Nanocluster explosions and quasimonoenergetic spectra by homogeneously distributed impurity ions

A plasma expansion into vacuum and the resultant ion acceleration are studied analytically and numerically. The expansion of an initially uniform spherical plasma (consisting of a nanocluster or microdroplet) with radius R_u0 and electron density n_u0 is driven by the explosion of hot electrons having an initial temperature T_e0. A self-similar solution describes the nonrelativistic expansion of a finite plasma mass with a full account of 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 formulas as a function of the normalized droplet radius, Λ_u= R_u0 λ_D, where λ_D = T_e0 4πn _u0e² is the Debye length. 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 attributed to the spherical geometry.