ABSTRACT
We review two approaches to x-ray lasing using ultra-short pulse (USP) driving lasers in which ionization of the lasant is not via electron collisions. In one approach, the large electric field of the laser beam ionizes a column of plasma along the laser axis. Both linear polarization, with subsequent recombination lasing, and circular polarization, with collisional excitation lasing, can be used. For either type of polarization, ionization induced refraction can limit the lasing length. We review recent experiments and theoretical calculations on ionization induced refraction and on the electron distribution following field ionization. Lasing has been observed with both types of pumping with the collisional variation having the largest gain-length product and the recombination variation having the shortest wavelength. We discuss various ideas to improve the gain coefficient of field ionized x-ray lasers. Another two-step approach to xray lasing with USP lasers involves the x-ray photoionization of inner-shell electrons. The first step is to produce a pulse of incoherent x rays with a very rapid rise time by heating a high-Z material with an USP laser. In the second step, a low energy filter removes x rays that can only ionize outer-shell electrons and the remaining x rays primarily ionize inner-shell electrons. This approach has not been demonstrated at x-ray wavelengths but modeling shows that lasing at 45 Å is possible with existing USP lasers and that lasing down to 15 Å should be possible with planned USP laser drivers.
