[en] The problem of the acceleration of impurity aluminum ions in a hydrogen plasma which is expanding into vacuum is solved by numerical simulation. Particular emphasis is placed on the role played by Coloumb collisions during the ion acceleration. The motion of the major plasma species is assumed to be collisionless, obeying the Vlasov kinetic equation. For the impurity component, the self-consistent-field effects and collisions with the ions of the major species and the electrons are taken into account. The conditions under which collisions play a governing role in the acceleration of the impurity ions are determined. Under these conditions, a steady-state energy spectrum is formed for the impurity ions comparatively rapidly. This spectrum then changes slowly as a result of ion acceleration by the self-consistent field. It is also shown that the dependence of the average energy of the various components on the charge weakens with increasing charge and with increasing plasma density