  Recent experiments have demonstrated the spontaneous evolution of a gas of
ultracold Rydberg atoms into an expanding ultracold plasma, as well as the
reverse process of plasma recombination into highly excited atomic states.
Treating the evolution of the plasma on the basis of kinetic equations, while
ionization/excitation and recombination are incorporated using rate equations,
we have investigated theoretically the Rydberg-to-plasma transition. Including
the influence of spatial correlations on the plasma dynamics in an approximate
way we find that ionic correlations change the results only quantitatively but
not qualitatively.
