Session UO7: ICF: X-Ray Diagnostics

High-Resolution X-Ray Imaging with Fresnel Zone Plates on the University of Rochester's OMEGA and OMEGA EP Laser Systems

Experiments performed on the OMEGA and OMEGA EP Laser Systems have utilized Fresnel zone plates (FZP's) to obtain x-ray images with a spatial resolution of as small as 1.6 $\mu $m, limited by the recording medium. Currently, single images are being obtained with either film, an x-ray charge-coupled device, or a framing camera at energies ranging from 2 to 8 keV. A time resolution of 100 ps is obtained by using a short-pulse backlighter or 30-ps time resolution is obtained using a framing camera with some compromise in spatial resolution. Example subjects, which have been imaged with FZP's, include shock-compressed, modulated surfaces that have undergone Rayleigh--Taylor unstable growth, self-emission from Cu-doped shells imploded by OMEGA, and implosions on OMEGA backlit by x-ray emission from Ti foils.   

Development of Dual High Energy Backlighting at the Omega and National Ignition Facility

The Advanced Radiographic Capability (ARC) is used to record radiographs of complex targets at the National Ignition Facility.~ This is achieved using point-projection backlighting where the ARC beamlets are focused onto a single Tungsten wire to generate a single snapshot of the hydrodynamic evolution of the target.~ In order to provide more accurate information, to reduce the requirement on repeatability of the target performance, and to reduce the number of experiments required to assemble a detailed time-sequence of the hydrodynamic evolution of the target, it is desired to be able to record two images per experiment with a time-separation of 3 ns.~ Here, we outline the development activities required to demonstrate dual radiograph capability using two wire sources in close proximity with a large time separation between them.~ The main challenge in this endeavor is evaluating and mitigating potential ``cross talk'' between the two wires.~ Recent Omega EP experiments have demonstrated that the emission from one backlighter detrimentally affects the performance of the second backlighter due to the proximity of the wires to each other as well as the long-time interval necessary between the two snapshots.~ NIF and Omega experiments are underway to test shielding options to mitigate the cross talk between the wires.   

Detecting Shell Contours in ICF Images using Neural Networks

X-ray imaging as a diagnostic is important in understanding the dynamics of ICF targets. Accurate Edge detection algorithms based on x-ray images are essential to constrain numerical models and predictions. Traditional edge detection methods are often hard to scale up to large datasets and are prone to error due to small signal-to-noise ratio and human biases. Neural networks such as Holistic Edge Detection are able to process large datasets systematically and therefore are less susceptible to ad hoc errors due to analyst bias. These networks are trained using deep supervision which generates hierarchies of edges. An issue with deeply supervised neural networks is that they require a large amount of labeled images for training. In order to compensate for small experimental data sets available from ICF, we use neural networks trained on standardized datasets. In some circumstances, the context of the images tend to drastically differ from ICF experiments. We plan on compensating this deficiency using simulation data corrupted with different noise levels.   

\textbf{Three-Dimensional Gated Hot-Spot X-ray Imaging on OMEGA}

The time-dependent morphology of the hot-spot x-ray emission from laser direct drive inertial confinement fusion capsules at stagnation provides a key observable in the assessment of implosion performance. The size, shape and offset of the stagnated fuel is the product of multidimensional effects arising from laser beam imbalance, capsule non-uniformity, the target stalk, and the initial target offset. Cryogenic deuterium-tritium layered implosions on the 60-beam, 30-kJ, 351-nm OMEGA Laser System are observed along two semi-orthogonal lines of sight (LOS) with gated x-ray imagers capable of 30-ps temporal resolution and \textless 10-$\mu $m spatial resolution. A third gated LOS imager, which is nearly orthogonal to the others, will be added to capture the three-dimensional nature of the hot spot. Synchronized observations of the hot-spot size and shape inferred from 4- to 9-keV thermal x rays measured along the two LOS will be presented. 3-D hydrodynamic simulations of the implosions with the code ASTER are used to assess what information of the hot-spot morphology can be inferred by combining multiple gated LOS imaging. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Further enhancements to the Single Line Of Sight x-ray framing camera to study the dynamics of the fuel-ablator interface of Inertial Confinement Fusion capsules.

The Single Line Of Sight (SLOS) framing camera has been used in conjunction with the Crystal Backlighter Imager at the National Ignition Facility to capture radiographs of an imploding ICF capsule with temporal and spatial resolutions never achieved before. By doing so, it has enabled physicists to study the hydrodynamic instabilities at the fuel-ablator interface and better understand one of the major obstacles on the path to ignition. X-ray imaging with SLOS comes with unique challenges, as image quality (signal to noise ratio) and image fidelity (linearity) both impose conflicting requirements on the desirable photocurrent established inside the drift tube. In this presentation, we will discuss the origin of these limitations and will present the alternatives that have already been implemented, as well as future ones, to achieve even better image quality both in the SLOS camera and in future SLOS-like systems.   

Overview of A New 3D MMI Ray-Tracing Tool

Inertial confinement fusion is one method used to obtain controlled thermonuclear burn through either direct or indirect ablation of a millimeter-scale capsule with the use of high-power lasers. Although there have been large strides made in understanding the physics involved in order to create reliable physics models and codes, simulations and experiments still show discrepancies. A factor in this mismatch is the asymmetry of the implosions that occur experimentally. The multi-monochromatic X-ray imager (MMI) is an instrument which gives spatially, spectrally, and temporally resolved arrays of narrow-band x-ray images which can be used to extract temperature, density, and mixing spatial profiles. A new 3D ray-tracing tool has been developed that allows for the designing and characterizing of MMI [D. T. Cliche and R. C. Mancini, Appl. Opt. \textbf{58}, 17 (2019)]. The tool also allows for the production of synthetic MMI data with a higher degree of fidelity previously obtained. This work was supported in part by Los Alamos National Laboratory (contract {\#}472892).   

Design of the National Ignition Facility Imaging and Spectroscopy Snout (ISS)

Current diagnostic capabilities at the National Ignition Facility (NIF) preclude the ability to record simultaneous neutron images and x-ray spectra along a similar line of sight. To better understand both 3D asymmetries and hot-spot mix, it would be advantageous to acquire these data on every ICF experiment. We present the design of the new ISS diagnostic that supports data collection of neutron pinhole images, 1D spatially-resolved x-ray spectra and time-resolved x-ray pinhole images at both low (10X) and high (55X). This new instrument is specific to the polar port of the NIF chamber, and it supports standard snout appendages, such as wedge range filters and solid radiochemical collection diagnostics. The x-ray spectrometer is configured to concurrently image two orthogonal lines of sight with spectral channels from 7.5 to 11.5 keV. Selectable crystal configurations also provide the ability to field one of three narrower band, higher resolution channels in place of a broadband channel. Full design details and expected performance will be discussed.   

Spectroscopic observation of geometric inversion in HED plasmas

The effects of plasma geometry on the plasma spectra have recently been proved important for diagnosing temperatures and densities in Dot Spectroscopy experiments at the National Ignition Facility (NIF). The cylindrical geometry of these targets causes the intensity of the optically thick lines to have a non-trivial angular dependence. The degree of anisotropy is a function of the aspect ratio (height-to-radius or H/R). We have performed experiments at OMEGA to understand this effect. In the experiments, we create a uniform cylindrical plasma with microdots of mid-Z materials in order to study K-shell spectroscopy. Time resolved imaging and spectroscopy data were obtained for two perpendicular views of the plasmas. Data show how the geometry of the plasma changing from disk-like to pipe-like affects the line ratios, in agreement with the escape factor approach proposed by Kerr \textit{et al. }in 2004. This effect shows the importance of the geometry of the plasma in the study of line ratios in High Energy Density (HED) Physics, and the potential of this analysis as a method for plasma characterization.   

Spectroscopic Diagnostics of Krypton Doped Symmetric Capsule Implosion Experiments on NIF

X-ray spectroscopy is used to diagnose plasma conditions of a symmetric capsule (symcap) target in ICF experiments on NIF. The DD gas inside the symcap target was doped with small traces of krypton. The high areal density shell of the symcap target has minimal attenuation of the krypton K-shell emission. The fraction of krypton dopant was selected to minimally perturb the implosion, but high enough to provide enough photons. The krypton He-alpha and He-beta line emission was measured using the absolutely calibrated dHIRES built by PPPL. Synthetic spectra generated from the NRL DRACHMA II code will be used to model the radiation to infer the plasma conditions. Drachma is a 1-D multi-zone non-LTE kinetics model with radiation transport. *Work supported by DOE/NNSA at NRL and U.S. DOE by LLNL under Contract No. DE-AC52-07NA27344.   

X-ray “burn through” studies for symmetry control considerations in support of magnetic-indirect-drive fusion platform development

Numerical simulations investigating the dependence of shine-shield material and thickness for mode-selective, time-dependent control of P2 and P4 Legendre mode asymmetries for magnetic-indirect-drive (MID) target designs are presented. The MID concept uses a plasma pinch to create x-rays within a primary hohlraum directing them into a secondary hohlraum where the resultant bath of x rays ablates and compresses a capsule containing deuterium and tritium fuel to high temperatures, creating fusion [T. W. L. Sanford et al., Physical Review Letters 83, 5511 (1999)]. A shine-shield is used to control the radiation flow from the primary to the secondary hohlraum and shape the implosion drive, including eliminating the pole-hot x-ray flux that would otherwise be seen by the capsule. We describe design options and the feasibility for exercising time-dependent control over low-mode asymmetries that are independent of hohlraum length.   

Pointing scheme for the NIF laser with "perfect" low mode uniformity

A new concept for designing the beam pointings for direct drive on the NIF laser is proposed. It is based on a calculation of the full 3D spherical harmonic laser absorption spectrum for an ideal surface absorbing target irradiated by identical beams of circular cross section aimed at target center. The model is used to find configurations of beam numbers and pointing angles that can zero out the amplitudes of the low mode number spectrum. The 192 beams of the NIF can be pointed in a five cone configuration that eliminates all the modes with principle mode numbers less than 12, (143 modes in all). In addition to zeroing the low modes, the scheme has the consequence of increasing individual beam smoothing such that modes not zeroed are reduced to very small levels -- and with relatively narrow beams. Thus with a beam to target ratio of, R$_{\mathrm{beam}}$/R$_{\mathrm{target}}\approx $ 0.4, the P$_{\mathrm{12\thinspace }}$mode amplitude is reduced by the factor of, 0.004, bringing the normalized amplitude to 0.15 {\%}. The main predictions of the analytic calculations are validated with two dimensional HYDRA simulations of the Revolver ablator shell. Research supported by the LDRD Program of Los Alamos National Laboratory under project number 20180051DR.   

Vacuum Ultraviolet and Visible Spectroscopy for Power Flow Studies on the 1 MA, 100 ns MAIZE LTD

Power flow studies on the 25-MA Z-Machine at Sandia have shown that magnetically insulated electron flow can separate from the vicinity of cathode surfaces, cross the anode-cathode gap, and lead to a loss of current delivered to the load. This presentation reports on efforts to develop spectroscopic diagnostics for power flow experiments on the Michigan’s 1-MA MAIZE facility to validate ongoing simulation studies. A vacuum ultraviolet (VUV) spectrometer will be used to measure the rate at which neutral constituents desorb out of the magnetically insulated transmission lines in experiments run with scaled anode-cathode gap spacing to obtain electric field intensities or current densities comparable to those found in certain regions on Z. The VUV region of the spectrum (100-200 nm) was chosen due to the expectation of low levels of background black-body emission, supported by preliminary simulations with PrismSPECT. Visible spectroscopy on MAIZE will allow comparison between VUV data on MAIZE and published visible spectroscopic data on the Z-machine.   

Effects of Pulsed X-rays on Cancer Cells compared with Conventional X-rays Irradiation.

It is demonstrated that pulsed X-rays (\textasciitilde 15ns) from a kJ plasma focus (PF-2kJ) device produce larger amount of cell death than conventional continuous X-rays in colorectal colon cancer line, DLD-1, for the similar amount of doses. At first, PF-2kJ was optimized for the maximum X-rays emission. Doses were characterized using thermoluminscent detectors (TLD-600). It was found that 60 pulses of X-rays provide about 0.7 Gy at a distance \textasciitilde 10.0 cm from the top of the PF-2kJ anode. Experiments were performed to irradiate (60 pulses of X-rays) DLD-1 cell line in vitro. Results show that 60 pulses of X-rays induce a significant cell death at 48 h post irradiation, which was \textasciitilde 3-fold greater than that was induced at 0.6 Gy obtained from a conventional continuous X-rays irradiation, and similar to the cell death evoked at 12 Gy (continuous X-rays). Dose rate for pulsed X-rays from PF-2kJ, was found \textasciitilde 10$^{\mathrm{7}}$ Gy/min that was larger than the conventional X-rays source (0.5 Gy/min). Thus, kJ plasma focus have potential applications in the area of cancer research. Results are encouraging to study the pulsed X-rays effects on different cancer cell lines, to study as a possible alternative of conventional radiation therapy.