathode spot. This opens a new approach to investigate strongly non-ideal plasmas by comparatively simple methods.