Session GO7: X-Ray Diagnostics and Measurement Techniques

Electron temperature from x-ray continuum measurements on the NIF

We report on measurements of the electron temperature within the hot spot of inertially confined, layered implosions on the NIF using a titanium differential filtering x-ray diagnostic. The electron temperature from x-ray emission is insensitive to non-thermal velocity flows as is the case with ion temperature measurements and is thus a critical parameter in interpreting stagnated hot spot conditions. Here we discuss measurements using titanium filters ranging from 10$\mu $m to 1mm in thickness with a sensitivity band of 10-30keV coupled with penumbral pinholes. The use of larger pinhole diameters increases x-ray fluence improving sensitivity of photon energies with minimal attenuation from the compressed fuel/shell. This diagnostic has been fielded on a series of cryogenic shots with DT ion temperatures ranging from 2-5keV. Analysis of the measurement will be presented along with a comparison against simulated electron temperatures and x-ray spectra as well as a comparison to DT ion temperature measurements. This work was performed under the auspices of U.S. DoE by LLNL under Contract No. DE-AC52-07NA27344.   

Development of a High Resolution X-ray Spectrometer on the National Ignition Facility

A high-resolution x-ray spectrometer has been designed, calibrated, and deployed on the National Ignition Facility (NIF) to measure plasma parameters for a Kr-doped surrogate capsule imploded at NIF conditions. Two conical crystals, each diffracting the He$\alpha $ and He$\beta $ complexes respectively, focus the spectra onto a steak camera photocathode for time-resolved measurements with a temporal resolution of \textless 20 ps. A third cylindrical crystal focuses the entire He$\alpha $ to He$\beta $ spectrum onto an image plate for a time-integrated spectrum to correlate the two streaked signals. The instrument was absolutely calibrated by the x-ray group at the Princeton Plasma Physics Laboratory using a micro-focus x-ray source. Detailed calibration procedures, including source and spectrum alignment, energy calibration, crystal performance evaluation, and measurement of the resolving power and the integrated reflectivity will be presented. Initial NIF experimental results will also be discussed.   

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 being developed for the NIF, which will allow ICF capsule implosions to be radiographed close to stagnation for the first time. This has not been possible using the previous pinhole-based area-backlighter configuration, as the self-emission from the capsule hotspot overwhelms the backlighter in the final stages of the implosion. CBI mitigates the broadband self-emission from the capsule hot spot by using the extremely narrow bandwidth (a few eV) inherent to imagers based on near-normal-incidence Bragg x-ray optics. The development of a diagnostic with the capability to image the capsule during the final stages of the implosion (r less than 200um) is important, as it will allow the shape, integrity and density of the shell to be measured, and will allow the evolution of features, such as the fill tube and capsule support structure, to be imaged close to bang time. The concept and operation of the 11.6keV CBI diagnostic will be discussed, and the first results from experiments on the NIF will be presented. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Progress in using imaging x-ray spectroscopy to diagnose interspecies ion separation in inertial confinement fusion experiments

We discuss the reconstruction of spatial profiles of ion densities and plasma conditions based on analyses of spatially resolved x-ray pinhole images of Ar-doped, D$_{\mathrm{2}}$-filled OMEGA direct-drive ICF implosions. The targets are 15-\textmu m spherical plastic shells filled with varying D$_{\mathrm{2}}$-Ar relative and total gas pressures. Ar K-shell spectral features are observed primarily between the time of first-shock convergence and slightly before neutron bang time, using a time- and space-integrated spectrometer (XRS2), streaked crystal spectrometer (SSCA), and up to three gated multi-monochromatic X-ray imagers (MMI) fielded along three different lines of sight. Uncertainties and validation of our analysis method using synthetic data are discussed. We report preliminary analysis of data from the IonSepMMI-17A OMEGA campaign, which adds a third MMI and aims for more-symmetric implosions than our prior campaign [1, 2]. Anticipated results are improved observations of strong and weak interspecies-ion-separation depending on the target fill. [1] S. C. Hsu et al, EPL \textbf{115}, 65001 (2016). [2] T. Joshi et al., PoP \textbf{24}, 056305 (2017). LA-UR-17-25780   

Inference of the electron temperature in ICF implosions from the hard X–ray spectral continuum

The NIF Continuum Spectrometer, scheduled to be first deployed in Fall of 2017, will infer the imploded core electron temperature from the free-free continuum self-emission spectra of photons with energies of 20 to 30 keV. However, this hard X-ray radiation is emitted by the tail of the electron distribution, which likely deviates from Maxwellian and thus obscures interpretation of the data. We investigate resulting modifications to the X-ray spectra. The logarithmic slope of the spectrum from the more realistic, non-thermal tail of the electron distribution is found to decrease more rapidly at higher photon energies, as compared to the perfectly Maxwellian case. Interpreting the spectrum with assumption of Maxwellian electrons enforced is shown to give an electron temperature that is lower than the actual one. Conversely, due to its connection with the non-thermal features in the electron distribution, hard X-ray emission can provide unprecedented information about kinetic processes in the hot DT core.   

Inferred UV Fluence Focal-Spot Profiles from Soft X-Ray Pinhole Camera Measurements on OMEGA

The drive uniformity of OMEGA cryogenic implosions is affected by UV beam{\-}fluence variations on target, which require careful monitoring at full laser power. This is routinely performed with multiple pinhole cameras equipped with charge-injection devices (CID's) that record the x-ray emission in the $\sim $3- to 7-keV photon energy range from an Au-coated target. The technique relies on the knowledge of the relation between x-ray fluence $F_{x} $ and UV fluence $F_{\mbox{UV}} , \quad F_{x} \sim F_{\mbox{UV}}^{\gamma } ,$ with a measured $\gamma =3.42$ for the CID-based diagnostic and 1-ns laser pulse.\footnote{F. J. Marshall \textit{et al.}, Phys. Plasmas \textbf{11}, 251 (2004).\par } It is demonstrated here that using a back-thinned charge-coupled--device camera with softer filtration for x-rays with photon energies \textless 2 keV and well calibrated pinhole provides a lower $\gamma \sim 2$ and a larger dynamic range in the measured UV fluence. Inferred UV fluence profiles were measured for 100-ps and 1-ns laser pulses and were compared to directly measured profiles from a UV equivalent-target-plane diagnostic. Good agreement between both techniques is reported for selected beams. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

X-ray imaging spectroscopic diagnostics on Nike

Electron temperature and density diagnostics of the laser plasma produced within the focal spot of the NRL's Nike laser are being explored with the help of X-ray imaging spectroscopy. Spectra of He-like and H-like ions were taken by Nike focusing spectrometers in a range of lower (1.8 kev, Si XIV) and higher (6.7 kev, Fe XXV) x-ray energies. Data that were obtained with spatial resolution were translated into the temperature and density as functions of distance from the target. As an example electron density was determined from He-like satellites to Ly-alpha in Si XIV. The dielectronic satellites with intensity ratios that are sensitive to collisional transfer of population between different triplet groups of double-excited states 2l2l' in Si XIII were observed with high spatial and spectral resolution Lineouts taken at different axial distances from the planar Si target show changing spectral shapes due to the different electron densities as determined by supporting non-LTE simulations. These shapes are relatively insensitive to the plasma temperature which was measured using different spectral lines.   

Conduction-Zone Measurements Using X-Ray Self-Emission Images

Time-gated soft x-ray self-emission images of directly driven implosions were measured to probe the hydrodynamic conditions between the critical-density surface and the ablation front of a CH target (conduction zone) at the beginning of a laser pulse. The self-emission at each point in the coronal plasma depends on the local electron temperature and the ion density, and the intensity measured at the diagnostic plane is the line-integrated emissivity through the target. Measured 2-D images of spherically symmetric implosions were angularly averaged and compared with synthetic self-emission profiles generated from 1-D hydrodynamic simulations to benchmark the hydrodynamic parameters in the corona. This comparison was performed for a range of times early in the implosion to study the formation and evolution of the conduction zone. This measurement is significant for inertial confinement fusion since it governs the length of time that the plasma is too small to provide substantial beam smoothing through thermal conduction, determining the laser imprint efficiency. The conduction zone has previously proven challenging to probe because the density is too high for optical diagnostics and because the temperature is too high for x-ray radiography.   

Crystal and source characterization for the Crystal Backlighter Imager capability at the National Ignition Facility

The Crystal Backlighter Imager (CBI) is a very narrow bandwidth (\textasciitilde 10 eV) x-ray radiography system that uses Bragg reflection from a spherically-curved crystal at near normal incidence. This diagnostic has the capability to image late in an ICF implosion because it only requires the brightness of the backlighter to be larger than the capsule self-emission in that narrow bandwidth. While the limited bandwidth is advantageous for this reason, it also requires that the effective energy of the backlighter atomic line is known to \textasciitilde 1 eV accuracy for proper crystal alignment. Any Doppler shift in the line energy must be understood for the imaging system to work. The work presented details characterization experiments done at the Jupiter Laser Facility with a Si (8 6 2) crystal that will be used with a Selenium backlighter in the NIF CBI diagnostic. We used the spherically-bent crystals to image a small (\textasciitilde 200 \textmu m) He$\alpha $ source generated by the Janus laser on a Se foil. Scanning Bragg angles over multiple shots allowed us to map out the spectral line intensity distribution for optimal alignment in NIF. A subsequent Doppler shift measurement using CBI on NIF will also be presented with complementary HYDRA modeling for both experiments.   

Analyzing x-ray hotspot images with Ince-Gaussian modes

X-ray images at the National Ignition Facility (NIF) provide important metrics regarding the shape of the hotspot along a given line-of-sight. The 17\% contour from peak brightness is usually used to infer the size of the hotspot as well as determine shape perturbations quantified through the Legendre coefficients $P_2$ and $P_4$. Unfortunately features that lie inside the contour such as those that could arise from tent or fill-tube perturbations are not easily captured. An analysis that takes into account the two-dimensional nature of the x-ray image is desirable. Ince-Gaussian modes (for short: Ince) offer such an analysis and could provide a new way to encode and understand the images recorded at NIF. The Ince modes are the solutions to the paraxial wave equation expressed in elliptical coordinates and thus form an orthonormal basis. Due to their elliptical nature they are suitable for decomposing images that have a non-zero $P_2$ or $P_4$ coefficient. We show that the Ince modes can be used to uncover structure that is missed by the contour analysis and how the modes aid in compressing images produced in large ensemble calculations. Finally a comparison is made to the Zernike modes which form an orthonormal basis on a circular disk.   

Recent Progress in Target Metrology at General Atomics

Targets are central to all ICF/HED programs. Many target specifications are so tight or specialized that the measurements cannot be performed on commercial equipment. General Atomics continues to provide on-demand target metrology development to support the evolving needs of the community. In this talk, we will present our latest efforts in new instrument design, equipment automation and data analysis technique development. Examples include a dark-field imaging algorithm to measure ablator defects down to 0.1um size required by Laboratory for Laser Energetics direct drive program, a full-surface wall-thickness mapper that enables polystyrene shell development, an integrated set up for GDP dome mapping and removal, an automated x-ray absorption spectroscope to improve the precision and accuracy of dopant measurement, a hohlraum interior surface inspection technique, optical transmission characterization of thin metallic films for micro-dots hohlraum diagnostic platform, new high-resolution NEXIV pin-hole array characterization, etc.   

Synthetic Pulse Dilation – PMT Model for high bandwidth gamma measurements

The Cherenkov mechanism used in Gas Cherenkov Detectors (GCD) is exceptionally fast. However, the temporal resolution of GCDs, such as the Gamma Reaction History diagnostic (GRH), is limited by the current state-of-the-art photomultiplier tube (PMT) to $\sim$100 ps. The new pulse dilation – PMT (PD-PMT) for NIF allows for a temporal resolution comparable to that of the gas cell, or of $\sim$10ps. Enhanced resolution will contribute to the quest for ignition in a crucial way through precision measurement of reaction history and areal density ($\roh$R) history, leading to better constrained models. Features such as onset of alpha heating, shock reverberations and burn truncation due to dynamically evolving failure modes will become visible for the first time. PD-PMT will be deployed on GCD-3 at NIF in 2018. Our synthetic PD-PMT model evaluates the capabilities of these future measurements, as well as minimum yield requirements for measurements performed in a well at 3.9 m from target chamber center (TCC), and within a diagnostic inserter at $\sim$0.2m from TCC.