  The expansion of non-ideal copper plasma into vacuum is analyzed for the
conditions typical to explosive electron emission in vacuum arcs. The
gas-dynamic model solves the Euler equations with an equation of state (EoS)
for weakly non-ideal plasma taking into account the ionization energy
correction due to electron-ion Coulomb coupling. We have obtained that the EoS
has insignificant influence on the plasma expansion, when the plasma properties
and composition change drastically. Based on our simulation results, the
validity of the 'frozen' state theory often used in the vacuum arcs plasma
diagnostics is questionable.
