Search

The time-resolved x-ray diffraction platform at the National Ignition Facility (NIF) fields electronic sensors closer to the exploding laser-driven target than any other NIF diagnostic in order to directly detect diffracted x rays from highly compressed materials. We document strategies to characterize and mitigate the unacceptably high background signals observed in this geometry. We specifically assess the possible effects of electromagnetic pulse, x-ray fluorescence, hot electrons, and sensor-specific non-x-ray artifacts. Significant background reduction is achieved by strategic shielding. [ABSTRACT FROM AUTHOR]

We present the development of an experimental platform that can collect four frames of x-ray diffraction data along a single line of sight during laser-driven, dynamic-compression experiments at the National Ignition Facility. The platform is comprised of a diagnostic imager built around ultrafast sensors with a 2-ns integration time, a custom target assembly that serves also to shield the imager, and a 10-ns duration, quasi-monochromatic x-ray source produced by laser-generated plasma. We demonstrate the performance with diffraction data for Pb ramp compressed to 150 GPa and illuminated by a Ge x-ray source that produces ∼7 × 1011, 10.25-keV photons/ns at the 400 μm diameter sample. [ABSTRACT FROM AUTHOR]

This study investigates methods to optimize quasi-monochromatic, ∼10 ns long x-ray sources (XRS) for time-resolved x-ray diffraction measurements of phase transitions during dynamic laser compression measurements at the National Ignition Facility (NIF). To support this, we produce continuous and pulsed XRS by irradiating a Ge foil with NIF lasers to achieve an intensity of 2 × 1015 W/cm2, optimizing the laser-to-x-ray conversion efficiency. Our x-ray source is dominated by Ge He-α line emission. We discuss methods to optimize the source to maintain a uniform XRS for ∼10 ns, mitigating cold plasma and higher energy x-ray emission lines. [ABSTRACT FROM AUTHOR]

We present the results of experiments to produce a 10 ns-long, quasi-monochromatic x-ray source. This effort is needed to support time-resolved x-ray diffraction (XRDt) measurements of phase transitions during laser-driven dynamic compression experiments at the National Ignition Facility. To record XRDt of phase transitions as they occur, we use high-speed (∼1 ns) gated hybrid CMOS detectors, which record multiple frames of data over a timescale of a few to tens of ns. Consequently, to make effective use of these imagers, XRDt needs the x-ray source to be narrow in energy and uniform in time as long as the sensors are active. The x-ray source is produced by a laser irradiated Ge foil. Our results indicate that the x-ray source lasts during the whole duration of the main laser pulse. Both time-resolved and time-integrated spectral data indicate that the line emission is dominated by the He-α complex over higher energy emission lines. Time-integrated spectra agree well with a one-dimensional Cartesian simulation using HYDRA that predicts a conversion efficiency of 0.56% when the incident intensity is 2 × 1015 W/cm2 on a Ge backlighter. [ABSTRACT FROM AUTHOR]

In pinhole-assisted point-projection x-ray radiography (or “backlighting”), pinholes are placed between the sample of interest and an x-ray source (or “backlighter”) to effectively limit the source size and hence improve the spatial resolution of the system. Pinholes are generally placed close to such x-ray backlighters to increase the field of view, leading to possible vaporization and pinhole closure due to x-ray driven ablation, thereby potentially limiting the usefulness of this method. An experimental study and modeling of time-dependent closure and resolution is presented. The pinhole closure time scale is studied for various pinhole sizes, pinhole-to-backlighter separations, and filtering conditions. In addition the time-dependent resolution is extracted from one-dimensional wire imaging prior to pinhole closure. Cylindrical hydrodynamic modeling of the pinhole closure shows reasonable agreement with data, giving us a predictive capability for pinhole closure in future experiments. [ABSTRACT FROM AUTHOR]

At the National Ignition Facility, the symmetry of the hot spot of imploding capsules is diagnosed by imaging the emitted x-rays using gated cameras and image plates. The symmetry of an implosion is an important factor in the yield generated from the resulting fusion process. The x-ray images are analyzed by decomposing the image intensity contours into Fourier and Legendre modes. This paper focuses on the additional protocols for the time-integrated shape analysis from image plates. For implosions with temperatures above ~4 keV, the hard x-ray background can be utilized to infer the temperature of the hot spot. [ABSTRACT FROM AUTHOR]

The performance of Inertial Confinement Fusion targets relies on the symmetric implosion of highly compressed fuel. X-ray area-backlit imaging is used to assess in-flight low mode 2D asymmetries of the shell. These time-resolved images of the shell exhibit features that can be related to the lift-off position of the membranes used to hold the capsule within the hohlraum. Here, we describe a systematic study of this membrane or "tent" thickness and its impact on the measured low modes for in-flight and self-emission images. The low mode amplitudes of the shell in-flight shape (P2 and P4) are weakly affected by the tent feature in time-resolved, backlit data. By contrast, time integrated self-emission images along the same axis exhibit a reversal in perceived P4 mode due to growth of a feature seeded by the tent, which can explain prior inconsistencies between the in-flight P4 and core P4, leading to a reevaluation of optimum hohlraum length. Simulations with a tent-like feature normalized to match the feature seen in the backlit images predict a very large impact on the capsule performance from the tent feature. [ABSTRACT FROM AUTHOR]

Two-dimensional radiographs of imploding fusion capsules are obtained at the National Ignition Facility by projection through a pinhole array onto a time-gated framing camera. Parallax among images in the image array makes it possible to distinguish contributions from the capsule and from the backlighter, permitting correction of backlighter non-uniformities within the capsule radiograph. Furthermore, precise determination of the imaging system geometry and implosion velocity enables combination of multiple images to reduce signal-to-noise and discover new capsule features. [ABSTRACT FROM AUTHOR]

Surrogate implosions play an important role at the National Ignition Facility (NIF) for isolating aspects of the complex physical processes associated with fully integrated ignition experiments. The newly developed CD Symcap platform has been designed to study gas-shell mix in indirectly driven, pure T2-gas filled CH-shell implosions equipped with 4 ~m thick CD layers. This configuration provides a direct nuclear signature of mix as the DT yield (above a characterized D contamination background) is produced by D from the CD layer in the shell, mixing into the T-gas core. The CD layer can be placed at different locations within the CH shell to probe the depth and extent of mix. CD layers placed flush with the gas-shell interface and recessed up to 8 ~m have shown that most of the mix occurs at the inner-shell surface. In addition, time-gated x-ray images of the hotspot show large brightly radiating objects traversing through the hotspot around bang-time, which are likely chunks of CH/CD plastic. This platform is a powerful new capability at the NIF for understanding mix, one of the key performance issues for ignition experiments. [ABSTRACT FROM AUTHOR]

A gated x-ray detector is under development for use at the National Ignition Facility that is intended to provide plasma emission images in the presence of neutron yields up to 1015 expected during inertial confinement fusion experiments with layered cryogenic targets. These images are expected to provide valuable time-resolved measurements of core and fuel symmetries. Additional capabilities of this instrument will include the ability to make spatially resolved electron temperature measurements. A description of this instrument and its operation is given with emphasis on features that differentiate it from previous designs. [ABSTRACT FROM AUTHOR]

We have designed and fabricated a structured streak camera photocathode to provide enhanced efficiency for high energy X-rays (1-12 keV). This gold coated photocathode was tested in a streak camera and compared side by side against a conventional flat thin film photocathode. Results show that the measured electron yield enhancement at energies ranging from 1 to 10 keV scales well with predictions, and that the total enhancement can be more than 3×. The spatial resolution of the streak camera does not show degradation in the structured region. We predict that the temporal resolution of the detector will also not be affected as it is currently dominated by the slit width. This demonstration with Au motivates exploration of comparable enhancements with CsI and may revolutionize X-ray streak camera photocathode design. [ABSTRACT FROM AUTHOR]

Single hit pulse height analysis is applied to National Ignition Facility x-ray framing cameras to quantify gain and gain variation in a single micro-channel plate-based instrument. This method allows the separation of gain from detectability in these photon-detecting devices. While pulse heights measured by standard-DC calibration methods follow the expected exponential distribution at the limit of a compound-Poisson process, gain-gated pulse heights follow a more complex distribution that may be approximated as a weighted sum of a few exponentials.We can reproduce this behavior with a simple statistical-sampling model. [ABSTRACT FROM AUTHOR]

We present initial characterization data from a new single-line-of-sight (SLOS) x-ray framing camera. The instrument uses an image-dissecting structure inside an electron optic tube to produce up to four simultaneous dc images from a single image incident on the cathode and a microchannel plate-based device to provide the temporal gating of those images. A series of gated images have been obtained using a short-pulse UV laser source, and the spatial resolution of those images is compared to those obtained using a more traditional-microchannel plate based system. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In an indirectly driven implosion, non-radial translational motion of the compressed fusion capsule is a signature of residual kinetic energy not coupled into the compressional heating of the target. A reduction in compression reduces the peak pressure and nuclear performance of the implosion. Measuring and reducing the residual motion of the implosion is therefore necessary to improve performance and isolate other effects that degrade performance. Using the gated x-ray diagnostic, the x-ray Bremsstrahlung emission from the compressed capsule is spatially and temporally resolved at x-ray energies of >8.7 keV, allowing for measurements of the residual velocity. Here details of the x-ray velocity measurement and fitting routine will be discussed and measurements will be compared to the velocities inferred from the neutron time of flight detectors. [ABSTRACT FROM AUTHOR]

The static x-ray imager at the National Ignition Facility is a pinhole camera using a CCD detector to obtain images of Hohlraum wall x-ray drive illumination patterns seen through the laser entrance hole (LEH). Carefully chosen filters, combined with the CCD response, allow recording images in the x-ray range of 3-5 keV with 60 μm spatial resolution. The routines used to obtain the apparent size of the backlit LEH and the location and intensity of beam spots are discussed and compared to predictions. A new soft x-ray channel centered at 870 eV (near the x-ray peak of a 300 eV temperature ignition Hohlraum) is discussed. [ABSTRACT FROM AUTHOR]

High-speed microstrip microchannel plate (MCP) x-ray framing cameras are a well established diagnostic for laser plasma experiments. Each frame acquired with these devices requires a separate image, and with most reasonable x-ray optics, a separate line of sight, causing potential parallax problems. Gated image tubes have a single line of sight capability, but the conventional designs have not been effectively extended to the short gating times of the microstrip-line MCP camera. A hybrid camera combining image tube and microstrip-line MCP technology has been under development at Lawrence Livermore National Lab in collaboration with University of Rochester Lab for Laser energetics, and KENTECH Instruments. The key feature of this single line of sight hybrid image tube is a deflection assembly that continuously divides the electrons from a single photocathode x-ray image into a set of four electron images. Temporal gating of these images is carried out using a microstrip-line microchannel plate framing camera module positioned at the image plane of the electron tube. Characterization measurements performed using both x rays from a Manson source and from laser generated plasmas, will be presented. Some implementation improvements will be discussed. [ABSTRACT FROM AUTHOR]

High-speed microchannel plate (MCP) x-ray framing cameras are a well established diagnostic for laser plasma experiments. Each frame acquired with these devices requires a separate image, and with most reasonable x-ray optics, a separate line of sight, causing potential parallax problems. Gated image tubes have a single line of sight capability, but the conventional designs have not been effectively extended to the short gating times of the microstrip MCP camera. A hybrid camera combining image tube and microstrip MCP technology has been under development at Lawrence Livermore National Laboratory in collaboration with the University of Rochester, Laboratory for Laser Energetics, and KENTECH Instruments. Characterization measurements using laser generated plasma x rays are presented. Simulations as a guide to improving this hybrid camera are also presented. [ABSTRACT FROM AUTHOR]

This letter describes the first use of a laser plasma x-ray source for the recording of surface reflection EXAFS (extended x-ray absorption fine structure), or reflexafs. A powerful technique in which a subnanosecond pulse of x rays is used to record the reflection spectrum over a range of incidence angles and wavelengths in a single shot is described. Reflexafs spectra for aluminum are presented which compare favorably with spectra recorded in transmission mode. [ABSTRACT FROM AUTHOR]

For energetic x rays (hv ≥ 3 keV), ring coded-aperture imaging gives better signal-to-noise ratio than equivalent-resolution pinhole cameras for inertial confinement fusion (ICF) targets. We have created a time-resolved ring coded-aperture microscope by combining a 4 × 3 array of annular apertures with a gated microchannel-ptate x-ray imager. The new instrument can produce 500-ps duration sequences of images with a temporal resolution of 80 ps and a spatial resolution of 5 to 6 µm. In demonstration experiments, coded images of imploded targets directly driven by the Omega laser at the Laboratory for Laser Energetics showed the formation and dissolution of the target cores with improved detail. Contour plots of the images indicate that the laser illumination pattern is imprinted in the imploded core region. The gated ring-aperture microscope will be useful in detailed studies of ICF target hydrodynamics. [ABSTRACT FROM AUTHOR]

A correction to the article "Image-plate sensitivity to x rays at 2 to 60 keV" in the 2019 issue, number 90, is presented which discusses two references concerning calibration and modeling of image plate (IP) sensitivity omitted in the article.

Measuring the shape of implosions is critical to inertial confinement fusion experiments at the National Ignition Facility. We have developed techniques that have proven successful for extracting shape information from images of x-ray self-emission recorded by a variety of diagnostic instruments for both DT-filled targets and low-yield surrogates. These key results help determine optimal laser and target parameters leading to ignition. We have compensated for instrumental response and have employed a variety of image processing methods to remove artifacts from the images while retaining salient features. The implosion shape has been characterized by decomposing intensity contours into Fourier and Legendre modes for different lines of sight. We also describe procedures we have developed for estimating uncertainties in these measurements. [ABSTRACT FROM AUTHOR]

Gated and streaked x-ray detectors generally require corrections in order to counteract instrumental effects in the data. The method of correcting for gain variations in gated cameras fielded at National Ignition Facility (NIF) is described. Four techniques for characterizing the gated x-ray detectors are described. The current principal method of characterizing x-ray instruments is the production of controlled x-ray emission by laser-generated plasmas as a dedicated shot at the NIF. A recently commissioned pulsed x-ray source has the potential to replace the other characterization systems. This x-ray source features a pulsed power source consisting of a Marx generator, capacitor bank that is charged in series and discharged in parallel, producing up to 300 kV. The pulsed x-ray source initially suffered from a large jitter (∼60 ns), but the recent addition of a pulsed laser to trigger the spark gap has reduced the jitter to ∼5 ns. Initial results show that this tool is a promising alternative to the other flat fielding techniques. [ABSTRACT FROM AUTHOR]

Initial laser-driven equation of state (EOS) experiments on liquid deuterium employed x-ray radiography to track the shock and particle speeds in the shock compressed sample. With the high pressures available with laser drivers we found that it is also possible to track the shock front directly with a velocity interferometer system for any reflector (VISAR) because the shock front reflects light across the visible spectrum with reflectance around 50% for shocks stronger than 50 GPa in liquid deuterium. We have observed similar reflectances in other dielectric samples, such as diamond, LiF, and water. The pressure required to produce a reflecting shock varies with each material. This phenomenon allows us to design impedance-matched EOS experiments using velocity interferometry to measure the propagation speed in the transparent shocked materials, and step breakout measurements to determine the speed in the pusher. In a different kind of experiment we have observed double shock compression in liquid deuterium by impacting a shock in liquid deuterium at a LiF anvil placed in the liquid sample. VISAR can be used to track the shock in the deuterium as well as the motion of the deuterium–LiF interface subsequent to impact. This allows us to diagnose double-shock states using the VISAR technique. As a final example VISAR can be used to track shock overtake events such as produced by shaped pulse compression or shock reverberation effects in the accelerating pusher. This capability is directly applicable to shock timing experiments needed to tune the drive pulse for inertial confinement fusion capsules on the National Ignition Facility. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The sensitivity of inertial confinement fusion implosions, of the type performed on the National Ignition Facility (NIF) [1], to low-mode flux asymmetries is investigated numerically. It is shown that large-amplitude, low-order mode shapes (Legendre polynomial P4), resulting from low-order flux asymmetries, cause spatial variations in capsule and fuel momentum that prevent the deuterium and tritium (DT) "ice" layer from being decelerated uniformly by the hot spot pressure. This reduces the transfer of implosion kinetic energy to internal energy of the central hot spot, thus reducing the neutron yield. Furthermore, synthetic gated x-ray images of the hot spot self-emission indicate that P4 shapes may be unquantifiable for DT layered capsules. Instead the positive P4 asymmetry "aliases" itself as an oblate P2 in the x-ray images. Correction of this apparent P2 distortion can further distort the implosion while creating a round x-ray image. Long wavelength asymmetries may be playing a significant role in the observed yield reduction of NIF DT implosions relative to detailed postshot two-dimensional simulations. [ABSTRACT FROM AUTHOR]

We have imaged hard x-ray (> 100 keV) bremsstrahlung emission from energetic electrons slowing in a plastic ablator shell during indirectly driven implosions at the National Ignition Facility. We measure 570 J in electrons with E > 100 keV impinging on the fusion capsule under ignition drive conditions. This translates into an acceptable increase in the adiabat a, defined as the ratio of total deuterium-tritium fuel pressure to Fermi pressure, of 3.5%. The hard x-ray observables are consistent with detailed radiative-hydrodynamics simulations, including the sourcing and transport of these high energy electrons. [ABSTRACT FROM AUTHOR]