There are significant advantages for using a compact capillary discharge soft x-ray laser (SXRL) with wavelength of 46.9 nm for mass spectrometry applications. The 26.4 eV energy photons provide efficient single-photon ionization while preserving the structure of molecules and clusters. The tens of nanometers absorption depth of the radiation coupled with the focusing of the laser beam to diameter of ∼100 nm result in the ablation of atto-liter scale craters which in turn enable high resolution mass spectral imaging of solid samples. In this paper we describe results on the analysis of composition depth-profiling of multilayer oxide stack and material studies in photoresists, ionic crystals, and magnesium corrosion products using SXRL ablation mass spectrometry, a method first demonstrated by our group. These materials are used in a variety of soft x-ray applications such as detectors, multilayer optics, and many more.
Mass spectrometry plays a vital role in the direct examination of the chemical composition of solids. We have introduced the use of soft x-ray laser ablation for mass spectrometry imaging. Here we demonstrate the method potential for composition depth profiling of multilayer stacks consisting of tens of nanometers thick metal and dielectric films.
The mechanisms responsible for an increase in collimation of laboratory plasma jets with higher atomic number was studied using soft x-ray laser interferometry and 2D model simulations. Dense plasma jets (Ne~ 1020 cm-3) were produced by irradiating V-shaped grooves of different materials (C, Al, and Cu) with 120 ps Ti:Sa laser pulses at peak intensities of 1 x 1012 W cm-2. High contrast soft x-ray interferograms of these plasmas were generated by combining a Mach-Zehnder interferometer that uses diffraction gratings as beam-splitters and a 46.9 nm table-top capillary discharge laser probe. A significant increase in jet collimation was observed for the higher Z materials. Simulations performed
with the radiation hydrodynamic code HYDRA attribute differences in jet collimation to an increased radiation cooling of the higher Z jets.
Soft x-ray interferometry was used to measure the evolution of dense converging plasmas created by laser irradiation of 500 μm diameter semi-cylindrical carbon targets. Optical laser pulses with an intensity of ~1×1012W cm-2 and 120 ps duration were used to heat the surface of the cavities. The dense plasma formed expands from the walls converging slightly off the semi-cylinder's axis, giving rise to a bright localized high density plasma region. A sequence of electron density maps were measure using a 46.9 nm wavelength tabletop capillary discharge soft x-ray laser probe and an amplitude division interferometer based on diffraction gratings. The measured density profiles are compared with simulations conducted using the multi-diminensional hydrodynamic code HYDRA. The benchmarked model was then used to simulate particle trajectories which reveal that the increase in electron density near the axis is mainly the result of the convergence of plasma that originated at the bottom of the groove during laser irradiation.
We have used soft x-ray laser interferometry to study dense colliding plasmas produced by laser irradiation of semi-cylindrical targets. Results are reported on the evolution of 1 mm long plasmas created by heating 500 μm diameter half holhraum copper targets with an intensity of ~1.6 1012 W.cm-2 from 120 ps duration laser pulses of 800 nm wavelength. The setup combines a robust high throughput amplitude division interferometer based on diffraction gratings with a 46.9 nm table-top capillary discharge laser. Series of high contrast interferograms were obtained depicting the evolution of the copper plasmas into a localized plasma that reaches densities above 1×1020 cm-3 when the plasmas collide near the center of the cavity. The technique allows the generation of high resolution density maps of colliding plasma with various degree of collisionality for comparison with code simulations.
We report clear evidence of the existence of multiply ionized plasmas with index of refraction greater than one at soft x-ray wavelengths. Moreover, it is shown to be a general phenomenon affecting broad spectral regions in numerous highly ionized plasmas. The experimental evidence consists of the observation of anomalous fringe shifts in soft x-ray laser interferograms of laser-created Al plasmas probed at 14.7 nm and of Ag and Sn laser-created plasmas probed at 46.9 nm. The comparison of measured and simulated interferograms shows that these anomalous fringe shifts result from the dominant contribution of low charge ions to the index of refraction. This usually neglected bound electron contribution can affect the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements and at different wavelengths.
We summarize results of several successful dense plasma diagnostics experiments realized combining two different kinds of table-top soft x-ray lasers with an amplitude division interferometer based on diffraction grating beam splitters. In the first set of experiments this robust high throughput diffraction grating interferometer (DGI) was used with a 46.9 nm portable capillary discharge laser to study the dynamics of line focus and point focus laser-created plasmas. The measured electron density profiles, which differ significantly from those expected from a classical expansion, unveil important twodimensional effects of the dynamics of these plasmas. A second DGI customized to operate in combination with a 14.7 nm Ni-like Pd transient gain laser was used to perform interferometry of line focus laser-created plasmas with picosecond time resolution. These measurements provide valuable new benchmarks for complex hydrodynamic codes and help bring new understanding of the dynamics of dense plasmas. The instrumentation and methodology we describe is scalable to significantly shorter wavelengths, and constitutes a promising scheme for extending interferometry to the study of very dense
plasmas such as those investigated for inertial confinment fusion.
We present within this paper a series of experiments, which yield new observations to further our understanding of the transient collisional x-ray laser medium. We use the recently developed technique of picosecond x-ray laser interferometry to probe the plasma conditions in which the x-ray laser is generated and propagates. This yields two dimensional electron density maps of the plasma taken at different times relative to the peak of the 600ps plasma-forming beam. In another experimental campaign, the output of the x-ray laser plasma column is imaged with a spherical multilayer mirror onto a CCD camera to give a two-dimensional intensity map of the x-ray laser output. Near-field imaging gives insights into refraction, output intensity and spatial mode structure. Combining these images with the density maps gives an indication of the electron density at which the x-ray laser is being emitted at (yielding insights into the effect of density gradients on beam propagation). Experimental observations coupled with simulations predict that most effective coupling of laser pump energy occurs when the duration of the main heating pulse is comparable to the gain lifetime (~10ps for Ni-like schemes). This can increase the output intensity by more than an order of magnitude relative to the case were the same pumping energy is delivered within a shorter heating pulse duration (< 3ps). We have also conducted an experiment in which the output of the x-ray laser was imaged onto the entrance slit of a high temporal resolution streak camera. This effectively takes a one-dimensional slice of the x-ray laser spatial profile and sweeps it in time. Under some conditions we observe rapid movement of the x-ray laser (~ 3um/ps) towards the target surface.
We report results of the development of capillary discharge driven metal-vapor plasma waveguides for the development of efficient laser-pumped soft x-ray lasers; and of the use of a previously developed capillary discharge Ne-like Ar 46.9 nm laser in study of the interaction of intense soft x-ray laser with materials. The guiding of a laser beam in a dense capillary discharge plasma channel containing a large density of Ag ions is reported. In term of applications we have conducted studies of materials modification and ablation with focalized 46.9 nm laser radiation at fluences between
0.1 and 100 J cm-2. The experiments demonstrated that the combined high repetition rate and high energy per pulse of the capillary discharge laser allows for the first time the processing of large surface areas with intense soft x-ray laser radiation. The damage threshold and damage mechanism of extreme ultraviolet Sc/Si multilayer mirror coatings was studied . Damage threshold fluences of ~ 0.08 J/cm2 were determined for coatings deposited on both borosilicate glass and Si substrates. Scanning and transmission electron microscopy, and small-angle X-ray diffraction techniques revealed the thermal nature of the damage mechanism. These results provide a benchmark for the use of Sc/Si multilayer mirrors in high fluence applications, and for the development of higher damage threshold mirrors. Soft x-ray laser ablation studies were also conducted for silicon and several plastic materials, including PMMA, Polyamide and PTFE.
We present results from soft x-ray laser interferometry/shadowgraphy of current-driven thin wire plasmas using a capillary discharge pumped 46.9 nm laser and a diffraction grating interferometer. We have obtained series of soft x-ray images of exploding Al wires 15 μm or 25 μm in diameter. The high photo-ionization cross sections of atoms and low charge ions at this wavelength allow the soft x-ray laser probe to detect the early stages of formation of a coronal plasma surrounding the wire core. Wires of 25 μm diameter excited by current pulses with a 78 A/ns increase rate are observed to expand uniformly. However, an increase in the rate of energy deposited per unit of mass is observed to give rise to significant instabilities. The simultaneous analysis of the fringe shift and soft x-ray absorption data suggests the coronal plasma contains a significant concentration of low Z impurities. The results illustrate that table-top soft x-ray lasers are a powerful new tool for the diagnostics of dense plasmas.
This paper gives an overview of recent soft x-ray laser research at Colorado State University. Progress related to capillary discharge source development includes the observation of emission from the 13.2-nm laser line of Nickel-like Cd in a plasma column generated by a high power capillary discharge. This result suggests it might be possible to extend capillary discharge lasers to significantly shorter wavelengths. In another approach to the generation of coherent soft x-ray radiation we analyzed the possibility of amplifying high order harmonic pulses in a discharge-pumped amplifier. The study of the already well- characterized 46.9-nm Ne-like Ar laser was extended with new spatial coherence and laser wavefront measurements, in work conducted in collaboration with U. California Berkeley and U. of Paris-Sud groups. In the field of applications, we have extended our previous results of plasma interferometry with a tabletop laser to plasma densities up to 0.9 x 1021 cm-3. Sequences of soft x-ray laser interferograms of plasmas generated by a Nd-YAG laser at intensities between 1 x 1011 W cm-2 and 7 x 1012 W cm-2 show the development and evolution of a concave electron density profile. The detailed mapping of this phenomenon with soft x-ray interferometry exemplifies the usefulness of compact soft x-ray lasers in increasing the understanding of high density plasmas.
The development of the transient collisional excitation x-ray laser scheme using tabletop laser systems with multiple pulse capability has progressed rapidly in the last three years. The high small-signal gain and strong x-ray output have been demonstrated for laser drive energies of typically less than 10 J. We report recent x-ray laser experiments on the Lawrence Livermore National Laboratory (LLNL) Compact Multipulse Terawatt (COMET) tabletop facility using this technique. In particular, the saturated output from the Ni-like Pd ion 4d - 4p x-ray laser at 146.8 angstrom has been well characterized and has potential towards a useable x-ray source in a number of applications. One important application of a short wavelength x-ray laser beam with picosecond pulse duration is the study of a high density laser-produced plasma. We report the implementation of a Mach-Zehnder type interferometer using diffraction grating optics as beam splitters designed for the Ni-like Pd laser and show results from probing a 600 ps heated plasma. In addition, gas puff targets are investigated as an x-ray laser gain medium and we report results of strong lasing on the n equals 3 - 3 transitions of Ne-like Ar.
We report an extension of previous tabletop soft x-ray laser interferometry work to plasma densities approaching the critical density. The evolution of line-focus and spot-focus plasmas created with Nd-YAG laser intensities of 0.1 and 7.0 TW/cm2 respectively were studied utilizing a 46.9-nm capillary discharge laser with a diffraction grating interferometer. In the latter case, the electron density was mapped to values up to 0.9x1021 cm-3 (90% of the critical density for the lambda equals 1.06 micrometers pump laser). The interferograms show the development of concave electron density profiles with a minimum on axis and pronounced side lobes. Hydrodynamic model simulations show that the concave profile is the result of the hydrodynamic and radiation effects that enlarge the ablated target area. The measurements exemplify how soft x-ray lasers can be used to probe high density plasmas for the validation of hydrodynamic codes.
Laser interferometry allows the recording of the electron density in a great variety of plasmas. However, the absorption and refraction imposes a limitation to the maximum density, plasma size and plasma gradient that can be measured with this technique. The development of compact soft x-ray laser sources gives the opportunity to extend the limits of plasma interferometry, probing plasmas with high densities and steep gradients. We present results of plasma interferometry using an amplitude division interferometer and a table top soft x-ray laser. The interferometer is a modified Mach-Zehnder configuration with diffraction gratings used as beam splitters. The soft x-ray laser is a 46.9 nm capillary discharge table-top laser. The set up was used to probe a laser-created plasma with a temporal resolution of approximately 1 ns and densities up to 6 1020 cm-3.
We report the first study of laser ablation and the demonstration of plasma interferometry with a tabletop soft x-ray laser. A capillary discharge pumped Ne-like Ar laser (46.9 nm) was focused using multilayer optics to significantly exceed the energy density necessary for the ablation of metals. Ablation in brass, stainless steel and aluminum samples is reported. The ablation patterns on brass were used in combination with ray tracing computations to characterize the focused soft x-ray laser beam. The radiation intensity within the 2 micrometer diameter central region of the focal spot is estimated to be approximately 1011 W/cm2, with an energy density of approximately 100 J/cm2. In a separate experiment we performed soft x-ray interferometry of a laser-created plasma using a table-top capillary discharge laser operating at 46.9 nm in combination with a novel amplitude division interferometer. The soft x-ray interferometer utilizes diffraction gratings as beam splitters in a Mach-Zehnder configuration to generate high contrast interferograms over a large field of view. This table-top system was used to probe a large-scale (3 mm long) plasma created by a Nd:YAG laser. The short wavelength of the probe laser has allowed mapping of the electron density in plasma regions with density gradients steeper than those that could be probed with the fourth harmonic of Nd:YAG for a plasma of this length.
We discuss the first demonstrations of plasma diagnostics using a tabletop soft x-ray laser. A very compact capillary discharge pumped Ne-like Ar laser operating 46.9 nm was used to perform shadowgraphy and interferometry experiments in discharge-created discharge plasma waveguide. In a second set of experiments we took advantage of the good spatial coherence of the capillary discharge laser to perform soft x-ray interferometry measurements. In a first experiment the laser was used in combination with a simple wavefront division interferometer based on Lloyd's mirror to map the electron density distribution in the cathode region of a pinch discharge. In our most recent experiment we used an amplitude division interferometer, which utilizes diffraction grating as beam splitters, to probe a large scale laser-created plasma. Both interferometer schemes can be adapted to operate at the wavelength corresponding to any of the presently available saturated soft x-ray lasers.
We have performed soft x-ray interferometry of a laser- created plasma using a novel amplitude division interferometer in combination with a tabletop capillary discharge laser operating at 46.9 nm. The soft x-ray interferometer utilizes diffraction gratings as beam splitters in a Mach-Zehnder configuration to generate high contrast interferograms over a large field of view. An advantage of this interferometer scheme is that it can be used at any of the wavelengths covered by presently available soft x-ray lasers. This table-top system was used to probe a large scale (approximately 3 mm long) plasma crated by a Nd:YAG laser. The short wavelength of the probe laser has allowed mapping of the electron density in plasma regions with density gradients stepper than those that could be probed with the fourth harmonic of Nd:YAG for a plasma of this length.
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