X-ray Laser Cavity Research Articles

X-ray laser resonator for the kilo-electron-volt range

We have designed, constructed, and tested an x-ray laser resonator operating in the hard x-ray, keV energy region. This ring x-ray laser cavity is formed by four highly oriented pyrolytic graphite crystals. The crystals are set at the Bragg angles that allow for the complete 360° round trip of the 2.37 Å, 5.23 keV Lα line of neodymium. In addition, we also present experimental data of a similar ring laser resonator that utilizes the Cr Kα, 5.41 keV, x-ray line to propagate through the four mirrors of the cavity. The specific properties of these x-ray laser resonator mirrors, including reflection losses and cavity arrangement, are presented.

Demonstration of X-Ray Amplification in an X-Ray Laser Cavity Pumped by a Pulse-Train Yttrium Aluminium Garnet Laser

X-ray lasings of a Li-like Al transition line at 15.47 nm in the recombination scheme have been investigated using a tabletop pulse-train yttrium aluminium garnet (YAG) laser system. A cavity experiment has been carried out with a resonator consisting of a flat and a concave Mo/Si multilayer mirrors. Clear enhancement of the X-ray output from the cavity has been demonstrated. It was found that the cavity output beam has a divergence of about 3 mrad and an absolute intensity of about 1×108 photons/shot. The temporal feature of the observed cavity output can be reproduced by a simple ray-trace calculation with multiple times reflection and the presence of a lasing medium.

Advances in multilayer X-ray optics

Abstract We present an overview of advances in multilayer X-ray optics which have been achieved in the past few years. Advances in fabrication, characterization, and theory are described. These mirrors are being used in a variety of synchrotron-related applications such as monochromators and projection lithography, as well as nonsynchrotron applications like astronomy and X-ray laser cavities. Although some of these coatings are produced routinely, much work remains to be done in producing higher quality multilayers with very thin layers (less than ∼ 20 A) and multilayers which are stable at high temperatures. Good multilayers with very thin layers are needed for near normal incidence optics at short wavelengths. Very stable multilayers are needed to withstand the high power loads which can be imposed by synchrotron radiation. New materials and deposition techniques are being applied to these problems. For example, molecular beam epitaxy is being applied to the growth of these multilayers in an effort to produce single-crystal superlattice mirrors. Since the performance of these mirrors is strongly dependent on atomic-scale interface quality, it is necessary to control the fabrication and perform characterization on this length scale. Characterization techniques which have made recent impact include high-resolution TEM, in situ ellipsometry, and energy dispersive X-ray diffraction. Surface analysis techniques such as STM, RHEED, LEED, AES, and XPS are being used to study interface formation and surface structure on a subnanometer scale.

Investigation of Damage to Multilayer Optics in X-Ray Laser Cavities: W/C, WRe/C, WC/C, Stainless-Steel/C, and Cr3C2/C Mirrors

In many applications, multilayer mirrors are exposed to damaging fluences of x rays. In x-ray laser cavities intense optical and broad-band x radiation, from the x-ray laser plasma amplifier, can damage multilayer mirrors on time scales of hundreds of picoseconds. We describe experiments using short duration (500 ps) bursts of soft x rays from a laser produced gold plasma to damage multilayer mirrors designed to reflect wavelengths close to 45 Å at normal incidence. The effect of the damaging x-ray flux on normal incidence reflectivity was time resolved for W/C, WRe/C, WC/C, 303-stainless-steel/C, and Cr3C2/C multilayers. The damage thresholds of the different mirrors were compared, and the Cr3C2/C mirrors were found to be the most resilient. The outer layers of the multilayers were observed to expand slowly as x rays were absorbed, and a more rapid expansion then preceded the total loss of reflectivity, at temperatures well below the melting temperature of the mirror components. It is believed that the dominant expansion mechanism is a change in the amorphous carbon layers to a more graphitic structure. The data are fit quite well by a model that assumes expansion of up to 25% in the thickness of the outermost carbon layers, followed by intermixing of the hotter layers. The rapid expansion has been observed to occur in times from 40 to 150 ps and may be the fastest resolution to date of the phenomenon of graphitization. The integrated reflectivity of the mirrors was observed to increase by up to a factor of 2.5 as they damaged; this reflectivity increase may be consistent with a reduction in the layer roughness.

X-ray optics for X-ray laser research applications

State of the art capabilities in soft X-ray lenses, multilayer mirrors, beamsplitters, and synthetically generated holograms are reviewed. Application of these capabilities in recent X-ray laser cavity experiments, and to the development of a soft X-ray interferometer and a high intensity (≥1013watt/cm2) soft X-ray laser are discussed.

X-Ray Lasers

Multicharged ions in laser-produced plasmas are able to provide soft X-ray amplification. Conditions under which hot plasma columns act as X-ray amplifyers are investigated in several laboratories. Plasma optical properties, especially in relation with refractive index gradient and radiation trapping, are of importance to reach large gain-length product values. Multilayer mirrors are now available for designing X-ray laser cavities. Regarding X-ray laser pumping, the theory of which requires detailed atomic-level population calculation to be joined to hydrodynamical plasma modelling, experiments set off now two types of mechanisms amongst many possible theoretical schemes. On the one hand, recombination, during fast plasma cooling, populates high-lying ion levels more rapidly than the first resonance levels. This is observed for the 2–3 transition of H-like ions, especially for the line at 182-A wavelength in carbon which has shown gain with a coefficient attaining 4 cm−1. This is also observed for 3–5 and 3–4 transitions of lithium-like ions, near 100-A wavelength. Gain coefficients are between 0.5 cm−1 and 2.5 cm−1. On the other hand, the balance between collisional excitation by plasma free electrons and fast radiative decay produces population inversions between 3s- and 3p-levels in Ne-like ions and between 4p- and 4d-levels in Ni-like ions. Ne-like selenium has provided a gain coefficient of 5 cm−1 at 206.3 A. Ni-like ions allow to extend this scheme to wavelenghts below 100 A. In view of their possible applications, X-ray lasers are expected to be by far the brightest X-ray sources in laboratory.

Time-resolved measurement of double-pass amplification of soft x rays

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.

Applications of microfabrication technology to x-ray laser cavities

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.

The Soft X-Ray Laser Program at Livermore

AbstractWe 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.

SOFT X-RAY LASER CAVITIES

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.