Nanocluster explosions and generation of quasimonoenergetic ions

Expansion of an initially uniform spherical plasma is studied analytically and numerically. A self-similar solution describes the nonrelativistic expansion of a finite plasma mass including charge separation effects. Such key features as the energy spectrum, maximum ion energy, and energy transfer efficiency from the electrons to the ions are given by simple analytic formulae. The solution predicts that impurity ions doped homogeneously in a droplet plasma are accelerated quasimonoenergetically by the electrostatic field generated by the charge separation. The prediction is confirmed by N-body particle simulations. The origin of the monoenergetic spectrum is therefore attributed to the spherical geometry.