OMEGA Laser System Research Articles

Demonstration of neutron-yield enhancement by laser preheating and magnetization of laser-driven cylindrical implosions

Magnetized liner inertial fusion (MagLIF) is a fusion concept that uses magnetized, preheated fuel to reduce the implosion velocities and convergence ratios required for ignition. A scaled, laser-driven experimental platform to study MagLIF has been demonstrated on the OMEGA laser system, providing comprehensive experimental data on MagLIF scaling, utilizing the higher shot rate on OMEGA compared to the Z machine. Using this platform, a broader experimental space for MagLIF has been studied. Presented in this article are experimental results that demonstrate that the combination of preheat and magnetization enhances the neutron yield by 470% compared to a reference implosion, significantly more than the yield enhancement by the field or preheat alone. These results are achieved while maintaining a relatively low convergence ratio (<20). The experiments were supported by one-, two-, and three-dimensional radiation-hydrodynamics simulations, all of which suggest that multiple sources of mix play different key roles depending on the scale of the MagLIF experiment.

Design of the high-yield time-gated x-ray hot-spot imager for OMEGA.

Time-resolved x-ray self-emission imaging of hot spots in inertial confinement fusion experiments along several lines of sight provides critical information on the pressure and the transient morphology of the hot spot on the University of Rochester's OMEGA Laser System. At least three quasi-orthogonal lines of sight are required to infer the tomographic information of the hot spots of deuterium-tritium cryogenic layered implosions. OMEGA currently has two time-gated x-ray hot-spot imagers: the time-resolved Kirkpatrick-Baez x-ray microscope and the single-line-of-sight, time-resolved x-ray imager (SLOS-TRXI). The time-gated x-ray hot-spot imager (XRHSI) is being developed for use on OMEGA as the third line of sight for the high-yield operation of up to 4 × 1014 neutrons. XRHSI follows the SLOS-TRXI concept; however, it will have improved spatial and temporal resolutions of 5μm and 20ps, respectively. The simultaneous operation of the three instruments will provide 3-D reconstructions of the assembled hot-spot fuel at various times through peak thermonuclear output. The technical approach consists of a pinhole array imager and demagnifying time-dilation drift tube that are coupled to two side-by-side hybrid complementary metal-oxide semiconductor (hCMOS) image sensors. To minimize the background and to harden the diagnostics, an angled drift-tube assembly shifting the hCMOS sensors out of the direct line of sight and neutron shielding will be applied. The technical design space for the instrument will be discussed and the conceptual design will be presented.

Beam-pointing verification using x-ray pinhole cameras on the 60-beam OMEGA laser.

On the OMEGA laser system, the beam-pointing accuracy is verified by irradiating a 4mm diameter Au-coated spherical target with ∼23 kJ of laser energy. Up to ten x-ray pinhole cameras record the x-ray emission from all 60-beam spots. A new set of algorithms has been developed to improve the accuracy of the pointing evaluation. An updated edge-finding procedure allows one to infer the center of the sphere with subpixel accuracy. A new approach was introduced to back-propagate the pixel locations on the 2D image to the 3D surface of the sphere. A fast Fourier transform-based de-noising method significantly improves the signal-to-noise of the data. Based on the beam-pointing analysis, hard-sphere calculations of the laser-drive illumination uniformity on the target surface and the decomposition of the illumination distribution into lower order modes (1-10) are evaluated.

Diagnosing magnetic fields in cylindrical implosions with oblique proton radiography

Two experiments at the OMEGA Laser System used oblique proton radiography to measure magnetic fields in cylindrical implosions with and without an applied axial magnetic field. Although the goal of both experiments was to measure the magnitude of the compressed axial magnetic field in the core of the implosion, this field was obfuscated by two features in the coronal plasma produced by the compression beams: an azimuthal self-generated magnetic field and small length scale, high-amplitude structures attributed to collisionless effects. In order to understand these features, synthetic radiographs are generated using fields produced by 3D HYDRA simulations. These synthetic radiographs reproduce the features of the experimental radiographs with the exception of the small-scale structures. A direct inversion algorithm is successfully applied to a synthetic radiograph but is only partially able to invert the experimental radiographs in part because some protons are blocked by the field coils. The origins of the radiograph features and their dependence on various experimental parameters are explored. The results of this analysis should inform future measurements of compressed axial magnetic fields in cylindrical implosions.

Effect of laser preheat in magnetized liner inertial fusion at OMEGA

Integrated magnetized liner inertial fusion (MagLIF) experiments on the OMEGA laser system are modeled to investigate the effects of laser preheat on implosion performance. In both simulations and experiments, preheating the fuel with a laser enhances the neutron yield, with the maximum yield occurring at an optimal preheat laser energy. Increasing the preheat energy past the optimal value reduces the neutron yield. In simulations, the rate of the drop in neutron yield depends on the inclusion of the Nernst effect. The Nernst effect advects the magnetic field away from the center of the fuel region during the preheat stage of MagLIF on OMEGA and weakens magnetic-flux compression. If the Nernst effect is not included, the simulated drop in yield past the optimal preheat laser energy is more gradual as opposed to the sharper drop seen in experiments. Simulations are able to model trend in measured ion temperatures seen in experiments. A mix model is used to show that including mix from the wall in the simulated fuel region further degrades the yield and lowers the optimal preheat laser energy. Simulations with mix predict that increasing the initial axial magnetic field may still enhance yield performance in integrated implosions.

Using statistical modeling to predict and understand fusion experiments

Over 300 cryogenic layered direct-drive inertial confinement fusion implosions have been successfully executed on the OMEGA Laser System in the last decade. However, extracting sufficient understanding from these experiments to develop new designs or to identify or mitigate degradation sources remains challenging. Recently, a statistical modeling approach was developed to successfully design and predict improved implosion experiments on OMEGA. Here, we show that one-dimensional simulations can be used to predict the outcomes of systematically perturbed three-dimensional simulations and that this statistical modeling approach can be used to identify or rule out physical mechanisms for some of the degradation sources observed on the OMEGA Laser System for direct-drive cryogenic inertial confinement fusion. In this instance, we investigate the fusion yield dependencies on the ion temperature asymmetries and laser beam size observed in experiments by comparing with trends in three-dimensional synthetic simulation databases. Using the statistical model on these systematically perturbed simulations, we find that the statistically inferred dependency on the measured ion temperature asymmetries is well explained by imposed ℓ=1 modes. However, we find that the dependency on the laser beam size is only dominated by the illumination non-uniformity for some extreme cases.

Inverse ray tracing on icosahedral tetrahedron grids for non-linear laser plasma interaction coupled to 3D radiation hydrodynamics

A novel approach to efficiently model 3-D laser plasma interactions at fluid scales is presented. This method, implemented in the IFRIIT propagation code developed at CELIA, relies on inverse ray tracing to compute laser fields at arbitrary locations in a plasma. This enables to describe the fields at high order in space compared to standard forward ray tracing approaches. In addition, inverse ray tracing enables the use of etalon integral methods to reconstruct caustic fields and greatly speeds up calculations of cross-beam energy transfer by decoupling the ray amplitude and ray phase calculations. A comparison of the inverse and forward methods for 3-D calculations of fields in presence or not of cross-beam energy transfer illustrates the significant advantages of the inverse method. Conversely, while the inverse method is well suited to most spherical plasma profiles, it currently cannot treat concave profiles or target holders. The coupling of IFRIIT with the 3-D ASTER radiative hydrodynamics code developed at the Laboratory for Laser Energetics is then presented. ASTER and IFRIIT resolve their respective equations on separate grids which communicate through interpolation. As such, IFRIIT uses a dedicated laser grid adapted to the computations at play, which also allows to use different parallelization methods for both codes: block decomposition for the hydrodynamics versus domain duplication for the laser. Applications to direct-drive implosions for inertial confinement fusion are presented, for which a geodesic icosahedron grid is implemented in IFRIIT. The performances of the ASTER/IFRIIT coupling are demonstrated by conducting simulations of cryogenic implosions performed on the OMEGA laser system, in presence of various sources of 3-D effects; laser port geometry, cross-beam energy transfer, beam imbalance and target mis-alignment. Comparison with neutron data, measured through bang-time, for a cryogenic implosion experiment shows an excellent agreement for the laser-plasma coupling.

High-resolution x-ray radiography with Fresnel zone plates on the University of Rochester's OMEGA Laser Systems.

Experiments performed at the Laboratory for Laser Energetics with a continuous-wave (cw) x-ray source and on the OMEGA and OMEGA EP Laser Systems [Boehly et al., Opt. Commun. 133, 495 (1997) and Waxer et al., Opt. Photonics News 16, 30 (2005)] have utilized a Fresnel zone plate (FZP) to obtain x-ray images with a spatial resolution as small as ∼1.5 μm. Such FZP images were obtained with a charge-coupled device or a framing camera at energies ranging from 4.5 keV to 6.7 keV using x-ray line emission from both the cw source and high-intensity, laser-beam-illuminated metal foils. In all cases, the resolution test results are determined from patterns and grids backlit by these sources. The resolutions obtained are shown to be due to a combination of the spectral content of the x-ray sources and detector resolution limited by the magnification of the images (14× to 22×). High-speed framing cameras were used to obtain FZP images with frame times as short as ∼30 ps. Double-shell implosions on OMEGA were backlit by laser-irradiated Fe foils, thus obtaining a framing-camera-limited, FZP-image resolution of ∼3 μm-4 μm.

A novel photomultiplier tube neutron time-of-flight detector.

A traditional neutron time-of-flight (nTOF) detector used in inertial confinement fusion consists of a scintillator coupled with a photomultiplier tube (PMT). The instrument response function (IRF) of such a detector is dominated by the scintillator-light decay. In DT implosions with neutron yield larger than 1013, a novel detector consisting of a microchannel-plate (MCP) photomultiplier tube in a housing without a scintillator (PMT nTOF) can be used to measure DT yield, ion temperature, and neutron velocity. Most of the neutron signals in PMT nTOF detectors are produced from neutron interaction with a PMT window. The direct interaction of neutrons with the MCP provides negligible contribution. The elimination of the scintillator removes the scintillator decay from the instrument response function and makes the IRF of the PMT nTOF detector faster, which makes the ion temperature and neutron velocity measurements more accurate. Three PMT nTOF detectors were deployed in the OMEGA laser system for the first time to diagnose inertial confinement fusion plasma. The design details, characteristics, and calibration results of these detectors in DT implosions on OMEGA are presented. Recommendations on the use of different PMTs for specific applications are provided.

Self-radiography of imploded shells on OMEGA based on additive-free multi-monochromatic continuum spectral analysis

Radiographs of pure-DT cryogenic imploding shells provide critical validation of progress toward ignition-scalable performance of inertial confinement fusion implosions [J. Nuckolls et al., Nature 239, 139 (1972)]. Cryogenic implosions on the OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] can be self-radiographed by their own core spectral emission near ≈2 keV. Utilizing the distinct spectral dependences of continuum emissivity and opacity, the projected optical-thickness distribution of imploded shells, i.e., the shell radiograph, can be distinguished from the structure of the core emission distribution in images. Importantly, this can be done without relying on spectral additives (shell dopants), as in previous applications of implosion self-radiography [V. A. Smalyuk et al., Phys. Rev. Lett. 87, 155002 (2001); L. A. Pickworth et al., ibid. 117, 035001 (2016)]. Demonstrations with simulated data show that this technique is remarkably well-suited to cryogenic implosions and can also be applied to self-radiography of imploded room-temperature CH shells at higher spectral energy (hv ≈ 3–5 keV) based on the very similar continuum spectrum of carbon. Experimental demonstration of additive-free self-radiography with warm CH shell implosions on OMEGA will provide an important proof of principle for future applications to cryogenic DT implosions.

Constraining physical models at gigabar pressures.

High-energy-density (HED) experiments in convergent geometry are able to test physical models at pressures beyond hundreds of millions of atmospheres. The measurements from these experiments are generally highly integrated and require unique analysis techniques to procure quantitative information. This work describes a methodology to constrain the physics in convergent HED experiments by adapting the methods common to many other fields of physics. As an example, a mechanical model of an imploding shell is constrained by data from a thin-shelled direct-drive exploding-pusher experiment on the OMEGA laser system using Bayesian inference, resulting in the reconstruction of the shell dynamics and energy transfer during the implosion. The model is tested by analyzing synthetic data from a one-dimensional hydrodynamics code and is sampled using a Markov chain Monte Carlo to generate the posterior distributions of the model parameters. The goal of this work is to demonstrate a general methodology that can be used to draw conclusions from a wide variety of HED experiments.

Effect of cross-beam energy transfer on target-offset asymmetry in direct-drive inertial confinement fusion implosions

The unintentional mispositioning of inertial confinement fusion (ICF) capsules from the center of laser beam convergence has long been shown in simulations to generate large ℓ=1 asymmetry and significantly degrade implosion symmetry and fusion yields. Experimental yields on the OMEGA laser system, however, have shown much less sensitivity to this initial target offset. This paper presents simulations of offset ICF implosions improved by including a physics model of cross-beam energy transfer (CBET), a mechanism of laser energy scattering from one beam to another. Room-temperature OMEGA implosion experiments with prescribed target offsets are simulated with and without CBET, illustrating that CBET mitigates the ℓ=1 implosion asymmetry from the target offset. Comparison of simulations to multiple complementary experimental observables indicates that the addition of CBET physics in offset simulations is necessary to match experimental results.

Optical characterization of the on-target OMEGA focal spot at high energy using the full-beam in-tank diagnostic.

The full-beam in-tank (FBIT) diagnostic has been deployed to directly measure the target-plane beam fluence profile, when operated at high energy, of the OMEGA Laser System at the University of Rochester's Laboratory for Laser Energetics. This paper presents the results of early measurements taken with this diagnostic and discusses an improvement that has overcome performance limitations discovered during the initial testing. The diagnostic gives new insight into the ability of the OMEGA Laser System to provide uniform fluence profiles that are consistent across all 60 beams in the laser. The ultimate goal of the FBIT diagnostic is to allow accurate assessment of the fluence uniformity on a spherical target in 60-beam implosion experiments.

A suite of neutron time-of-flight detectors to measure hot-spot motion in direct-drive inertial confinement fusion experiments on OMEGA

A suite of six neutron time of flight detectors capable of measuring the primary DT neutron energy spectrum has recently been completed on the OMEGA Laser System. The detectors are positioned along multiple quasi-orthogonal lines of sight (LOS’s) in either a stand-alone, collinear, or antipodal configuration. The collinear detector configuration enables one to measure the neutron velocity along the detector LOS independent of the neutron-production history, while the antipodal detector configuration enables one to directly measure of the hot-spot velocity along the detector axis and Gamow velocity shift due to the plasma ion temperature. Using these six measurements of the primary DT neutron energy spectrum, the neutron-averaged hot-spot velocity and Gamow velocity shift of DT fusion neutrons have been measured in cryogenic experiments on OMEGA for the first time. The detector suite is operable between DT neutron yields of 1 × 1013 to 2 × 1014 with an uncertainty of ∼14 km/s in the neutron-averaged hot-spot velocity measurement.

Deuteron breakup induced by 14-MeV neutrons from inertial confinement fusion

Measurements are reported for the angle-averaged double-differential cross section ${\ensuremath{\langle}{d}^{2}\ensuremath{\sigma}/\phantom{\ensuremath{\sigma}d{E}_{n}d{\mathrm{\ensuremath{\Omega}}}_{n}}\phantom{\rule{0.0pt}{0ex}}d{E}_{n}d{\mathrm{\ensuremath{\Omega}}}_{n}\ensuremath{\rangle}}_{{0}^{\ensuremath{\circ}}l\ensuremath{\theta}l7.{4}^{\ensuremath{\circ}}}$ for the breakup reaction $^{2}\mathrm{H}$($n,2n$)$^{1}\mathrm{H}$, induced by 14-MeV neutrons generated using an inertial confinement fusion platform. A bright neutron source, created on the OMEGA Laser System [Boehly et al., Opt. Commun. 133, 495 (1997)] with a luminosity of $L={10}^{24}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, was used to irradiate deuterated targets. The absolute yields and energy spectra from the breakup neutrons emitted in a forward-angle geometry ($\ensuremath{\theta}={0}^{\ensuremath{\circ}}$ to 7.4\ifmmode^\circ\else\textdegree\fi{}) were detected with a sensitive, high-dynamic-range neutron time-of-flight spectrometer. The cross-section data, measured for neutron energy range from 0.5 to 10 MeV, is well reproduced by a theoretical calculation employing realistic nucleon--nucleon and three-nucleon forces.

Exchange-correlation thermal effects in shocked deuterium: Softening the principal Hugoniot and thermophysical properties

Exchange-correlation (XC) thermal effects for transport and optical properties of deuterium along the principal Hugoniot are investigated. The study is performed using ab initio molecular dynamics simulations within the Mermin-Kohn-Sham density functional theory. XC thermal effects are taken into account via the temperature-dependent Karasiev-Dufty-Trickey generalized gradient approximation functional [V. V. Karasiev et al., Phys. Rev. Lett. 120, 076401 (2018)]. We find that XC thermal effects account for the softening of the Hugoniot at pressures $Pg250$ GPa and improve agreement with recent experimental measurements. Also, XC thermal effects lead to the reflectivity increase by about 2% for shock speeds above 20 km/s. The calculated reflectivity for shock speeds up to 50 km/s is in excellent agreement with recent experimental measurements on the Omega Laser System. The dc conductivity is increased by about 4% due to XC thermal effects. The system evolution along the Hugoniot crosses the so-called warm-dense-matter regime, and XC thermal effects must be taken into account to accurately predict the thermophysical properties across warm-dense conditions.

Direct-drive measurements of laser-imprint-induced shock velocity nonuniformities.

Perturbations in the velocity profile of a laser-ablation-driven shock wave seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been measured. Direct measurements of these velocity perturbations were recorded using a two-dimensional high-resolution velocimeter probing plastic material shocked by a 100-ps picket laser pulse from the OMEGA laser system. The measured results for experiments with one, two, and five overlapping beams incident on the target clearly demonstrate a reduction in long-wavelength (>25-μm) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These experimental measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint for laser direct drive inertial confinement fusion research since they highlight the significant (factor of 3) underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation, such as multiphoton ionization are neglected.

Tripled yield in direct-drive laser fusion through statistical modelling.

Focusing laser light onto a very small target can produce the conditions for laboratory-scale nuclear fusion of hydrogen isotopes. The lack of accurate predictive models, which are essential for the design of high-performance laser-fusion experiments, is a major obstacle to achieving thermonuclear ignition. Here we report a statistical approach that was used to design and quantitatively predict the results of implosions of solid deuterium-tritium targets carried out with the 30-kilojoule OMEGA laser system, leading to tripling of the fusion yield to its highest value so far for direct-drive laser fusion. When scaled to the laser energies of the National Ignition Facility (1.9 megajoules), these targets are predicted to produce a fusion energy output of about 500 kilojoules-several times larger than the fusion yields currently achieved at that facility. This approach could guide the exploration of the vast parameter space of thermonuclear ignition conditions and enhance our understanding of laser-fusion physics.

Microphysics studies for direct-drive inertial confinement fusion

Accurate and self-consistent knowledge of material properties under high-energy-density (HED) conditions is crucial to reliably understand and design inertial confinement fusion (ICF) targets through radiation–hydrodynamic simulations. For direct-drive ICF target designs, the fuel deuterium–tritium mixtures and ablator materials can undergo a wide range of density and temperature conditions. Their properties under extreme HED conditions, including the equation of state, thermal conductivity, opacity, and stopping power, are the necessary inputs for ICF simulations. To improve the predictive capability of radiation–hydrodynamic codes for direct-drive ICF simulations, we have performed systematic ab initio studies on the static, transport, and optical properties of deuterium (D2) and ablator materials such as polystyrene (CH), beryllium (Be), and silicon (Si), using first-principles methods. The obtained material properties, being favorably compared with existing experimental data, have been implemented into radiation–hydrodynamic codes. This article gives a brief review on how these microphysics studies affect the 1D radiation–hydrodynamic predictions of direct-drive ICF implosions on the OMEGA Laser System.

The National Direct-Drive Inertial Confinement Fusion Program

The National Direct-Drive Inertial Confinement Fusion Program consists of the 100 Gbar Campaign on the 30 kJ, 351 nm, 60-beam OMEGA Laser System and the megajoule direct-drive (MJDD) Campaign on the 1.8 MJ, 351 nm, 192-beam National Ignition Facility (NIF). The main goals of the 100 Gbar Campaign are to demonstrate and understand the physics for hot-spot conditions and formation relevant for ignition at the MJ scale, while the MJDD Campaign seeks to understand the laser plasma interactions, energy coupling, and laser imprint for ignition-scale direct-drive coronal plasmas. An overview of the multiyear, systematic effort that is underway for the National Direct-Drive Inertial Confinement Fusion Program (including laser, target, and diagnostic improvements in progress), as well as recent results from the 100 Gbar Campaign on OMEGA and the MJDD Campaign on NIF is presented in this paper.