We report the first demonstration of a high average power tabletop soft x-ray laser. An average laser output power of ഠ1 mW ͑.2 3 10 14 photons͞s͒ was generated at 46.9 nm in Ne-like Ar using a very compact tabletop discharge. The spatially coherent average power emitted by this 26.5 eV laser is comparable to that generated at this photon energy in a similar bandwidth ͑Dl͞l 10 24 ͒ by a thirdgeneration synchrotron beam line. Lasing was obtained at a repetition rate of 7 Hz with an average output energy of 135 mJ͞pulse by exciting a plasma column in a ceramic capillary with a fast current pulse. This very compact high-repetition-rate laser source makes intense short-wavelength coherent radiation accessible to a wide variety of new applications. [S0031-9007(98)08022-3]

Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-pass amplif ication in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain -length product of this plasma column allows for soft-x-ray amplif ication in a highly saturated regime, resulting in efficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of ഠ1 3 10 23 photons͑͞s mm 2 mrad 2 0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sources to date. 

We report the observation of discharge-pumped extreme ultraviolet lasing in collisionally excited ions of a material ablated from a solid target. Excitation of sulfur plasmas by a capillary discharge resulted in amplification of the Jϭ0 -1 line of Ne-like sulfur at 60.8 nm, with a gain coefficient of 0.45 cm Ϫ1 and a gain-length product of 7.5. Overheating of the electron temperature and transient population effects are computed to make a significant contribution to the measured gain. ͓S1050-2947͑97͒01702-2͔PACS number͑s͒: 42.55.VcSince their first realization in 1984 ͓1͔, soft x-ray and extreme ultraviolet lasers based on the collisional excitation of Ne-like ions have been pumped by laser drivers. Laser action in this isoelectronic series has been achieved in elements from a wide range of atomic numbers, from silver ͓2͔ down to silicon ͓3͔. Alternatively, a recent experiment demonstrated large amplification at 46.9 nm in a dischargepumped argon plasma ͓4͔, opening a new route to the development of simpler and more compact ultrashort wavelength lasers based on excitation by capillary discharges. That argon capillary discharge experiment also constituted the first observation of soft x-ray amplification on the Ne-like sequence utilizing a gaseous target. However, only a few elements are available in gaseous form under normal laboratory conditions. Therefore, the observation of lasing in materials produced from solid targets is important to extend the spectral range of discharge-pumped soft-x-ray lasers. In this paper we report the demonstration of lasing action in Ne-like sulfur ions by capillary discharge excitation of sulfur vapor produced by discharge ablation of a solid target.

We have performed systematic measurements and numerical modeling of the divergence and exit beam size of a 46.9 nm Ne-like Ar capillary discharge pumped soft-x-ray amplifier. Two-dimensional near-field and far-field measurements were obtained over a wide range of discharge parameters. The spot size and divergence of the laser beam were observed to increase significantly with decreasing discharge pressure. Simultaneously, the beam intensity distribution changed from a single peak pattern to an annular profile. These effects are shown to be the result of increased refraction of the soft-x-ray beam, caused by larger density gradients in the plasma columns of the lower pressure discharges. The spatial images are nearly cylindrically symmetric, and have less structure than those reported in the literature for laser-driven soft-x-ray lasers. Beam profiles synthesized by model calculations are found to be in good agreement with the recorded images. The obtained images in combination with the model computations clarify the origin of the different beam patterns that are observed in capillary discharge soft-x-ray laser experiments. ͓S1050-2947͑98͒09208-7͔

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