Evaluation of bent-crystal x-ray backlighting and microscopy techniques for the Sandia Z machine

Quartz conditioning crystal for X-ray rocking curve topography

A large α-quartz crystal designed to condition the monochromatic beam at beamline 1-BM of the Advanced Photon Source is presented. The purpose of this crystal was to provide a precise match to the Bragg angle of quartz crystals that are commonly used to make analyzers for plasma diagnostics. In order to characterize these analyzers, area detectors need to be positioned at distances from the analyzer at upwards of 1000 mm. As a result of this precise matching, a Bragg-reflected beam from such an analyzer is precisely parallel to the beam incident on the conditioning crystal. This situation alleviates the need to adjust the position of the area detector as the distance between the analyzer and the area detector is varied. That is, there is no need to account for vertical displacement of the diffracted-beam image as a function of this distance. Additionally, verification that the analyzer is set to the correct Bragg reflection is obtained by scanning this distance, because only for a spurious reflection will there be a vertical displacement. This is a very useful check. To commission the conditioning crystal, diffraction from a high-quality flat quartz crystal was mapped using a CCD. Bragg diffraction from the 40\overline 40 reflection at 8.5 keV was studied over an area of 23 × 31 mm. The theoretical Darwin width of the flat sample in this case was 4.7 µrad. An FWHM value near 6 µrad was measured over almost the entire mapped area. These data demonstrate that the resolution function for this four-crystal arrangement is ∼4 µrad. Data are also presented for a 0.1 mm-thick α-quartz wafer pressed into a concave form, having a nominal radius of 500 mm and intended for use as an analyzer. Because analyzers are bent crystals, diffraction occurs in narrow bands. When a multiple exposure is made as a function of rocking angle a striped pattern is obtained, which is commonly referred to as a zebra-stripe pattern. A series of zebra stripes from the 30\overline 3\overline 3 Bragg reflection of the bent wafer over its 25 mm-diameter area were recorded on a CCD. The zebra-stripe pattern was analyzed to show a uniform bending to a radius of 497.0 ± 0.3 mm, in support of the nominal value. An r.m.s. slope error of 7 µrad was also obtained from this analysis.

The Crystal Backlighter Imager: A spherically bent crystal imager for radiography on the National Ignition Facility

The Crystal Backlighter Imager (CBI) is a quasi-monochromatic, near-normal incidence, spherically bent crystal imager developed for the National Ignition Facility (NIF), which will allow inertial confinement fusion capsule implosions to be radiographed close to stagnation. This is not possible using the standard pinhole-based area-backlighter configuration, as the self-emission from the capsule hotspot overwhelms the backlighter signal in the final stages of the implosion. The CBI mitigates the broadband self-emission from the capsule hot spot by using the extremely narrow bandwidth inherent to near-normal-incidence Bragg diffraction. Implementing a backlighter system based on near-normal reflection in the NIF chamber presents unique challenges, requiring the CBI to adopt novel engineering and operational strategies. The CBI currently operates with an 11.6 keV backlighter, making it the highest energy radiography diagnostic based on spherically bent crystals to date. For a given velocity, Doppler shift is proportional to the emitted photon energy. At 11.6 keV, the ablation velocity of the backlighter plasma results in a Doppler shift that is significant compared to the bandwidth of the instrument and the width of the atomic line, requiring that the shift be measured to high accuracy and the optics aligned accordingly to compensate. Experiments will be presented that used the CBI itself to measure the backlighter Doppler shift to an accuracy of better than 1 eV. These experiments also measured the spatial resolution of CBI radiographs at 7.0 μm, close to theoretical predictions. Finally, results will be presented from an experiment in which the CBI radiographed a capsule implosion driven by a 1 MJ NIF laser pulse, demonstrating a significant (>100) improvement in the backlighter to self-emission ratio compared to the pinhole-based area-backlighter configuration.

Characterization of shaped Bragg crystal assemblies for narrowband x-ray imaging

X-ray imaging using shaped crystals in Bragg reflection is a powerful technique used in high-energy-density physics experiments. The characterization of these crystal assemblies with conventional x-ray sources is very difficult because of the required angular resolution of the order of ∼10 μrad and the narrow bandwidth of the crystal. The 10-J, 1-ps Multi-Terawatt (MTW) laser at the Laboratory for Laser Energetics was used to characterize a set of Bragg crystal assemblies. The small spot size (of the order of 5 μm) and the high power (>10¹⁸ W/cm²) of this laser make it possible to measure the spatial resolution at the intended photon energy. A set of six crystals from two different vendors was checked on MTW, showing an unexpectedly large variation in spatial resolution of up to a factor of 4.

On evaluating x-ray imaging crystals with synchrotron radiation

Bent crystals used in diagnostics of plasmas combine x-rays diffracted from across the crystal. Therefore imperfections in the resulting 1-D spectrum or 2-D image are not the best way to find out why one particular crystal may differ in its performance from another and what, if anything, to do about it. Instead, here we want to measure the diffraction locally, with the necessary resolution. Nominally monochromatic and unidirectional radiation from the synchrotron's standard x-ray optics proved to be insufficient for the purpose. Here much better radiation comes from the x-ray topography setup at the x-ray optics testing beamline 1-BM at the Advanced Photon Source, thanks to a specially designed quartz conditioning crystal. Some worrisome features in a bent crystal's diffraction have thereby disappeared, while minor fabrication flaws remain highly visible.

A 7.2 keV spherical x-ray crystal backlighter for two-frame, two-color backlighting at Sandia's Z Pulsed Power Facility

Many experiments on Sandia National Laboratories' Z Pulsed Power Facility-a 30 MA, 100 ns rise-time, pulsed-power driver-use a monochromatic quartz crystal backlighter system at 1.865 keV (Si He_α) or 6.151 keV (Mn He_α) x-ray energy to radiograph an imploding liner (cylindrical tube) or wire array z-pinch. The x-ray source is generated by the Z-Beamlet laser, which provides two 527-nm, 1 kJ, 1-ns laser pulses. Radiographs of imploding, thick-walled beryllium liners at convergence ratios C_R above 15 [C_R=r_i(0)/r_i(t)] using the 6.151-keV backlighter system were too opaque to identify the inner radius r_i of the liner with high confidence, demonstrating the need for a higher-energy x-ray radiography system. Here, we present a 7.242 keV backlighter system using a Ge(335) spherical crystal with the Co He_α resonance line. This system operates at a similar Bragg angle as the existing 1.865 keV and 6.151 keV backlighters, enhancing our capabilities for two-color, two-frame radiography without modifying the system integration at Z. The first data taken at Z include 6.2-keV and 7.2-keV two-color radiographs as well as radiographs of low-convergence (C_R about 4-5), high-areal-density liner implosions.

X-ray focusing properties of doubly bent crystals

The focusing properties of several bent crystal geometries, including the newly introduced Pestehe & Askari general point-focusing system [Pestehe & Askari Germi (2012),Opt. Soc. Am. A,29, 68–77; Pestehe & Askari Germi (2012),J. Appl. Cryst.45, 890–901], on an arbitrarily positioned detector plane are investigated and illustrated. The properties of the focal points and the generated images are theoretically related to the local, ℓ, and directional, γ, positions of the detector plane for a given position of a point source on the Rowland circle. A general relation is derived for the detector positioning to obtain a specially focused image. This formula for the polar position of the detector plane, given by the two ℓ and γ variables, enables the exact determination of the system astigmatism and the exact calculation of the sagittal and meridional image positions for the spectrometer under study. The astigmatisms of the above-mentioned bent crystal geometries have been studied, and their sagittal and meridional focal positions and characteristics have been obtained and are illustrated. It is also shown that there is a possibility of designing a spectrometer to focus rays from a linear source onto a point on the Rowland circle.

Systematic search for spherical crystal X-ray microscopes matching 1-25 keV spectral line sources

Spherical-crystal microscopes are used as high-resolution imaging devices for monochromatic x-ray radiography or for imaging the source itself. Crystals and Miller indices (hkl) have to be matched such that the resulting lattice spacing d is close to half the spectral wavelength used for imaging, to fulfill the Bragg equation with a Bragg angle near 90^∘ which reduces astigmatism. Only a few suitable crystal and spectral-line combinations have been identified for applications in the literature, suggesting that x-ray imaging using spherical crystals is constrained to a few chance matches. In this article, after performing a systematic, automated search over more than 9 × 10⁶ possible combinations for x-ray energies between 1 and 25 keV, for six crystals with arbitrary Miller-index combinations hkl between 0 and 20, we show that a matching, efficient crystal and spectral-line pair can be found for almost every He_α or K_α x-ray source for the elements Ne to Sn. Using the data presented here it should be possible to find a suitable imaging combination using an x-ray source that is specifically selected for a particular purpose, instead of relying on the limited number of existing crystal imaging systems that have been identified to date.

X-ray backlighting of imploding aluminium liners on PTS facility

The x-ray backlighting systems, including a 1.865 keV (Si He_α line) spherically bent crystal imaging system and an ∼8.3 keV (Cu He_α line) point-projection imaging system, newly fielded on the Primary Test Stand facility are introduced and its preliminary experimental results in radiography of the aluminium (Al) liners with seeded sinusoidal perturbations are presented. The x-ray backlighter source is created using a 1 TW, 1 kJ Nd: glass high power laser, kilo-joule laser system, recently constructed at China Academy of Engineering Physics. The ablation melt and instability of the imploding Al liner outer edge under the driving current of ∼7.5 MA are successfully observed using these two backlighting systems, respectively.

Construction of a quartz spherical analyzer: application to high-resolution analysis of the NiKα emission spectrum

The construction and characterization of a focusing X-ray spherical analyzer based on α-quartz 4{\bar 4}04 are presented. The performance of the analyzer was demonstrated by applying it to a high-resolution X-ray spectroscopy study of theKα1,2emission spectrum of Ni. An analytical representation based on physical grounds was assumed to model the shape of the X-ray emission lines. Satellite structures assigned to 3dspectator hole transitions were resolved and determined as well as their relative contribution to the emission spectrum. The present results on 1s−13d−1shake probabilities support a recently proposed calculation framework based on a multi-configuration atomic model.

Toroidal silicon polarization analyzer for resonant inelastic x-ray scattering

Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe for studying electronic excitations in materials. Standard high energy RIXS measurements do not measure the polarization of the scattered x-rays, which is unfortunate since it carries information about the nature and symmetry of the excitations involved in the scattering process. Here we report the fabrication of thin Si-based polarization analyzers with a double-concave toroidal surface, useful for L-edge RIXS studies in heavier atoms such as the 5-d transition metals.

Four-color laser diagnostics for Z-pinch and laser-produced plasma

Four-color laser diagnostics were developed for Z-pinch and laser plasma at the 1 MA pulsed power generator. Four harmonics of the Nd:YAG laser at wavelengths of 1064, 532, 266, and 213 nm were produced during the cascade conversion in three nonlinear crystals and propagated together in one beampath. Deep UV probing allows better penetration of the dense plasma. Laser probing at four wavelengths allows observation of plasma in a wide range of densities in one shot of the diagnostic laser. Examples of four-color laser shadowgraphy and interferometry of the wire-array load and laser plasma interaction are presented and discussed.

Spherical quartz crystals investigated with synchrotron radiation

The quality of x-ray spectra and images obtained from plasmas with spherically bent crystals depends in part on the crystal's x-ray diffraction across the entire crystal surface. We employ the energy selectivity and high intensity of synchrotron radiation to examine typical spherical crystals from alpha-quartz for their diffraction quality, in a perpendicular geometry that is particularly convenient to examine sagittal focusing. The crystal's local diffraction is not ideal: the most noticeable problems come from isolated regions that so far have failed to correlate with visible imperfections. Excluding diffraction from such problem spots has little effect on the focus beyond a decrease in background.

Performance of bent-crystal x-ray microscopes for high energy density physics research

We present calculations for the field of view (FOV), image fluence, image monochromaticity, spectral acceptance, and image aberrations for spherical crystal microscopes, which are used as self-emission imaging or backlighter systems at large-scale high energy density physics facilities. Our analytic results are benchmarked with ray-tracing calculations as well as with experimental measurements from the 6.151 keV backlighter system at Sandia National Laboratories. The analytic expressions can be used for x-ray source positions anywhere between the Rowland circle and object plane. This enables quick optimization of the performance of proposed but untested, bent-crystal microscope systems to find the best compromise between FOV, image fluence, and spatial resolution for a particular application.

Performance and Mix Measurements of Indirect Drive Cu-Doped Be Implosions

The ablator couples energy between the driver and fusion fuel in inertial confinement fusion (ICF). Because of its low opacity, high solid density, and material properties, beryllium has long been considered an ideal ablator for ICF ignition experiments at the National Ignition Facility. We report here the first indirect drive Be implosions driven with shaped laser pulses and diagnosed with fusion yield at the OMEGA laser. The results show good performance with an average DD neutron yield of ∼2×10^{9} at a convergence ratio of R_{0}/R∼10 and little impact due to the growth of hydrodynamic instabilities and mix. In addition, the effect of adding an inner liner of W between the Be and DD is demonstrated.

Analysis and implementation of a space resolving spherical crystal spectrometer for x-ray Thomson scattering experiments

The application of a space-resolving spectrometer to X-ray Thomson Scattering (XRTS) experiments has the potential to advance the study of warm dense matter. This has motivated the design of a spherical crystal spectrometer, which is a doubly focusing geometry with an overall high sensitivity and the capability of providing high-resolution, space-resolved spectra. A detailed analysis of the image fluence and crystal throughput in this geometry is carried out and analytical estimates of these quantities are presented. This analysis informed the design of a new spectrometer intended for future XRTS experiments on the Z-machine. The new spectrometer collects 6 keV x-rays with a spherically bent Ge (422) crystal and focuses the collected x-rays onto the Rowland circle. The spectrometer was built and then tested with a foam target. The resulting high-quality spectra prove that a spherical spectrometer is a viable diagnostic for XRTS experiments.

X-ray tests of a two-dimensional stigmatic imaging scheme with variable magnifications

A two-dimensional stigmatic x-ray imaging scheme, consisting of two spherically bent crystals, one concave and one convex, was recently proposed [M. Bitter et al., Rev. Sci. Instrum. 83, 10E527 (2012)]. The Bragg angles and the radii of curvature of the two crystals of this imaging scheme are matched to eliminate the astigmatism and to satisfy the Bragg condition across both crystal surfaces for a given x-ray energy. In this paper, we consider more general configurations of this imaging scheme, which allow us to vary the magnification for a given pair of crystals and x-ray energy. The stigmatic imaging scheme has been validated for the first time by imaging x-rays generated by a micro-focus x-ray source with source size of 8.4 μm validated by knife-edge measurements. Results are presented from imaging the tungsten Lα1 emission at 8.3976 keV, using a convex Si-422 crystal and a concave Si-533 crystal with 2d-spacings of 2.21707 Å and 1.65635 Å and radii of curvature of 500 ± 1 mm and 823 ± 1 mm, respectively, showing a spatial resolution of 54.9 μm. This imaging scheme is expected to be of interest for the two-dimensional imaging of laser produced plasmas.

Monochromatic radiography of high energy density physics experiments on the MAGPIE generator

A monochromatic X-ray backlighter based on Bragg reflection from a spherically bent quartz crystal has been developed for the MAGPIE pulsed power generator at Imperial College (1.4 MA, 240 ns) [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (2005)]. This instrument has been used to diagnose high energy density physics experiments with 1.865 keV radiation (Silicon He-α) from a laser plasma source driven by a ∼7 J, 1 ns pulse from the Cerberus laser. The design of the diagnostic, its characterisation and performance, and initial results in which the instrument was used to radiograph a shock physics experiment on MAGPIE are discussed.

Research on a logarithmically bent Laue crystal analyzer for X-ray monochromatic backlight imaging

A new logarithmically bent Laue imaging crystal analyzer (LBLICA) was proposed to obtain the monochromatic image of plasmas and exhibited a great potential for application in the Inertial Confinement Fusion experiment over a large field of view (FOV) and with a high spatial resolution. The imaging geometry of the LBLICA has been discussed. According to the Bragg condition and the equation of the logarithmic spiral, the key image parameters of the crystal analyzer, including the system magnification, the spatial resolution, and the FOV, have been analyzed theoretically. An experiment has been performed with a Cu target X-ray tube as a backlighter to backlight a mesh grid consisting of 50-μm Cu wires, and the monochromatic image of the grid has been obtained with a spatial resolution of approximately 30 μm.

Kirkpatrick-Baez microscope for hard X-ray imaging of fast ignition experiments

A Kirkpatrick-Baez X-ray microscope has been developed for use on the Titan laser facility at the Lawrence Livermore National Laboratory in Fast Ignition experiments. It was developed as a broadband alternative to narrow band Bragg crystal imagers for imaging Kα emission from tracer layers. A re-entrant design is employed which allows for alignment from outside the chamber. The mirrors are coated with Pt and operate at a grazing incident angle of 0.5° providing higher resolution than an equal brightness pinhole and sufficient bandwidth to image thermally shifted characteristic Kα emission from heated Cu tracer layers in Fast Ignition experiments. The superpolished substrates (<1 Å rms roughness) had a final visible wavelength roughness of 1.7 Å after coating, and exhibited a reflectivity corresponding to an X-ray wavelength roughness of 7 ± 1 Å. A unique feature of this design is that during experiments, the unfiltered direct signal along with the one-dimensional reflections are retained on the detector in order to enable a live indication of alignment and incident angle. The broad spectral window from 4 to 9 keV enables simultaneous observation of emission from several spectral regions of interest, which has been demonstrated to be particularly useful for cone-wire targets. An experimentally measured resolution of 15 μm has been obtained at the center of the field of view.

Investigation of plasma instabilities in the stagnated Z pinch

High-resolution laser probing diagnostics at a wavelength of 266 nm allow observation of the internal structure and instabilities in dense stagnated Z pinches, typically hidden by trailing material. The internal structure of the 1-MA Z pinch includes strong kink and sausage instabilities, loops, flares, and disruptions. Mid- and small-scale density perturbations develop in the precursor and main pinch. The three-dimensional shape and dynamics of the wire-array Z pinch are predetermined by the initial configuration of the wire array. Cylindrical, linear, and star wire-array Z pinches present different sets of instabilities seeded to the pinch at the implosion stage. Prolonged implosion of trailing mass can enhance x-ray production in wire arrays. Fast plasma motion with a velocity >100 km/s was observed in the Z pinch at stagnation with two-frame shadowgraphy. Development of instabilities in wire arrays is in agreement with three-dimensional magnetohydrodynamic simulations.

Imaging x-ray Thomson scattering spectrometer design and demonstration (invited)

In many laboratory astrophysics experiments, intense laser irradiation creates novel material conditions with large, one-dimensional gradients in the temperature, density, and ionization state. X-ray Thomson scattering is a powerful technique for measuring these plasma parameters. However, the scattered signal has previously been measured with little or no spatial resolution, which limits the ability to diagnose inhomogeneous plasmas. We report on the development of a new imaging x-ray Thomson spectrometer (IXTS) for the Omega laser facility. The diffraction of x-rays from a toroidally curved crystal creates high-resolution images that are spatially resolved along a one-dimensional profile while spectrally dispersing the radiation. This focusing geometry allows for high brightness while localizing noise sources and improving the linearity of the dispersion. Preliminary results are presented from a scattering experiment that used the IXTS to measure the temperature profile of a shocked carbon foam.

Measurement of the ionization state and electron temperature of plasma during the ablation stage of a wire-array Z pinch using absorption spectroscopy

Wire-array plasmas were investigated in the nonradiative ablation stage via x-ray absorption spectroscopy. A laser-produced Sm plasma was used to backlight Al wire arrays. The Sm spectrum was simultaneously observed by two spectrometers: one recorded the unattenuated spectrum and the other the transmission spectrum with 1.45-1.55 keV K-shell absorption lines. Analysis of absorption spectra revealed electron temperature in the range of 10-30 eV and the presence of F-, O-, N- and C-like Al ions in the absorbing plasma. A comparison of this electron temperature with the postprocessed absorption spectra of a 2D MHD simulation yields results in general agreement with the data analysis.

Note: Diagnosing femtosecond laser-solid interactions with monochromatic Kα imager and x-ray pinhole camera

An x-ray pinhole camera and a monochromatic K(α) imager are used to measure the interactions of intense femtosecond laser pulses with Cu foil targets. The two diagnostics give different features in the spot size and the laser energy scaling, which are resulted from different physical processes. Under our experimental conditions, the K(α) emission is mainly excited by the fast electrons transporting inside the cold bulk target. In contrast, the x-ray pinhole signals are dominated by the broadband thermal x-ray emission from the hot plasma at the front target surface.