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Capillary plasmas with large length-to-diameter ratios (1ld > 100) are proposed as amplification media for soft-x- ray and XUV radiation by direct discharge excitation. The capillary geometry provides a small volume and an adequate resistance for ohmic heating. Heat conduction to the capillary walls provides rapid cooling of the plasma during the decay of the excitation pulse, resulting in a large recombination rate and a population inversion. A time- dependent collisional-radiative model of the capillary plasma predicts gains of the order of 5 cm- 1 in the 18.2-nm line of C VI. Recently soft-x-ray lasers have been demonstrated by using dense plasmas created by high-energy lasers as the gain medium.1' 2 In these successful experiments, electron-ion recombination played an important role in achieving the population inversion.', 3 Other schemes suitable for short-wavelength amplification have been demonstrated at wavelengths near 100 nm.4 -6 All of them use a laser as the pumping source. In this Letter we discuss the possibility of creating soft-x-ray lasers in recombining capillary plasmas cre- ated by a fast discharge. The capillary geometry that we propose allows the creation of plasmas with the characteristics required to explore the amplification of soft x rays and XUV radiation by using a discharge. The capillary advantages relate to the requirements for the generation of a dense highly ionized plasma as well as its subsequent rapid cooling. Both conditions are necessary to achieve the large recombination rates associated with the gains required in the soft-x-ray and XUV regions of the spectrum, where high-reflec- tivity resonant cavities are not yet available. In addi- tion, the capillary limits the plasma radius, helping to avoid self-absorption of the lower-level transition. In this scheme a dense and nearly totally ionized plasma with a large length-to-diameter ratio (ld > 100) is created from a fast discharge by ionizing mate- rial ablated from the capillary walls and from gas that might be introduced into the capillary. Subsequent rapid plasma cooling by heat conduction to the capil- lary walls, enthalphy loss by particle diffusion, and radiation results in the recombination of highly ion- ized species into ions of lower charge, creating popula- tion inversions. Several types of discharge have been used in the excitation of relatively low-density plasmas (Ne < 1015 cm-3) to obtain visible and infrared recombination laser radiation. They include axial and transverse

Proposal for soft-x-ray and XUV lasers in capillary discharges