POPULATION-INVERSION IN THE RECOMBINATION OF OPTICALLY-IONIZED PLASMAS

We examine the possibility of using a high-intensity optical field in conjunction with a gas target to produce a highly-ionized plasma filament suitable for recombination XUV lasers in both transient and quasi-steady-state regimes. A distinction is made between low Z ions which can be stripped to the desired ionization state at nonrelativistic intensities and higher Z ions which require relativistic intensities to produce the desired ionization. In the nonrelativistic case (E(i) < 500 eV), it is shown that electron thermal conduction is extremely effective in cooling approximately-10-mu-m diameter filaments imbedded in cold background plasma at densities required for quasi-steady-state gain on 4-3 and 3-2 transitions. These filaments can be cooled to the required temperatures with very little hydrodynamic motion and with little sensitivity to the initial temperature and typically reach small-signal gain much in excess of that usually predicted in expansion-cooled systems. In the relativistic case, self-focusing of the ionizing laser radiation may lead to very small diameter, electron-cavitated filaments which will undergo a space-charge-driven expansion (Coulomb explosion) on the time scale of an ion plasma period, resulting in the emission of extremely high currents of moderate energy (E-almost-equal-to-1/8 Zm(e)c2) ions. We discuss the implications of such filamentation for the scaling of the present type of recombination laser to short wavelengths.