A spherical crystal imager for OMEGA EP

Transport of an intense proton beam from a cone-structured target through plastic foam with unique proton source modeling

Laser-accelerated proton beams are applicable to several research areas within high-energy density science, including warm dense matter generation, proton radiography, and inertial confinement fusion, which all involve transport of the beam through matter. We report on experimental measurements of intense proton beam transport through plastic foam blocks. The intense proton beam was accelerated by the 10ps, 700J OMEGA EP laser irradiating a curved foil target, and focused by an attached hollow cone. The protons then entered the foam block of density 0.38g/cm^{3} and thickness 0.55 or 1.00mm. At the rear of the foam block, a Cu layer revealed the cross section of the intense beam via proton- and hot electron-induced Cu-K_{α} emission. Images of x-ray emission show a bright spot on the rear Cu film indicative of a forward-directed beam without major breakup. 2D fluid-PIC simulations of the transport were conducted using a unique multi-injection source model incorporating energy-dependent beam divergence. Along with postprocessed calculations of the Cu-K_{α} emission profile, simulations showed that protons retain their ballistic transport through the foam and are able to heat the foam up to several keV in temperature. The total experimental emission profile for the 1.0mm foam agrees qualitatively with the simulated profile, suggesting that the protons indeed retain their beamlike qualities.

Improved imaging using Mn He-α x rays at OMEGA EP

In this paper, we report on a crystal based x-ray imaging system fielded at the OMEGA EP laser facility. This new system has a pointing accuracy of +/100 μm, a temporal resolution down to 100 ps (depending on backlighter characteristics), variable magnification, and a spatial resolution of 21.9 µm at the object plane at a magnification of 15×. The system is designed to use a crystal along the crystal plane that satisfies the Bragg condition for the x ray of interest. The thin crystal is then bent into a spherical geometry and attached to a glass backing substrate to hold it in the diagnostic, and the x rays are imaged onto a charge coupled device. We report on data acquired with the new Los Alamos National Laboratory supplied spherical quartz crystal to image the Mn He-α 6.15 keV line emission.

Shock Ignition Laser-Plasma Interactions in Ignition-Scale Plasmas

We use a subignition scale laser, the 30 kJ Omega, and a novel shallow-cone target to study laser-plasma interactions at the ablation-plasma density scale lengths and laser intensities anticipated for direct drive shock-ignition implosions at National Ignition Facility scale. Our results show that, under these conditions, the dominant instability is convective stimulated Raman scatter with experimental evidence of two plasmon decay (TPD) only when the density scale length is reduced. Particle-in-cell simulations indicate this is due to TPD being shifted to lower densities, removing the experimental back-scatter signature and reducing the hot-electron temperature. The experimental laser energy-coupling to hot electrons was found to be 1%-2.5%, with electron temperatures between 35 and 45 keV. Radiation-hydrodynamics simulations employing these hot-electron characteristics indicate that they should not preheat the fuel in MJ-scale shock ignition experiments.

Four-channel toroidal crystal x-ray imager for laser-produced plasmas

The motion law of complex fluids under extreme conditions is an important aspect of high energy density physics research. It has been demonstrated that using multi-channel curved crystals and a framing camera to observe the laser-produced target pellets doped with tracer elements is an appropriate method for investigating this law. This paper presents a feasible design scheme for a multi-channel toroidal imager, with the ray trace model used to verify the rationality of the evaluation method and the aberration of single toroidal crystal imaging. We demonstrate that the field of view (FOV) consistency of the four-channel Ge(400) toroidal crystal imager is less than 50 µm, while the best spatial resolution is ∼4 µm and the FOV of each channel is >2.2 mm.

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.

X-ray backlighter requirements for refraction-based electron density diagnostics through Talbot-Lau deflectometry

Talbot-Lau x-ray interferometers can map electron density gradients in High Energy Density (HED) samples. In the deflectometer configuration, it can provide refraction, attenuation, elemental composition, and scatter information from a single image. X-ray backlighters in Talbot-Lau deflectometry must meet specific requirements regarding source size and x-ray spectra, amongst others, to accurately diagnose a wide range of HED experiments. 8 keV sources produced in the high-power laser and pulsed power environment were evaluated as x-ray backlighters for Talbot-Lau x-ray deflectometry. In high-power laser experiments, K-shell emission was produced by irradiating copper targets (500 × 500 × 12.5 μm³ foils, 20 μm diameter wire, and >10 μm diameter spheres) with 30 J, 8-30 ps laser pulses and a 25 μm copper wire with a 60 J, 10 ps laser pulse. In the pulsed power environment, single (2 × 40 μm) and double (4 × 25 μm) copper x-pinches were driven at ∼1 kA/ns. Moiré fringe formation was demonstrated for all x-ray sources explored, and detector performance was evaluated for x-ray films, x-ray CCDs, and imaging plates in context of spatial resolution, x-ray emission, and fringe contrast.

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.

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.

Eight-channel Kirkpatrick-Baez microscope for multiframe x-ray imaging diagnostics in laser plasma experiments

Because grazing-incidence Kirkpatrick-Baez (KB) microscopes have better resolution and collection efficiency than pinhole cameras, they have been widely used for x-ray imaging diagnostics of laser inertial confinement fusion. The assembly and adjustment of a multichannel KB microscope must meet stringent requirements for image resolution and reproducible alignment. In the present study, an eight-channel KB microscope was developed for diagnostics by imaging self-emission x-rays with a framing camera at the Shenguang-II Update (SGII-Update) laser facility. A consistent object field of view is ensured in the eight channels using an assembly method based on conical reference cones, which also allow the intervals between the eight images to be tuned to couple with the microstrips of the x-ray framing camera. The eight-channel KB microscope was adjusted via real-time x-ray imaging experiments in the laboratory. This paper describes the details of the eight-channel KB microscope, its optical and multilayer design, the assembly and alignment methods, and results of imaging in the laboratory and at the SGII-Update.

An x-ray backlit Talbot-Lau deflectometer for high-energy-density electron density diagnostics

X-ray phase-contrast techniques can measure electron density gradients in high-energy-density plasmas through refraction induced phase shifts. An 8 keV Talbot-Lau interferometer consisting of free standing ultrathin gratings was deployed at an ultra-short, high-intensity laser system using K-shell emission from a 1-30 J, 8 ps laser pulse focused on thin Cu foil targets. Grating survival was demonstrated for 30 J, 8 ps laser pulses. The first x-ray deflectometry images obtained under laser backlighting showed up to 25% image contrast and thus enabled detection of electron areal density gradients with a maximum value of 8.1 ± 0.5 × 10(23) cm(-3) in a low-Z millimeter sized sample. An electron density profile was obtained from refraction measurements with an error of <8%. The 50 ± 15 μm spatial resolution achieved across the full field of view was found to be limited by the x-ray source-size, similar to conventional radiography.

High Miller-index germanium crystals for high-energy x-ray imaging applications

Near-normal-incidence bent crystals are widely used for x-ray imaging applications. Advantages include high collection solid angle and potentially high efficiency for narrow-band sources, while disadvantages include relatively large (several Å) interatomic spacings and a limited number of suitable matches between a crystal 2d value and an integral multiple of useful emission line wavelengths. The disadvantages become more significant at x-ray energies >10  keV. The former disadvantage can be mitigated by using high-order reflections from crystal planes having low Miller indices, but both disadvantages can be mitigated by using low-order reflections from crystal planes having high Miller indices. We report here on integrated reflectivity measurements we performed of Ge (15,7,7) (2d=0.6296  Å), a candidate for imaging Ru He-α (θ(B)=87°). We find good agreement with calculations, and the data show a multitude of closely spaced reflections with slightly different Bragg angles including a fifth-order reflection of Ge (3,1,1) that has comparable reflectivity. This demonstrates that arbitrary choices of Miller indices in Ge crystals can be used to fine-tune Bragg angles for near-normal-incidence x-ray imaging at tens of kiloelectron volt x-ray energies with minimal lower-energy contamination from lower-order reflections, and that existing calculational tools can be used to reliably estimate integrated reflectivity.

Observation of Single-Mode, Kelvin-Helmholtz Instability in a Supersonic Flow

We report the first observation, in a supersonic flow, of the evolution of the Kelvin-Helmholtz instability from a single-mode initial condition. To obtain these data, we used a novel experimental system to produce a steady shock wave of unprecedented duration in a laser-driven experiment. The shocked, flowing material creates a shear layer between two plasmas at high energy density. We measured the resulting interface structure using radiography. Hydrodynamic simulations reproduce the large-scale structures very well and the medium-scale structures fairly well, and imply that we observed the expected reduction in growth rate for supersonic shear flow.

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.

Soft x-ray backlighting of cryogenic implosions using a narrowband crystal imaging system (invited)

A high-performance cryogenic DT inertial confinement fusion implosion experiment is an especially challenging backlighting configuration because of the high self-emission of the core at stagnation and the low opacity of the DT shell. High-energy petawatt lasers such as OMEGA EP promise significantly improved backlighting capabilities by generating high x-ray intensities and short emission times. A narrowband x-ray imager with an astigmatism-corrected bent quartz crystal for the Si Heα line at ∼1.86 keV was developed to record backlit images of cryogenic direct-drive implosions. A time-gated recording system minimized the self-emission of the imploding target. A fast target-insertion system capable of moving the backlighter target ∼7 cm in ∼100 ms was developed to avoid interference with the cryogenic shroud system. With backlighter laser energies of ∼1.25 kJ at a 10-ps pulse duration, the radiographic images show a high signal-to-background ratio of >100:1 and a spatial resolution of the order of 10 μm. The backlit images can be used to assess the symmetry of the implosions close to stagnation and the mix of ablator material into the dense shell.

Note: Characterization of a high-photon-energy X-ray imager

The Bragg angle, rocking curve, and reflection efficiency of a quartz crystal x-ray imager (Miller indices 234) were measured at photon energy of 15.6909 keV, corresponding to the K(α2) line of Zr, using the X15A beamline at the National Synchrotron Light Source at Brookhaven National Laboratory. One flat and three spherically curved samples were tested. The peak reflectivity of the best-performing crystal was determined to be (3.6 ± 0.7) × 10(-4) with a rocking-curve full width at half maximum of 0.09°. The Zr K(α2) emission was imaged from a hot Zr plasma generated by a 10-J multiterawatt laser.

Soft x-ray backlighting of direct-drive implosions using a spherical crystal imager on OMEGA

Using a spherically bent quartz crystal for the Si He(α) line at ~1.865 keV, a narrowband x-ray imager has been deployed at the Omega Laser Facility to record backlit images of direct-drive laser implosions. The crystal was cut along the 1011 planes for a 2d spacing of 0.687 nm, resulting in a Bragg angle of 83.9°. Apertures in front of the crystal were used to control the astigmatism of the imaging system. The backlit images show a high signal-to-background ratio of >10:1 with backlighter laser energies ≥1.5 kJ at a 10-ps pulse duration and a spatial resolution of better than 20 μm.

Note: spatial resolution of Fuji BAS-TR and BAS-SR imaging plates

The spatial resolution of two types of imaging plates, Fuji BAS-TR and Fuji BAS-SR, has been measured using a knife-edge x-ray source of 8-keV Cu K(α) radiation. The values for the spatial resolution, defined as the distance between 10% and 90% levels of the edge spread function, are 94 μm and 109 μm, respectively. The resolution values are important for quantitative analysis of x-ray and particle imaging and spectroscopic diagnostics.