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Rep-rated diagnostics are a necessity for present and future laser facilities focused on the study of high-energy-density (HED) phenomena. Electromagnetic fields in HED systems are of particular interest to the community, but are inherently difficult to diagnose due to the high mass-densities involved. By utilizing high-energy protons that penetrate through dense plasmas, fields may be visualized by detecting variations in proton fluence caused by the Lorenz interaction of the protons with the field. Typical detecting media used for proton imaging are single-use and do not scale well to rep-rated operation. We propose here the use of a scintillator-based detector and characterize the energy and spatial responses to demonstrate its feasibility as a viable option for rep-rated proton imaging and other diagnostics.

As high-intensity short-pulse lasers that can operate at high-repetition-rate (HRR) (>10 Hz) come online around the world, the high-energy-density (HED) science they enable will experience a radical paradigm shift. The >10^3x increase in shot rate over today’s shot-per-hour drivers translates into dramatically faster data acquisition and more experiments, and thus the potential to significantly accelerate the advancement of HED science. We will present the vision and ongoing work to realize a HRR framework that allows for rapidly delivered optimal experiments by bringing together feedback laser control loops, high-throughput targetry and diagnostics, cognitive simulation, enhanced HED codes, and advanced data analytics.

Application of deep learning to shaped, short-pulse laser-driven ion acceleration. Using a neural network as a universal approximator function, i.e., a surrogate model, we can map out large areas of parameter space. The neural network is informed by a large dataset of about 1,000, mid-fidelity particle-in-cell simulations modeling instances of Target-Normal Sheath Acceleration. The neural-network-based function allows us to rapidly explore regions of interest in search of optimal input parameters and features of interest.

High intensity, high-repetition rate (HRR) lasers, that is lasers that can operate on the order of 1 Hz or faster, are quickly coming on-line around the world. High intensity lasers have long been an impactful tool in high energy density (HED) science since they are capable of creating matter at extreme temperatures and pressures relevant to this field. The advent of HRR technology enhances to this capability since HRR enables these types of these experiments to be performed faster, thus leading to an acceleration in the rate of learning in fundamental HED science. However, in order to use the full potential of HRR systems, high repetition rate diagnostics in addition to real-time analysis tools must be developed to process experimental measurements and outputs at a rate that matches the laser. Towards this goal, we present an automated machine learning based analysis for a synthetic X-ray spectrometer, which is a common diagnostic in HED experiments.

High-intensity short-pulse lasers are being pushed further as applications continue to demand higher laser intensities. Uses such as radiography and laser-driven particle acceleration require these higher intensities to produce the necessary x-ray and particle fluxes. Achieving these intensities, however, is limited by the damage threshold of costly optics and the complexity of target chambers. This is evidenced by the Advanced Radiographic Capability (ARC) short-pulse laser at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory, producing four high-energy ≈ 1 k J laser pulses at 30 ps pulse duration, being limited to an intensity of 10 18 W / c m 2 by the large focal spot size of ≈ 100 µ m . Due to the setup complexity of NIF, changing the location of the final focusing parabola in order to improve the focal spot size is not an option. This leads to the possible use of disposable ellipsoidal plasma mirrors (PMs) placed within the chamber, close to the target in an attempt to refocus the four ARC beams. However, the behavior of PMs at these relatively long pulse durations (tens of picoseconds) is not well characterized. The results from the COMET laser at the Jupiter Laser Facility carried out at 0.5 to 20 ps pulse durations on flat mirrors are presented as a necessary first step towards focusing curved mirrors. The findings show defocusing at longer pulse durations and higher intensities, with less degradation when using aluminum coated mirrors.

A single shot, complete spatiotemporal measurement of the complex electric field E(x,y,z,t) emitted by a high power (>0.1 TW) laser is demonstrated for the first time. Color movies of the laser's electric field are retrieved, and the temporal, spectral, and spatial field features are examined.

The mode-1 x-ray drive asymmetry of indirect-drive Inertial Confinement Fusion(ICF) implosions at the National Ignition Facility(NIF) has been estimated using a simple static ViewFactor model. The model takes as input measured laser performance data in the foot and peak, the hohlraum configuration, and laser to hohlraum pointing. These estimates are compared with neutron time-of-flight measurements of directionality and magnitude of the resultant hotspot bulk velocity (~20–100 μm/ns) for 39 NIF shots using High Density Carbon (HDC) ablators and show strong correlation on a statistically significant number of shots. The most important factors identified so far are random quad-to-quad peak power laser imbalances, the presence of lossy diagnostic windows and gaps on the hohlraum waist, capsule sag and capsule thickness mode 1. Typical mode-1 asymmetry in drive is currently ~0.5% for many of these sources on their own and, when summed, can lead to a neutron hotspot velocity of up to 100 μm /ns and a reduction in yield of 35% for current NIF DT layered implosions. Our goal is to identify, quantify and mitigate all potential sources of mode-1 asymmetry (which also include target and laser alignment imperfections, foot power and Cross Beam Energy Transfer imbalances) to enable higher quality implosions on NIF.

We present a deep learning based framework for real-time analysis of a differential filter based x-ray spectrometer that is common on short-pulse laser experiments. The analysis framework was trained with a large repository of synthetic data to retrieve key experimental metrics, such as slope temperature. With traditional analysis methods, these quantities would have to be extracted from data using a time-intensive and manual analysis. This framework was developed for a specific diagnostic, but may be applicable to a wide variety of diagnostics common to laser experiments and thus will be especially crucial to the development of high-repetition rate (HRR) diagnostics for HRR laser systems that are coming online.

One of the pressing demands in our western society is the safety and maintenance of our nuclear legacy. In Germany the dismantling, safe processing and storage of nuclear waste have resulted in a multi-national research program. One of the findings was that nondestructive testing methods and material selective imaging of compound large objects is possible using thermal and fast neutrons. They also identified that a powerful, safe, and compact neutron source would be required. Since the advent of ultra-intense lasers many applications have been investigated using the unique parameter of laser-driven secondary sources. Recently, we have demonstrated the realization of a short-pulse laser- driven neutron source with beam intensities orders of magnitude above earlier attempts. Those sources can lead to a compact and potentially mobile neutron source with a large number of applications. The neutron source is based on a so-called pitcher-catcher approach, where a short burst of energetic, laser-driven deuteron ions are converted into a short pulse of neutrons in a subsequent converter material. Here, in addition to neutron conversion via (d,n) reactions the breakup of energetic deuterons and a non-thermal pre-compound reaction in the converter material result in a directed beam of neutrons and an increase in neutron numbers compared to earlier attempts. The neutron record number so far were produced using high-contrast, high-energy short pulse lasers of a few hundred femtosecond pulse duration and invoking relativistic transparency in solids as a new ion acceleration mechanism. I will present the underlying mechanism of creating an intense pulsed and highly directed beam of neutrons using ultra-intense lasers and the recent experimental results using laser systems in the US and in Europe. Furthermore, I will focus on a few examples of using such sources for applications that are either important for the security of our countries or will have large economical potential in industrial applications. These range from the remote sensing of illicit nuclear material in cargo to the non-destructive analysis of large civil constructions using compact laser systems. The initial ion pulse that is converted into neutrons is of only a few picosecond duration and thus the neutron burst is generated with sub-nanosecond pulse length. This allows for precise energy resolution of the neutrons using time-of flight techniques. As the ion acceleration is of only a few millimeter in length, replacing ten's to hundred's of meter of conventional acceleration length the source is extremely compact. As the ion beams are extremely intense and do not suffer from space charge deterioration the fast neutron source is capable of point projection imaging with an initial source size of around a millimeter. As the pulse duration is short, high energy resolution can be achieved at a much reduced time-of-flight length offering more compact systems. As future laser systems largely increase not only peak power, but also already offer repetition rates up the kHz applications in research and industry become visible. To fully take advantage of those new sources new detector systems should keep up with enhanced spatial and temporal resolution. Also they need to withstand the unavoidable x-ray flash and electro-magnetic pulse originating from the laser plasma interaction. We have started to explore novel detectors and tested them at high-energy short pulse laser systems for fast and thermal neutron beams.

New short-pulse kilojoule, Petawatt-class lasers, which have recently come online and are coupled to large-scale, many-beam long-pulse facilities, undoubtedly serve as very exciting tools to capture transformational science opportunities in high energy density physics. These short-pulse lasers also happen to reside in a unique laser regime: very high-energy (kilojoule), relatively long (multi-picosecond) pulse-lengths, and large (10s of micron) focal spots, where their use in driving energetic particle beams is largely unexplored. Proton acceleration via Target Normal Sheath Acceleration (TNSA) using the Advanced Radiographic Capability (ARC) short-pulse laser at the National Ignition Facility in the Lawrence Livermore National Laboratory is demonstrated for the first time, and protons of up to 18 MeV are measured using laser irradiation of >1 ps pulse-lengths and quasi-relativistic (∼1018 W/cm2) intensities. This is indicative of a super-ponderomotive electron acceleration mechanism that sustains acceleration over long (multi-picosecond) time-scales and allows for proton energies to be achieved far beyond what the well-established scalings of proton acceleration via TNSA would predict at these modest intensities. Furthermore, the characteristics of the ARC laser (large ∼100 μm diameter focal spot, flat spatial profile, multi-picosecond, relatively low prepulse) provide acceleration conditions that allow for the investigation of 1D-like particle acceleration. A high flux ∼ 50 J of laser-accelerated protons is experimentally demonstrated. A new capability in multi-picosecond particle-in-cell simulation is applied to model the data, corroborating the high proton energies and elucidating the physics of multi-picosecond particle acceleration.

Developments in machine learning promise to ameliorate some of the challenges of modeling complex physical systems through neural-network-based surrogate models. High-intensity, short-pulse lasers can be used to accelerate ions to mega-electronvolt energies, but to model such interactions requires computationally expensive techniques such as particle-in-cell simulations. Multilayer neural networks allow one to take a relatively sparse ensemble of simulations and generate a surrogate model that can be used to rapidly search the parameter space of interest. In this work, we created an ensemble of over 1,000 simulations modeling laser-driven ion acceleration and developed a surrogate to study the resulting parameter space. A neural-network-based approach allows for rapid feature discovery not possible for traditional parameter scans given the computational cost. A notable observation made during this study was the dependence of ion energy on the pre-plasma gradient length scale. While this methodology harbors great promise for ion acceleration, it has ready application to all topics in which large-scale parameter scans are restricted by significant computational cost or relatively large, but sparse, domains.

A pixelated scintillator has been designed, fabricated, and tested using a laser-accelerated proton source for use in proton diagnostics at rep-rated laser facilities. The work presented here demonstrates the enhanced spatial resolution of thin, organic scintillators through a novel pixelation technique. Experimental measurements using laser-generated protons incident onto 130 μm-thick scintillators indicate a >20% reduction in the scintillator point spread function (PSF) for the detectors tested. The best performing pixelated detector reduced the ∼200 μm PSF of the stock material to ∼150 μm. The fabrication technique may be tailored to reduce the pixel size and achieve higher spatial resolutions.

We are reporting on a series of indirect-drive 0.9-scale CH capsule implosions (inner radius = 840 μm) fielded in low gas-fill (0.6 mg/cm3) hohlraums of 6.72 mm diameter at the National Ignition Facility. Thanks to the 11%-reduction of the capsule size at a given hohlraum diameter compared to previously tested full-scale capsules, we achieved good hot spot symmetry control near 33% cone-fraction and without the need to invoke cross beam energy transfer. As a result, we achieved a hot spot pressure of 280 ± 40 Gbar, which is the highest pressure demonstrated in layered DT implosions with CH capsules to date. Pushing this design to higher velocity resulted in a reduction of neutron yield. Highly resolved capsule simulations suggest that higher Au M-shell preheat resulted in an increase in Atwood number at the ablator–ice interface, which leads to increased fuel-ablator instability and mixing. The results reported here provide important scaling information for next-generation CH designs.

Picosecond-scale laser–matter interactions using compound parabolic concentrators have demonstrated strongly relativistic ponderomotive effects with ∼ 10 × increase in x-ray source brightness, positron production and multi-MeV proton acceleration versus flat targets, using a marginally relativistic intensity laser.

Understanding hydrodynamic phenomena driven by fast electron heating is important for a range of applications including fast electron collimation schemes for fast ignition and the production and study of hot, dense matter. In this work, detailed numerical simulations modelling the heating, hydrodynamic evolution, and extreme ultra-violet (XUV) emission in combination with experimental XUV images indicate shock waves of exceptional strength (200 Mbar) launched due to rapid heating of materials via a petawatt laser. We discuss in detail the production of synthetic XUV images and how they assist us in interpreting experimental XUV images captured at 256 eV using a multi-layer spherical mirror.

We examine systematic errors in x-ray imaging by pinhole optics for quantifying uncertainties in the measurement of convergence and asymmetry in inertial confinement fusion implosions. We present a quantitative model for the total resolution of a pinhole optic with an imaging detector that more effectively describes the effect of diffraction than models that treat geometry and diffraction as independent. This model can be used to predict loss of shape detail due to imaging across the transition from geometric to diffractive optics. We find that fractional error in observable shapes is proportional to the total resolution element we present and inversely proportional to the length scale of the asymmetry being observed. We have experimentally validated our results by imaging a single object with differently sized pinholes and with different magnifications.

X-ray penumbral imaging has been successfully fielded on a variety of inertial confinement fusion (ICF) capsule implosion experiments on the National Ignition Facility (NIF). We have demonstrated sub-5 μm resolution imaging of stagnated plasma cores (hot spots) at x-ray energies from 6 to 30 keV. These measurements are crucial for improving our understanding of the hot deuterium-tritium fuel assembly, which can be affected by various mechanisms, including complex 3-D perturbations caused by the support tent, fill tube or capsule surface roughness. Here we present the progress on several approaches to improve x-ray penumbral imaging experiments on the NIF. We will discuss experimental setups that include penumbral imaging from multiple lines-of-sight, target mounted penumbral apertures and variably filtered penumbral images. Such setups will improve the signal-to-noise ratio and the spatial imaging resolution, with the goal of enabling spatially resolved measurements of the hot spot electron temperature and material mix in ICF implosions.

We have developed and fielded x-ray penumbral imaging on the National Ignition Facility in order to enable sub-10 μm resolution imaging of stagnated plasma cores (hot spots) of spherically shock compressed spheres and shell implosion targets. By utilizing circular tungsten and tantalum apertures with diameters ranging from 20 μm to 2 mm, in combination with image plate and gated x-ray detectors as well as imaging magnifications ranging from 4 to 64, we have demonstrated high-resolution imaging of hot spot plasmas at x-ray energies above 5 keV. Here we give an overview of the experimental design criteria involved and demonstrate the most relevant influences on the reconstruction of x-ray penumbral images, as well as mitigation strategies of image degrading effects like over-exposed pixels, artifacts, and photon limited source emission. We describe experimental results showing the advantages of x-ray penumbral imaging over conventional Fraunhofer and photon limited pinhole imaging and showcase how internal hot spot microstructures can be resolved.

We report on measurements of the electron temperature in the hotspot of inertially confined, layered, spherical implosions on the National Ignition Facility using a differential filtering diagnostic. Measurements of the DT and DD ion temperatures using neutron time-of-flight detectors are complicated by the contribution of hot spot motion to the peak width, which produce an apparent temperature higher than the thermal temperature. The electron temperature is not sensitive to this non-thermal velocity and is thus a valuable input to interpreting the stagnated hot spot conditions. Here we show that the current differential filtering diagnostic provides insufficient temperature resolution for the hot spot temperatures of interest. We then propose a new differential filter configuration utilizing larger pinhole size to increase spectral fluence, as well as thicker filtration. This new configuration will improve measurement uncertainty by more than a factor of three, allowing for a more accurate hotspot temperature.

Fast electron energy spectra have been measured for a range of intensities between 1018 and 1021 W cm-2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5 to 5 ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40° in p -polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions. © 2009 American Institute of Physics.

Analyses of high foot implosions show that performance is limited by the radiation drive environment, i.e., the hohlraum. Reported here are significant improvements in the radiation environment, which result in an enhancement in implosion performance. Using a longer, larger case-to-capsule ratio hohlraum at lower gas fill density improves the symmetry control of a high foot implosion. Moreover, for the first time, these hohlraums produce reduced levels of hot electrons, generated by laser-plasma interactions, which are at levels comparable to near-vacuum hohlraums, and well within specifications. Further, there is a noteworthy increase in laser energy coupling to the hohlraum, and discrepancies with simulated radiation production are markedly reduced. At fixed laser energy, high foot implosions driven with this improved hohlraum have achieved a 1.4×increase in stagnation pressure, with an accompanying relative increase in fusion yield of 50% as compared to a reference experiment with the same laser energy.

The HED 2-Shock implosion campaign was developed on the National Ignition Facility as a relatively robust and well-beha ved nearly one-dimensional, low convergence, symmetric platform by employing a very high foot temperature and a large case-to-capsule (hohlraum-to-capsule) ratio. The results of these experiments investigating capsule implosion phenomena such as interface hydrodynamic mixing, convergence effects, and shape effects are being used for the validation of hydro-c odes and to develop a systematic understanding of performance degradation mechanisms. A sequence of experiments was initially completed to tune the 2-Shock implosion round, with little shape swing from in-flight to stagnation. Then, hohlraum size and hohlraum gas fill were systematically changed to study the effect on shape and symmetry evolution. Details and results of these experiments are described.

A new capability at the National Ignition Facility (NIF) has been implemented to measure the temperature of x-ray emitting sources. Although it is designed primarily for Inertial Confinement Fusion (ICF), it can be used for any hot emitting source that is well modeled. The electron temperature (Te) of the hot spot within the core of imploded ICF capsules is an effective indicator of implosion performance. Currently, there are spatially and temporally integrated Te inferences using image plates. A temporally resolved measurement of Te will help elucidate the mechanisms for hot spot heating and cooling such as conduction to fuel, alpha-heating, mix, and radiative losses. To determine the temporally resolved Te of hot spots, specific filters are added to an existing x-ray streak camera "streaked polar instrumentation for diagnosing energetic radiation" to probe the emission spectrum during the x-ray burn history of implosions at the NIF. One of the difficulties in inferring the hot spot temperature is the attenuation of the emission due to opacity from the shell and fuel. Therefore, a series of increasingly thick titanium filters were implemented to isolate the emission in specific energy regions that are sensitive to temperatures above 3 keV while not significantly influenced by the shell/fuel attenuation. Additionally, a relatively thin zinc filter was used to measure the contribution of colder emission sources. Since the signal levels of the emission through the thicker filters are relatively poor, a dual slit (aperture) was designed to increase the detected signal at the higher end of the spectrum. Herein, the design of the filters and slit is described, an overview of the solving technique is provided, and the initial electron temperature results are reported.

High laser energy (>1.2 MJ) implosion experiments on the National Ignition Facility show that low mode implosion symmetry is highly dependent on an expanding high-Z wall “bubble” plasma feature. The bubble is caused by the early time deposition of laser beams incident on the interior near the entrance of the cylindrical hohlraum (outer cone beams). It absorbs beams designated for the waist of the hohlraum (inner cone beams) causing a redistribution of x-ray flux on the capsule. From measurements, we are able to quantify the absorption and expansion of this bubble. Measurements show that the resulting hot spot is more oblate when there is more inner beam absorption in the bubble. We find absorption in the bubble to be between 51 ± 3% and 62 ± 2%. This bubble absorption is found to evolve predictably as a function of the early time outer cone laser pulse fluence and the pulse length. From this, a phenomenological model of the effective drive symmetry and subsequent implosion shape is found indicating a very strong dependence of implosion shape on early time laser fluence and laser pulse duration.High laser energy (>1.2 MJ) implosion experiments on the National Ignition Facility show that low mode implosion symmetry is highly dependent on an expanding high-Z wall “bubble” plasma feature. The bubble is caused by the early time deposition of laser beams incident on the interior near the entrance of the cylindrical hohlraum (outer cone beams). It absorbs beams designated for the waist of the hohlraum (inner cone beams) causing a redistribution of x-ray flux on the capsule. From measurements, we are able to quantify the absorption and expansion of this bubble. Measurements show that the resulting hot spot is more oblate when there is more inner beam absorption in the bubble. We find absorption in the bubble to be between 51 ± 3% and 62 ± 2%. This bubble absorption is found to evolve predictably as a function of the early time outer cone laser pulse fluence and the pulse length. From this, a phenomenological model of the effective drive symmetry and subsequent implosion shape is found indicating a very...

We present a data-based model for low mode asymmetry in low gas-fill hohlraum experiments on the National Ignition Facility {NIF [Moses et al., Fusion Sci. Technol. 69, 1 (2016)]} laser. This model is based on the hypothesis that the asymmetry in these low fill hohlraums is dominated by the hydrodynamics of the expanding, low density, high-Z (gold or uranium) “bubble,” which occurs where the intense outer cone laser beams hit the high-Z hohlraum wall. We developed a simple model which states that the implosion symmetry becomes more oblate as the high-Z bubble size becomes large compared to the hohlraum radius or the capsule size becomes large compared to the hohlraum radius. This simple model captures the trends that we see in data that span much of the parameter space of interest for NIF ignition experiments. We are now using this model as a constraint on new designs for experiments on the NIF.

By the time an inertially confined fusion (ICF) implosion has converged a factor of 20, its surface area has shrunk 400 × , making it an inefficient x-ray energy absorber. So, ICF implosions are traditionally designed to have the laser drive shut off at a time, toff, well before bang-time, tBT, for a coast-time of t coast = t B T − t o f f > 1 ns. High-foot implosions on NIF showed a strong dependence of many key ICF performance quantities on reduced coast-time (by extending the duration of laser power after the peak power is first reached), most notably stagnation pressure and fusion yield. Herein we show that the ablation pressure, pabl, which drives high-foot implosions, is essentially triangular in temporal shape, and that reducing tcoast boosts pabl by as much as ∼ 2 × prior to stagnation thus increasing fuel and hot-spot compression and implosion speed. One-dimensional simulations are used to track hydrodynamic characteristics for implosions with various coast-times and various assumed rates of hohl...

When used with 1.06-mm beryllium (Be) capsules on the National Ignition Facility, gold hohlraums with the inner diameter of 5.75 mm and helium gas fill density of 1.6 mg/cm3 exhibit significant drive degradation due to laser energy backscatter (of order 14%–17%) and “missing” X-ray drive energy (about 32% during the main pulse). Also, hard to simulate cross-beam energy transfer (CBET) must be used to control the implosion symmetry. Larger, 6.72-mm hohlraums with fill densities ≤0.6 mg/cm3 generally offer improved drive efficiency, reduced hot-electron preheat, and better control of the implosion symmetry without CBET. Recently, we carried out an exploratory campaign to evaluate performance of 1.06-mm Be capsules in such hohlraums and determine optimal hohlraum parameters. Specifically, we performed a hohlraum fill-density scan with a three-shock, 9.5-ns laser pulse and found that an appropriate axial laser repointing and azimuthal outer-quad splitting resulted in significantly improved hohlraum energetics...

We have developed an experimental platform for absolute equation of state measurements up to Gbar pressures on the National Ignition Facility (NIF) within the Fundamental Science Program. We use a symmetry-tuned hohlraum drive to launch a spherical shock wave into a solid CH sphere. Streaked radiography is the primary diagnostic to measure the density change at the shock front as the pressure increases towards smaller radii. At shock stagnation in the center of the capsule, we observe a short and bright x-ray self emission from high density (∼50 g/cm(3)) plasma at ∼1 keV. Here, we present results obtained with penumbral imaging which has been carried out to characterize the size of the hot spot emission. This allows extending existing NIF diagnostic capabilities for spatial resolution (currently ∼10 μm) at higher sensitivity. At peak emission we find the hot spot radius to be as small as 5.8 +/- 1 μm, corresponding to a convergence ratio of 200.

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.

The neutron imaging system at the National Ignition Facility (NIF) is an important diagnostic tool for measuring the two-dimensional size and shape of the neutrons produced in the burning deuterium-tritium plasma during the ignition stage of inertial confinement fusion (ICF) implosions at NIF. Since the neutron source is small (∼100 μm) and neutrons are deeply penetrating (>3 cm) in all materials, the apertures used to achieve the desired 10-μm resolution are 20-cm long, single-sided tapers in gold. These apertures, which have triangular cross sections, produce distortions in the image, and the extended nature of the pinhole results in a non-stationary or spatially varying point spread function across the pinhole field of view. In this work, we have used iterative Maximum Likelihood techniques to remove the non-stationary distortions introduced by the aperture to reconstruct the underlying neutron source distributions. We present the detailed algorithms used for these reconstructions, the stopping criteria used and reconstructed sources from data collected at NIF with a discussion of the neutron imaging performance in light of other diagnostics.

This work was supported in part by DTRA under Basic Research Award No. HDTRA1-10-10077. Here, we show that defocusing of the laser in the interaction of a picosecond duration, 1.053 μm wavelength, high energy pulse with a cone-wire target does not significantly affect the laser energy coupling efficiency, but does result in a drop in the fast electron effective temperature. This may be beneficial for fast ignition, since not only were more electrons with lower energies seen in the experiment but also the lower prepulse intensity will reduce the amount of preplasma present on arrival of the main pulse, reducing the distance the hot electrons have to travel. We used the Vulcan Petawatt Laser at the Rutherford Appleton Laboratory and gold cone targets with approximately 1 mm long, 40 μm diameter copper wires attached to their tip. Diagnostics included a quartz crystal imager, a pair of highly oriented pyrolytic graphite crystal spectrometers and a calibrated CCD operating in the single photon counting regime, all of which looked at the copper Kα emission from the wire. A short pulse optical probe, delayed 400 ps relative to the main pulse was employed to diagnose the extent of plasma expansion around the wire. A ray-tracing code modeled the change in intensity on the interior surface of the cone with laser defocusing. Using a model for the wire copper Kα emission coupled to a hybrid Vlasov-Fokker-Planck code, we ran a series of simulations, holding the total energy in electrons constant whilst varying the electron temperature, which support the experimental conclusions. Publisher PDF

We have derived a 3-dimensional synthetic model for NIF implosion conditions, by predicting and optimizing fits to a broad set of x-ray and nuclear diagnostics obtained on each shot. By matching x-ray images, burn width, neutron time-of-flight ion temperature, yield, and fuel r, we obtain nearly unique constraints on conditions in the hotspot and fuel in a model that is entirely consistent with the observables. This model allows us to determine hotspot density, pressure, areal density (r), total energy, and other ignition-relevant parameters not available from any single diagnostic. This article describes the model and its application to National Ignition Facility (NIF) tritium-hydrogen-deuterium (THD) and DT implosion data, and provides an explanation for the large yield and r degradation compared to numerical code predictions. To optimize the target and laser parameters for ignition (1, 2), it is important to understand the implosion conditions achieved in a given experiment, independent of the design predictions. To this end, we developed and applied a model to estimate the performance of an implosion and assess temperature, density, and composition distributions within the assembled core, solely from the experimental data taken during the shot. Using radiative, equation of state, nuclear fusion relations for relevant materials, and an approximation of pressure equilibrium within the hotspot (3), we derive a 3-dimensional representation of the capsule density and temperature profiles at stagnation, by predicting and optimizing fits to a broad set of x-ray and nuclear diagnostics. This model allows us to determine hotspot density, pressure, areal density (r), total energy, and other ignition-relevant parameters not available from any single diagnostic. This approach has been validated by comparing results with radiation-hydrodynamic simulations and has produced semi-quantitative (10-20%) agreement. We begin by constructing a 3D model for the temperature and density conditions of the imploded core at neutron bang time, defined as the time of peak neutron and 10-20keV x-ray production in the capsule (4). We further assume that this imploded core, including both hotspot and fuel, is at a uniform pressure Phs. So for a given density profile (r, , ), we derive an associated temperature profile using the hydrogenic equation-of-state, which includes effects of Fermi degeneracy in the dense shell (5). This description produces a relatively complete characterization of the implosion temperature and density distributions, and from this we can calculate the complete properties of the x-ray emission and nuclear fusion processes to predict the ensemble of diagnostic signatures from the implosion. Given an assumed pressure and density profile, we calculate the waist and polar x-ray emission images, using a free-free emissivity derived from the VISTA opacity model, and a Detailed This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Periodic sensitivity calibration of imaging plates (IP) is crucial for quantitative understanding of x-ray data obtained at the National Ignition Facility (NIF). To test the x-ray sensitivity of the IPs and the scanners, we developed an x-ray exposure station based on radioactive isotopes (55Fe, 109Cd, and 241Am). This apparatus provides a convenient setup for a periodical test of the IP’s and the scanners. On NIF implosion experiments with deuterium-tritium mixture fuel, the neutrons produced in the capsule hit the imaging plates and impose background signal. Therefore it is also important to know the neutron sensitivity of the IPs. The sensitivity for 14 MeV neutrons was measured on high neutron yield shots at the OMEGA laser facility. The measured sensitivities were compared with the results of Monte Carlo simulations.

A high resolution (E/ΔE = 1200-1800) Bragg crystal x-ray spectrometer is being developed to measure plasma parameters in National Ignition Facility experiments. The instrument will be a diagnostic instrument manipulator positioned cassette designed mainly to infer electron density in compressed capsules from Stark broadening of the helium-β (1s2-1s3p) lines of krypton and electron temperature from the relative intensities of dielectronic satellites. Two conically shaped crystals will diffract and focus (1) the Kr Heβ complex and (2) the Heα (1s2-1s2p) and Lyα (1s-2p) complexes onto a streak camera photocathode for time resolved measurement, and a third cylindrical or conical crystal will focus the full Heα to Heβ spectral range onto an image plate to provide a time integrated calibration spectrum. Calculations of source x-ray intensity, spectrometer throughput, and spectral resolution are presented. Details of the conical-crystal focusing properties as well as the status of the instrumental design are also presented.

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.

The dilation x-ray imager (DIXI) [T. J. Hilsabeck et al., Rev. Sci. Instrum. 81, 10E317 (2010); S. R. Nagel et al., ibid. 83, 10E116 (2012); S. R. Nagel et al., ibid. 85, 11E504 (2014)] is a high-speed x-ray framing camera that uses the pulse-dilation technique to achieve a temporal resolution of less than 10 ps. This is a 10 × improvement over conventional framing cameras currently employed on the National Ignition Facility (NIF) (100 ps resolution), and otherwise only achievable with 1D streaked imaging. A side effect of the dramatically reduced gate width is the comparatively lower detected signal level. Therefore we implement a Poisson noise reduction with non-local principal component analysis method [J. Salmon et al., J. Math. Imaging Vision 48, 279294 (2014)] to improve the robustness of the DIXI data analysis. Here we present results on ignition-relevant experiments at the NIF using DIXI. In particular we focus on establishing that/when DIXI gives reliable shape metrics (P0, P2, and P4 Legendre modes, and their temporal evolution/swings).

© 2016 Author(s). We are developing x-ray Thomson scattering for applications in implosion experiments at the National Ignition Facility. In particular we have designed and fielded MACS, a high-efficiency, gated x-ray spectrometer at 7.5-10 keV [T. Döppner et al., Rev. Sci. Instrum. 85, 11D617 (2014)]. Here we report on two new Bragg crystals based on Highly Oriented Pyrolytic Graphite (HOPG), a flat crystal and a dual-section cylindrically curved crystal. We have performed in situ calibration measurements using a brass foil target, and we used the flat HOPG crystal to measure Mo K-shell emission at 18 keV in 2nd order diffraction. Such high photon energy line emission will be required to penetrate and probe ultra-high-density plasmas or plasmas of mid-Z elements.

A technique for measuring residual motion during the stagnation phase of an indirectly driven inertial confinement experiment has been implemented. This method infers a velocity from spatially and temporally resolved images of the X-ray emission from two orthogonal lines of sight. This work investigates the accuracy of recovering spatially resolved velocities from the X-ray emission data. A detailed analytical and numerical modeling of the X-ray emission measurement shows that the accuracy of this method increases as the displacement that results from a residual velocity increase. For the typical experimental configuration, signal-to-noise ratios, and duration of X-ray emission, it is estimated that the fractional error in the inferred velocity rises above 50% as the velocity of emission falls below 24 μm/ns. By inputting measured parameters into this model, error estimates of the residual velocity as inferred from the X-ray emission measurements are now able to be generated for experimental data. Details...

The first indirect drive implosion experiments using Beryllium (Be) capsules at the National Ignition Facility confirm the superior ablation properties and elucidate possible Be-ablator issues such as hohlraum filling by ablator material. Since the 1990s, Be has been the preferred Inertial Confinement Fusion (ICF) ablator because of its higher mass ablation rate compared to that of carbon-based ablators. This enables ICF target designs with higher implosion velocities at lower radiation temperatures and improved hydrodynamic stability through greater ablative stabilization. Recent experiments to demonstrate the viability of Be ablator target designs measured the backscattered laser energy, capsule implosion velocity, core implosion shape from self-emission, and in-flight capsule shape from backlit imaging. The laser backscatter is similar to that from comparable plastic (CH) targets under the same hohlraum conditions. Implosion velocity measurements from backlit streaked radiography show that laser energy coupling to the hohlraum wall is comparable to plastic ablators. The measured implosion shape indicates no significant reduction of laser energy from the inner laser cone beams reaching the hohlraum wall as compared with plastic and high-density carbon ablators. These results indicate that the high mass ablation rate for beryllium capsules does not significantly alter hohlraum energetics. In addition, these data, together with data for low fill-density hohlraum performance, indicate that laser power multipliers, required to reconcile simulations with experimental observations, are likely due to our limited understanding of the hohlraum rather than the capsule physics since similar multipliers are needed for both Be and CH capsules as seen in experiments.

The near vacuum campaign on the National Ignition Facility has concentrated its efforts over the last year on finding the optimum target geometry to drive a symmetric implosion at high convergence ratio (30×). As the hohlraum walls are not tamped with gas, the hohlraum is filling with gold plasma and the challenge resides in depositing enough energy in the hohlraum before it fills up. Hohlraum filling is believed to cause symmetry swings late in the pulse that are detrimental to the symmetry of the hot spot at high convergence. This paper describes a series of experiments carried out to examine the effect of increasing the distance between the hohlraum wall and the capsule (case to capsule ratio) on the symmetry of the hot spot. These experiments have shown that smaller Case to Capsule Ratio (CCR of 2.87 and 3.1) resulted in oblate implosions that could not be tuned round. Larger CCR (3.4) led to a prolate implosion at convergence 30× implying that inner beam propagation at large CCR is not impeded by the expanding hohlraum plasma. A Case to Capsule ratio of 3.4 is a promising geometry to design a round implosion but in a smaller hohlraum where the hohlraum losses are lower, enabling a wider cone fraction range to adjust symmetry.

We present results of the beryllium experimental campaign on the implosion symmetry properties of beryllium capsules at the National Ignition Facility (NIF) [1]. These indirect drive experiments measure both the inflight and core self-emission implosion symmetry. The inflight symmetry of the ablator before stagnation is measured using a backlight imaging technique. A copper backlighter was used to measure the transmissions (or backlit absorption) of the copper doped beryllium shells. Images of the x-ray emission from the core around bang time provide a measure of the symmetry near peak compression. Both pieces of information about the 2D symmetry are used to infer the drive and velocity uniformity enabling us to predictably adjust the properties of the incident laser, mainly the time dependent ratio of the inner beam cone power to the outer laser beam powers, to achieve proper symmetry of the implosion. Results from these experiments show inner beam propagation is not degraded compared to similar implosions with CH ablators. Variations in the shape compared with implosions using CH ablators also provides information about the cross beam energy transfer used to adjust the equatorial shape and thus infer information about the differences in plasma conditions near the laser entrance holes. Experimental results of the implosion shape for beryllium capsules will be presented along with comparisons relative to CH ablators.

Encouraging results have been obtained using a strong first shock during the implosion of carbon-based ablator ignition capsules. These "high-foot" implosion results show that capsule performance deviates from 1D expectations as laser power and energy are increased. A possible cause of this deviation is the disruption of the hot spot by jets originating in the capsule fill tube. Nominally, a 10 μm outside diameter glass (SiO2) fill tube is used in these implosions. Simulations indicate that a thin coating of Au on this glass tube may lessen the hotspot disruption. These results and other mitigation strategies will be presented.

We introduce a new quasi 1-D implosion experimental platform at the National Ignition Facility designed to validate physics models as well as to study various Inertial Confinement Fusion aspects such as implosion symmetry, convergence, hydrodynamic instabilities, and shock timing. The platform has been developed to maintain shell sphericity throughout the compression phase and produce a round hot core at stagnation. This platform utilizes a 2-shock 1 MJ pulse with 340 TW peak power in a near-vacuum Au Hohlraum and a CH ablator capsule uniformly doped with 1% Si. We have performed several inflight radiography, symmetry capsule, and shock timing experiments in order to tune the symmetry of the capsule to near round throughout several epochs of the implosion. Adjusting the relative powers of the inner and outer cones of beams has allowed us to control the drive at the poles and equator of the capsule, thus providing the mechanism to achieve a spherical capsule convergence. Details and results of the tuning e...

Direct-drive-implosion experiments from both OMEGA and the National Ignition Facility (NIF) are critical to gain confidence in ignition predictions on the NIF. Adequate performance of hydrodynamically scaled 1.8-MJ ignition designs must be obtained on OMEGA at 26 kJ. Implosions on the NIF must be used to identify and mitigate the effect of laser-plasma interactions (LPI's) on hydrodynamic parameters at the NIF scale. Results from spherically driven OMEGA cryogenic implosion experiments are described. Mitigation of nonuniformity sources and cross-beam energy transfer (CBET) is important for improving target performance on OMEGA. Initial polar-driven implosion experiments on the NIF have provided valuable measurements of trajectory and symmetry. Simulations that include the effect of CBET more closely reproduce the observed velocity.

Accurately assessing and optimizing the implosion performance of inertial confinement fusion capsules is a crucial step to achieving ignition on the NIF. We have applied differential filtering (matched Ross filter pairs) to provide broadband time-integrated absolute x-ray self-emission images of the imploded core of cryogenic layered implosions. This diagnostic measures the temperature- and density-sensitive bremsstrahlung emission and provides estimates of hot spot mass, mix mass, and pressure.

We have conducted experiments on both the Vulcan and Titan laser facilities to study hot electron generation and transport in the context of fast ignition. Cu wires attached to Al cones were used to investigate the effect on coupling efficiency of plasma surround and the pre-formed plasma inside the cone. We found that with thin cones 15% of laser energy is coupled to the 40μm diameter wire emulating a 40μm fast ignition spot. Thick cone walls, simulating plasma in fast ignition, reduce coupling by x4. An increase of pre-pulse level inside the cone by a factor of 50 reduces coupling by a factor of 3. © 2010 IOP Publishing Ltd.