Population inversion in the recombination of optically-ionized plasmas

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 is examined. 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 ~10-μm diameter filaments imbedded in cold background plasma. In the relativistic case, self-focusing of the ionizing laser radiation may lead to a 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≈1/8 Zm_e c²) ions. The implications of such filamentation for the scaling of the present type of recombination laser to short wavelengths are discussed