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Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

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Purpose-To describe the frequencies of and risk factors for ocular complications and poor visual acuity at presentation in a cohort of patients with juvenile idiopathic arthritis (JIA)-associated uveitis.Design-Cross-sectional study.Setting Single-center, academic practice. Procedures Data on patients diagnosed with JIA-associated uveitis were entered retrospectively into a database and analyzed. Study populationOutcome measures Visual acuity of 20/50 or worse or 20/200 or worse, and presence of ocular complications (including cataract, posterior synechiae, band keratopathy, elevated intraocular pressure, hypotony, macular edema, and epiretinal membrane) at presentation.Results-At presentation, ocular complications were seen in 67% of eyes affected by JIAassociated uveitis. Presence of ≥ 1+ anterior chamber flare, a positive antinuclear antibody, and a shorter duration between the diagnosis of arthritis and uveitis were significantly associated with the presence of ocular complication. The frequencies of 20/50 or worse and of 20/200 or worse visual acuities at presentation in affected eyes were 36% and 24%, respectively. The presence of ≥ 1+ anterior chamber flare and a history of intraocular surgery prior to presentation were significantly associated with 20/50 or worse and 20/200 or worse vision. Presence of posterior synechiae also was associated with 20/200 or worse vision at presentation. The main causes of poor vision at presentation for affected eyes and better-seeing eyes were cataract, band keratopathy within the visual axis, and glaucoma.Conclusions-Ocular complications and poor vision at presentation were common in our patients with JIA-related uveitis.

The newly commissioned Orion laser system has been used to study dense plasmas created by a combination of short pulse laser heating and compression by laser driven shocks. Thus the plasma density was systematically varied between 1 and 10  g/cc by using aluminum samples buried in plastic foils or diamond sheets. The aluminum was heated to electron temperatures between 500 and 700 eV allowing the plasma conditions to be diagnosed by K-shell emission spectroscopy. The K-shell spectra show the effect of the ionization potential depression as a function of density. The data are compared to simulated spectra which account for the change in the ionization potential by the commonly used Stewart and Pyatt prescription and an alternative due to Ecker and Kröll suggested by recent x-ray free-electron laser experiments. The experimental data are in closer agreement with simulations using the model of Stewart and Pyatt.

We have demonstrated a 10 Hz Ni-like Mo x-ray laser operating at 18.9 nm with 150 mJ total pump energy by employing a novel pumping scheme. The grazing-incidence scheme is described, where a picosecond pulse is incident at a grazing angle to a Mo plasma column produced by a slab target irradiated by a 200 ps laser pulse. This scheme uses refraction of the short pulse at a predetermined electron density to increase absorption to pump a specific gain region. The higher coupling efficiency inherent to this scheme allows a reduction in the pump energy where 70 mJ long pulse energy and 80 mJ short pulse energy are sufficient to produce lasing at a 10 Hz repetition rate. Under these conditions and by optimizing the delay between the pulses, we achieve strong amplification and close to saturation for 4 mm long targets.

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