We report a recent experiment where the first hard x-ray beam line, X-ray Pump Probe (XPP) instrument using the
SLAC National Accelerator Laboratory's Linac Coherent Light Source (LCLS) free electron laser, was used to heat thin
foils to high energy densities ~ 107 J/cm3. An intense 9 keV, 60 fs (FWHM) duration beam with energy of 2 - 4 mJ at
the XPP beam line was focused using beryllium lenses to an irradiance approaching 1016 Wcm-2. Targets of 0.5 - 3.5 μm
thick foils of Ag and Cu were studied using a suite of diagnostics including Fourier Domain Interferometry, energy
calorimetry and grating and crystal spectrometers. The experimental details and spectroscopic results from the campaign
will be described. Preliminary results indicate that the target is heated relatively uniformly to a temperature lower than
20 eV.
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.
Short pulse (< 100 fs) tunable X-ray and VUV laser sources, based on the free electron laser (FEL) concept, will be a
watershed for high energy density research in several areas. These new 4th generation light sources will have extremely
high fields and short wavelength (~0.1 nm) with peak spectral brightness -photons/(s/mrad2/mm2/0.1% bandwidth- 1010
greater than 3rd generation light sources. We briefly discuss several applications: the creation of warm dense matter
(WDM), probing of near solid density plasmas, and laser-plasma spectroscopy of ions in plasmas. The study of dense
plasmas has been severely hampered by the fact that laser-based probes that can directly access the matter in this regime
have been unavailable and these new 4th generation sources will remove these restrictions. Finally, we present the plans
for a user-oriented set of facilities that will incorporate high-energy, intense short-pulse, and x-ray lasers at the first x-ray
FEL, the LCLS to be opened at SLAC in 2009.
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.
Laser-based plasma spectroscopic techniques have been used with great success to determine the line shapes of atomic transitions in plasmas, study the population kinetics of atomic systems embedded in plasmas, and look at the redistribution of radiation. However, the possibilities for optical lasers end for plasmas with ne > 1022 cm-3 as light propagation is severely altered by the plasma. The construction of the Tesla Test Facility (TTF) at DESY (Deutsche Elektronen-Synchrotron), a short pulse tunable free electron laser in the vacuum-ultraviolet and soft X-ray regime (VUV FEL), based on the SASE (self amplified spontaneous emission) process, will provide a major advance in the capability for dense plasma-related research. This source will provide 1013 photons in a 200 fs duration pulse that is tunable from ~6 nm to 100 nm. Since an VUV FEL will not have the limitation associated with optical lasers the entire field of high density plasmas kinetics in laser produced plasma will then be available to study with the tunable source. Thus, one will be able to use this and other FEL x-ray sources to pump individual transitions creating enhanced population in the excited states that can be easily monitored. We show two case studies illuminating different aspects of plasma spectroscopy.
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.
During recent months we have continued investigations of many different aspects of x-ray lasers to characterize and improve the source and applications. This work has included temporal characterization of existing laser-heated x-ray lasers under a wide range of pumping conditions. We have also looked into more details at different applications of x-ray lasers among which was the interferometry of laser-produced and capillary discharge plasmas in several irradiation conditions for different target Z materials. The reduction of pump energy remains the most important for the generation of new compact x-ray lasers. Numerical studies show that there are some ways to improve several of the key parameters of x-ray lasers specifically repetition rates and efficiency.
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 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.
In this work we report our numerical modeling results of laser-generated transient inversion and capillary discharge X-ray lasers. In the search for more efficient X-ray lasers we look closely at other approaches in conjunction with experiments at LLNL. In the search for improved X-ray lasers we perform modeling and experimental investigations of low density targets including gas puff targets. We have found the importance of plasma kinetics in transient X-ray lasers by expanding the physical model beyond hydrodynamics approach with Particle In Cell (PIC) and Fokker-Planck codes. The evidence of the Langdon effect was inferred from the recent experimental data obtained with the Ni-like Pd X-ray laser. We continue modeling different kinds of capillary discharge plasma configurations directed toward shorter wavelength X-ray lasers, plasma diagnostics and other applications.
Recent transient collisional excitation x-ray laser experiments are reported using the COMET tabletop laser driver at the Lawrence Livermore National Laboratory. Ne- like and Ni-like ion x-ray laser schemes have been investigated with a combination of long 600 ps and short approximately 1 ps high power laser pulses with 5 - 10 J total energy. We show small signal gain saturation for x-ray lasers when a reflection echelon traveling wave geometry is utilized. A gain length product of 18 has been achieved for the Ni-like Pd 4dyields4p J equals 0 - 1 line at 147 angstroms, with an estimated output of approximately 10 (mu) J. Strong lasing on the 119 angstroms Ni-like Sn line has also been observed. To our knowledge this is the first time gain saturation has been achieved on a tabletop laser driven scheme and is the shortest wavelength table-top x-ray laser demonstrated to date. In addition, we present preliminary results of the characterization of the line focus uniformity for a Ne-like ion scheme using L-shell spectroscopy.
We present experimental results of a high efficiency Ne-like Fe transient collisional excitation x-ray laser using the COMET 15 TW table-top laser system at LLNL. The plasma formation, ionization and collisional excitation of the x- ray laser have been optimized using two sequential laser pulses: a plasma formation beam with 5 J energy of 600 ps duration and a pump beam with 5 J energy of 1.2 ps duration. Since the observation of strong lasing on the 255 angstroms 3p - 3s J equals 0 - 1 transition of Ne-like Fe, we have achieved high gains of 35 cm-1 and saturation of the x-ray laser. A five-stage traveling wave excitation enhances the strongest Fe 3p - 3s 255 angstroms lasing line as well as additional x-ray lines. A careful characterization of the plasma column conditions using L-shell spectroscopy indicates the degree of ionization along the line focus.
An investigation of the rapid rise time of incoherent x-ray emission from targets heated by an ultra-short pulse (USP) high-intensity optical laser was conducted for use as the x- ray source for inner-shell photo-ionized (ISPI) x-ray lasing. Previous studies considered front-side x-ray emission; however, ISPI x-ray lasing requires a filtered x-ray source. Modeling using the hydrodynamics/atomic kinetics code LASNEX of a 40 fs USP driving laser with an intensity of 1017 W/cm2 incident on a flat target of thin Au layered on a Be filter is presented. The filter has a modest influence on the x-ray emission of the Au via conduction cooling but has a large effect on the backside spectrum by removing low energy x-rays as the Au emission passes through the filter. The use of such a filtered source is shown to provide the needed x rays to achieve high gain in C at 45 angstrom.
Pumping of proposed inner-shell photo-ionized (ISPI) x-ray lasers places stringent requirements on the optical pump source. We investigate these requirements for an example x-ray laser (XRL) in Carbon lasing on the 2p - 1s transition at 45 angstrom. Competing with this lasing transition is the very fast auger decay rate out of the upper lasing state, such that the x-ray laser would self-terminate on a femto-second time scale. XRL gain may be demonstrated if pump energy is delivered in a time short when compared to the auger rate. The fast self-termination also demands that we sequentially pump the length of the x-ray laser at the group velocity of the x- ray laser. This is the classical traveling wave requirement. It imposes a condition on the pumping source that the phase angle of the pump laser be precisely de-coupled from the pulse front angle. At high light intensities, this must be performed with a vacuum grating delay line. We also include a discussion of issues related to pump energy delivery, i.e. pulse-front curvature, temporal blurring and pulse fidelity. An all- reflective optical system with low aberration is investigated to see if it fulfills the requirements. It is expected that these designs together with new high energy (greater than 1 J) ultra-short pulse (less than 40 fs) pump lasers now under construction, may fulfill our pump energy conditions and produce a tabletop x-ray laser.
An investigation of the rapid rise time of x-ray emission from targets heated by an ultrashort- pulse high-intensity optical laser was conducted for use as a pump for inner-shell photo- ionized x-ray lasing. Results of x-ray rise times from instantaneously heated Au rod targets show little benefit for using optical pulse widths less than 30 fs. Gain calculations for inner- shell photo-ionized lasing show that large gains can be obtained for pulse widths between 30 and 100 fs. Calculated spectra, using the hydrodynamic/atomic kinetics code LASNEX, from a 1 J, 65 fs FWHM pulse optical laser incident on a structured Au target gave a gain of 11.5 cm-1 in C at 45 angstrom.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.