  Ultracold plasmas are a promising candidate for the creation of
strongly-coupled Coulomb systems. Unfortunately, the values of the coupling
parameter Gamma_e actually achieved after photoionization of the neutral atoms
remain relatively small because of the considerable intrinsic heating of the
electrons. A conceivable way to get around this obstacle might be to utilize a
spontaneous ionization of the ultracold Rydberg gas, where the initial kinetic
energies could be much less. However, the spontaneous avalanche ionization will
result in a very inhomogeneous distribution (clusterization) of the ions, which
can change the efficiency of the electron relaxation in the vicinity of such
clusters substantially. In the present work, this hypothesis is tested by an
extensive set of numerical simulations. As a result, it is found that despite a
less initial kinetic energy, the subsequent relaxation of the electron
velocities in the clusterized plasmas proceeds much more violently than in the
case of the statistically-uniform ionic distribution. The electron temperature,
firstly, experiences a sharp initial jump (presumably, caused by the
"virialization" of energies of the charged particles) and, secondly, exhibits a
gradual subsequent increase (presumably, associated with a multi-particle
recombination of the electrons at the ionic clusters). As a possible tool to
reduce the anomalous temperature increase, we considered also a two-step plasma
formation, involving the blockaded Rydberg states. This leads to a suppression
of the clusterization due to a quasi-regular distribution of ions. In such a
case, according to the numerical simulations, the subsequent evolution of the
electron temperature proceeds more gently, approximately with the same rate as
in the statistically-uniform ionic distribution.
