<title>Tabletop x-ray laser pumped with subnanosecond and picosecond pulses</title>

Laser energy requirements for the creation of active plasma media with Ne- and Ni-like ions with a `long' subnanosecond prepulse (100 - 1000 ps) and a `short' picosecond main pulse (0.1 - 10 ps) have been determined for foil and low density targets (foams, gases, vapors, etc.). RADEX hydrodynamics and atomic kinetics calculations show that X-ray laser action in the 200 - 400 A can be obtained with 1 - 2 J in 100 - 1000 ps prepulse followed of an approximately 1 ps pulse of similar energy. Transient gains of approximately 100 cm<SUP>-1</SUP> level have been calculated. For the wavelengths 38 - 46 A (`red' edge of `water window' spectral band) a short pulse of only approximately 5 - 10 J is required.