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We report the first demonstration to our knowledge of multipass amplification of soft x rays. A gain medium of neonlike selenium ions was placed within a resonant cavity composed of a multilayer mirror and a beam splitter designed for normal-incidence use at the 20.63- and 20.96-nm laser lines of the neonlike selenium. The laser-cavity output was time resolved and exhibited three distinct temporal components identifiable as the single-, double-, and triple-pass amplified emission. In these experiments, multipass amplification was limited by the finite duration of the gain medium.

We report observations of amplified spontaneous emission at soft x-ray wavelengths. An optical laser ionized thin foils of selenium to produce a population inversion of the $2{p}^{5}3p$ and $2{p}^{5}3s$ levels of the neonlike ion. Using three time-resolved, spectroscopic measurements we demonstrated gain-length products up to 6.5 and gain coefficients of 5.5\ifmmode\pm\else\textpm\fi{}1.0 ${\mathrm{cm}}^{\ensuremath{-}1}$ for the $J=2 \mathrm{to} 1$ lines at 206.3 and 209.6 \AA{}. We also observed considerable amplification for the same transitions in yttrium at 155.0 and 157.1 \AA{}.

Recent progress in experimental laboratory soft X-ray laser research at Lawrence Livermore National Laboratory (LLNL) is reviewed. Research at LLNL in this area has concentrated on further characterising and understanding neon-like X-ray laser plasmas, investigating soft X-ray amplification at shorter wavelengths, and demonstrating examples of X-ray laser applications. For the standard 200 AA neon-like selenium collisional excitation laser, the output source size as well as the beam time history, divergence, energy and spatial profile have been measured. Gain has been demonstrated at wavelengths as short as 50.3 AA in nickel-like ytterbium. Several recombination X-ray laser schemes have also been investigated. X-ray laser holography, cavity operation of an X-ray laser, and the capability to point and focus the output laser beam have been demonstrated.

We report an explicit demonstration of double pass amplification of soft X-ray laser radiation at 131 A, the shortest wavelength demonstration to date. Using efficient ( R =60%), narrowband ( λ Δλ =13 ), normal incidence, multilayer mirrors, double pass amplification was achieved on the 131 A and 132.7 A laser lines of neon-like molybdenum. The laser output was time resolved, exhibiting two distinct temporal components clearly identifiable as the single pass and double pass amplified emission.

We report the absolute conversion efficiency ξx from the incident laser light energy to x‐ray photons for laser‐produced plasmas. Potential x‐ray backlighting (radiography) line sources having photon energies from 1.4 to 8.6 keV are studied as a function of laser wavelength, pulsewidth, and intensity. The laser intensity and pulsewidth range from 1014 to 1016 W/cm2, 100 ps to 2 ns and include incident wavelengths of 1.06, 0.53, and 0.35 μm. We found that K‐shell x‐ray line emission ξx : (1) decreases with increasing x‐ray energy, (2) decreases with increasing laser intensity, (3) decreases rapidly with pulselength, and (4) moderately increases with decreasing laser wavelength. On the contrary, for Au M band emission, at a fixed laser intensity and pulsewidth, ξx significantly increases (∼25×) upon decreasing the laser wavelength from 1.06 to 0.35 μm.

We report progress in the development of multilayer components for use in multiple pass soft x-ray laser cavities operating in the 100A to 300A spectral range. Our work includes fabrication and characterization of multilayer components; simple resonant cavity design; damage threshold assessment for multilayers in the x-ray laser environment; and multipass cavity experiments for efficiency enhancement and transverse mode selection. 14 refs., 9 figs.

We have measured the spatial extent of heliumlike and hydrogenlike argon x‐ray lines that were emitted from the fuel region of an imploded laser fusion target directly illuminated by the Argus laser. The diameter of the fuel region measured in this way agrees with that obtained from images of the thermonuclear burn region recorded simultaneously by an alpha‐particle zone plate camera. Time‐averaged electron temperatures in the target core were determined from argon line intensity ratios and compared to ion temperatures determined from measurements of neutron yield.

X -ray laser cavity experiments*N. M. Ceglio, D. P. Gaines, J. Trebes,A. M. Hawryluk, D. G. Stearns, G. L. HoweLawrence Livermore National Laboratory, University of CaliforniaP. O. Box 5508, L -473, Livermore, California 94550AbstractWe report progress toward the development of multipass, soft x -ray laser cavitiesoperating in a spectral range around 200 A. Experimental results on the characteri-zation of normal incidence multilayer mirrors, the survival of multilayer mirrors in thehostile x -ray laser environment, and the performance of double pass cavities at 206 A to209 A are presented.IntroductionThere are a number of strong motivations for x -ray laser cavity development: enhance-ment of laser output efficiency, correction of refractive effects associated with inhomo-geneities in the gain medium, and improvement of the spatial coherence of the output

Significant progress toward the goal of producing high Power, high coherence x-ray lasers has been made. Lasing at wavelengths as low as 66 A has been achieved in a nickel like laser scheme which is scalable to sub-44 A wavelengths. In addition, x-ray laser cavities, x-ray holography, and an applications beamline have been demonstrated.

Recent demonstrations of the generation of amplified spontaneous emission (ASE) at soft x-ray wavelengths have spurred interest in normal incidence multilayer structures for use as x-ray laser (XRL) cavity mirrors. Calculations indicate that cavity mirrors can provide significant enhancement of XRL output, and drive the oscillations to the saturation limit. Novel ideas for cavity output coupling may expand the versatility of XRL devices, while encouraging the marriage of lithographic and multilayer technologies to the general benefit of x-ray optics. Preliminary tests of a double pass cavity have already been conducted. These tests point out the importance of detailed cavity design issues, such as precision mirror alignment, mirror damage dynamics, and detailed mirror calibration to the future success of this field.

Recent advances in the development of lasers at soft x-ray wavelengths has spurred increasing interest in the production of cavity components using multilayer technology. We have established a comprehensive capability to design, fabricate, and characterize multilayer x-ray optics directed towards the goal of building the first x-ray laser cavity. High quality multilayer structures have been fabricated using magnetron sputtering. In addition, we have applied microfabrication technology to create freestanding beamsplitters and three-dimensional diffracting structures, as is discussed in another paper at this conference. The x-ray reflectivity and transmission of the multilayer components have been measured using synchrotron radiation. We have also characterized the microstructure of these devices using high-resolution transmission electron microscopy (TEM). This information provides structural parameters that are incorporated into computer codes to calculate the theoretical performance of the multilayer components. Comparison of the calculated reflectivity and transmission with the measured performance of the multilayer optics provides insight into the physics of these devices. In addition, a successful modeling capability allows us to iterate the fabrication cycle, modifying the design of the multilayer components to optimize their performance.

Data are presented from three different x-ray laser inversion schemes tested on the Novette laser facility at LLNL: collisional excitation of neon-like selenium (λ = 206, 209, 220A); resonant photoexcitation of hydrogen-like fluorine (λ = 81A); and recombination of hydrogen-like magnesium (λ = 130A). The data illustrate the measurement capabilities of a new, time resolved imaging spectrometer (TGSS-EM), and provide insight to the dynamics and emission characteristicts of a new class of laser produced plasmas.

Recent experiments conducted with the multi-terrawatt Novette laser have demonstrated substantial amplification of soft x rays in laser-produced plasmas. Gains at wavelengths of 206 and 209 A in neonlike selenium and at 153 A in neonlike yttrium have been observed.

Gold transmission gratings with 0.3 um spatial period, 0.6 μm thick have been fabricated, and are being used for the spectroscopy of high temperature, laser produced plasmas. A transmission grating has been coupled with a soft x-ray streak camera, producing the first time resolved (20 psec resolution) continuous x-ray spectrum over the range 0.1-1.5 keV from a laser produced plasma. In audition, a transmission grating has been coupled with a high resolution, Winter-design x-ray microscope to produce an imaging spectrometer of unprecedented spatial and spectral resolution. In laboratory experiments the imaging spectrometer demonstrated a spectral resolving power, λ of 200 and a spacial resolution of 1 μm.

A reflectometer has been set up at the radiometric laboratory of PTB at the Berlin electron storage ring BESSY. Special attention has been paid to optimize the instrumentation for the characterization of multilayer structures suitable for laboratory soft x-ray laser research. The instrumentation is described and its performance demonstrated showing examples of optical component characterization.

The development of a new time-resolved x-ray spectrometer is reported in which a free-standing x-ray transmission grating is coupled to a soft x-ray streak camera. The instrument measures continuous x-ray spectra with 20-psec temporal resolution and moderate spectral resolution (deltalambda/= 1 A) over a broad spectral range (0.1-5 keV) with high sensitivity and large information recording capacity. Its capabilities are well suited to investigation of laser-generated plasmas, and they nicely complement the characteristics of other time-resolved spectroscopic techniques presently in use. The transmission grating spectrometer has been used on a variety of laser-plasma experiments. We report the first measurements of the temporal variation of continuous low-energy x-ray spectra from laser-irradiated disk targets.

Two-dimensional modeling of magnesium exploding-foil experiments shows that spontaneous emission from regions outside of line focus makes the measurement of induced emission and gain difficult. An improved target design is proposed that solves the spontaneous-emission problem by replacing the foil by a ribbon. The new targets consist of an aluminum ribbon on a Formvar sheet with the potential 4–3 lasing line at lll A for H-like Al. We comment on what conditions are required for lasing in the 3–2 line of H-like Al at 39 A.

We have fabricated new, efficient x‐ray optical components (x‐ray mirrors, beamsplitters, and highly dispersive multilayer mirrors) and have used them to build an x‐ray laser cavity. With our laser cavity, we have demonstrated multipass amplification of soft x rays with significant enhancement (>20×) of the x‐ray laser emission. This is an important first step in the ultimate conversion of x‐ray amplifier media into true x‐ray lasers with optimized coherence and divergence properties. We used advanced microfabrication techniques to fabricate the key laser cavity components which require precise layering of alternating materials, thin‐film membrane technology, and precise patterning. The x‐ray laser requirements, fabrication details, and characterization results for these cavity components are discussed.

We describe the experiments and supporting theoretical modelling to develop and characterize soft x-ray lasers. The x-ray lasers are created in dense plasmas produced by optical laser irradiation of solid targets with line focussed beams. We use mainly thin foil targets, which upon appropriate illumination, produce rather uniform plasmas. We consider laser schemes pumped by electron collisional excitation and dielectronic recombination in Ne-like and Ni-like ions, and schemes pumped by collisional and radiative recombination following rapid cooling for H-like and Li-like ions.Experimental measurements of the time and space resolved spectra taken both along the lasing axis and at other viewing angles, in addition to data on the angular pattern of x-ray laser radiation and on the absorption and scattering of the optical laser light are presented. These data allow us the determine the characteristics of the plasmas which have been created, as well as the properties of the x-ray lasers, such as the gain coefficients for the inverted transitions, and their spatial and temporal distributions. The modelling includes calculations of the absorption of the optical laser light, the hearing and hydrodynamics of the targets and the evolution of the atomic level populations within the plasma. Transfer of the emitted radiation is calculated, including resonance line trapping, amplification for inverted transitions, and refraction of the x-ray laser beam due to electron density gradients. Results are used to optimize x-ray laser designs before the experiments and to interpret the measured spectra.The latest experimental results from the NOVA laser facility on the performance of several laser schemes and on the use of multilayer mirrors to produce x-ray laser cavities are reported. These results arc compared to the models to test and improve our understanding of the complex physics involved in making x-ray lasers. Based on current experiments, we show how the modelling can be use to design shorter wavelength and more efficient schemes for use in applications such as x-ray holography.

We report the first time-resolved measurements of emission from a double-pass soft x-ray laser cavity. In these experiments the output signal from a selenium x-ray laser had two temporal components clearly identifiable as the single- and double-pass emission, with the double-pass amplified signal more intense than the single pass. In addition to an unequivocal demonstration of double-pass amplification of soft x rays, the data provide information about of the time-dependent gain in these x-ray laser media, suggesting an effective gain-length profile which rises more slowly and falls-off more rapidly than predicted by state of the art hydrodynamics and kinetics codes.