  The collisionless expansion of spherical plasmas composed of cold ions and
hot electrons is analyzed using a novel kinetic model, with special emphasis on
the influence of the electron dynamics. Simple, general laws are found,
relating the relevant expansion features to the initial conditions of the
plasma, determined from a single dimensionless parameter. A transition is
identified in the behavior of the ion energy spectrum, which is monotonic only
for high electron temperatures, otherwise exhibiting a local peak far from the
cutoff energy.
