Free expansion of a low-density laser plasma

Abstract

Expansion of a laser plasma, formed by multistage selective ionisation of metal vapours in the atomic vapour laser isotope separation (AVLIS) method, is considered. The main feature of such a plasma is that the mean free path of particles in the plasma is much greater and the Debye radius is much smaller than the transverse dimensions of the ionisation region. A numerical solution of equations of two-fluid hydrodynamics is used to show that, when expansion becomes steady, the average-mass velocity is equal to about half the most probable velocity of the particles which have the temperature of electrons and the mass of ions. Plasma expansion becomes steady in a time approximately 1.5 times longer than the time of flight across the initial transverse size of the ionisation region at the steady expansion velocity. It is therefore reasonable to expect efficient ion extraction by free expansion in laser isotope separation by the AVLIS method.