Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Hot-electron preheat and mitigation in polar-direct-drive experiments at the National Ignition Facility

Target preheat by superthermal electrons from laser-plasma instabilities is a major obstacle to achieving thermonuclear ignition via direct-drive inertial confinement fusion at the National Ignition Facility (NIF). Polar-direct-drive surrogate plastic implosion experiments were performed on the NIF to quantify preheat levels at an ignition-relevant scale and develop mitigation strategies. The experiments were used to infer the hot-electron temperature, energy fraction, and divergence, and to directly measure the spatial hot-electron energy deposition profile inside the imploding shell. Silicon layers buried in the ablator are shown to mitigate the growth of laser-plasma instabilities and reduce preheat, providing a promising path forward for ignition designs at an on-target intensity of about 10^{15}W/cm^{2}.

Measurements of low-mode asymmetries in the areal density of laser-direct-drive deuterium-tritium cryogenic implosions on OMEGA using neutron spectroscopy

Areal density is one of the key parameters that determines the confinement time in inertial confinement fusion experiments, and low-mode asymmetries in the compressed fuel are detrimental to the implosion performance. The energy spectra from the scattering of the primary deuterium-tritium (DT) neutrons off the compressed cold fuel assembly are used to investigate low-mode nonuniformities in direct-drive cryogenic DT implosions at the Omega Laser Facility. For spherically symmetric implosions, the shape of the energy spectrum is primarily determined by the elastic and inelastic scattering cross sections for both neutron-deuterium and neutron-tritium kinematic interactions. Two highly collimated lines of sight, which are positioned at nearly orthogonal locations around the OMEGA target chamber, record the neutron time-of-flight signal in the current mode. An evolutionary algorithm is being used to extract a model-independent energy spectrum of the scattered neutrons from the experimental neutron time-of-flight data and is used to infer the modal spatial variations (l = 1) in the areal density. Experimental observations of the low-mode variations of the cold-fuel assembly (ρL₀ + ρL₁) show good agreement with a recently developed model, indicating a departure from the spherical symmetry of the compressed DT fuel assembly. Another key signature that has been observed in the presence of a low-mode variation is the broadening of the kinematic end-point due to the anisotropy of the dense fuel conditions.

Development of an x-ray radiography platform to study laser-direct-drive energy coupling at the National Ignition Facility

A platform has been developed to study laser-direct-drive energy coupling at the National Ignition Facility (NIF) using a plastic sphere target irradiated in a polar-direct-drive geometry to launch a spherically converging shock wave. To diagnose this system evolution, eight NIF laser beams are directed onto a curved Cu foil to generate He_α line emission at a photon energy of 8.4 keV. These x rays are collected by a 100-ps gated x-ray imager in the opposing port to produce temporally gated radiographs. The platform is capable of acquiring images during and after the laser drive launches the shock wave. A backlighter profile is fit to the radiographs, and the resulting transmission images are Abel inverted to infer radial density profiles of the shock front and to track its temporal evolution. The measurements provide experimental shock trajectories and radial density profiles that are compared to 2D radiation-hydrodynamic simulations using cross-beam energy transfer and nonlocal heat-transport models.

Diagnosis of the imploding shell asymmetry in polar-direct-drive deuterium-tritium cryogenic target implosions on OMEGA

We discuss the analyses of gated, x-ray imaging data from polar-direct-drive experiments with cryogenically layered deuterium-tritium targets on the OMEGA laser. The in-flight shell asymmetries were diagnosed at various times during the implosion, which was caused by the beam pointing geometry and preimposed variations in the energy partition between the different groups of laser beams. The shape of the ablation surface during the acceleration phase of the implosion was measured along two different lines of sight, and a Legendre mode (ℓ-mode) decomposition was applied for modes of up to ten to investigate shell asymmetries. A clear causal relationship between the imposed beam imbalance and the shape of the in-flight shell asymmetries was observed. The imploded shell with a balanced energy ratio shows smaller values of the amplitudes of ℓ-mode 2 compared to that from implosions with an imbalanced ring energy ratio. The amplitudes of ℓ-modes 4 and 6 are the same within the measurement uncertainty with respect to the change in beam energy ratio.

A knock-on deuteron imager for measurements of fuel and hotspot asymmetry in direct-drive inertial confinement fusion implosions (invited)

A knock-on deuteron imager (KoDI) has been implemented to measure the fuel and hotspot asymmetry of cryogenic inertial confinement fusion implosions on OMEGA. Energetic neutrons produced by D-T fusion elastically scatter ("knock on") deuterons from the fuel layer with a probability that depends on ρR. Deuterons above 10 MeV are produced by near-forward scattering, and imaging them is equivalent to time-integrated neutron imaging of the hotspot. Deuterons below 6 MeV are produced by a combination of side scattering and ranging in the fuel, and encode information about the spatial distribution of the dense fuel. The KoDI instrument consists of a multi-penumbral aperture positioned 10-20 cm from the implosion using a ten-inch manipulator and a detector pack at 350 cm from the implosion to record penumbral images with magnification of up to 35×. Range filters and the intrinsic properties of CR-39 are used to distinguish different charged-particle images by energy along the same line of sight. Image plates fielded behind the CR-39 record a 10 keV x-ray image using the same aperture. A maximum-likelihood reconstruction algorithm has been implemented to infer the source from the projected penumbral images. The effects of scattering and aperture charging on the instrument point-spread function are assessed. Synthetic data are used to validate the reconstruction algorithm and assess an appropriate termination criterion. Significant aperture charging has been observed in the initial experimental dataset, and increases with aperture distance from the implosion, consistent with a simple model of charging by laser-driven EMP.

Three-dimensional hot-spot x-ray emission tomography from cryogenic deuterium-tritium direct-drive implosions on OMEGA

A three-dimensional model of the hot-spot x-ray emission has been developed and applied to the study of low-mode drive asymmetries in direct-drive inertial confinement fusion implosions on OMEGA with cryogenic deuterium-tritium targets. The steady-state model assumes an optically thin plasma and the data from four x-ray diagnostics along quasi-orthogonal lines of sight are used to obtain a tomographic reconstruction of the hot spot. A quantitative analysis of the hot-spot shape is achieved by projecting the x-ray emission into the diagnostic planes and comparing this projection to the measurements. The model was validated with radiation-hydrodynamic simulations assuming a mode-2 laser illumination perturbation resulting in an elliptically shaped hot spot, which was accurately reconstructed by the model using synthetic x-ray images. This technique was applied to experimental data from implosions in polar-direct-drive illumination geometry with a deliberate laser-drive asymmetry, and the hot-spot emission was reconstructed using spherical-harmonic modes of up to ℓ = 3. A 10% stronger drive on the equator relative to that on the poles resulted in a prolate-shaped hot spot at stagnation with a large negative A_2,0 coefficient of A_2,0 = -0.47 ± 0.03, directly connecting the modal contribution of the hot-spot shape with the modal contribution in laser-drive asymmetry.

Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion.

Experimentally Inferred Fusion Yield Dependencies of OMEGA Inertial Confinement Fusion Implosions

Statistical modeling of experimental and simulation databases has enabled the development of an accurate predictive capability for deuterium-tritium layered cryogenic implosions at the OMEGA laser [V. Gopalaswamy et al.,Nature 565, 581 (2019)10.1038/s41586-019-0877-0]. In this letter, a physics-based statistical mapping framework is described and used to uncover the dependencies of the fusion yield. This model is used to identify and quantify the degradation mechanisms of the fusion yield in direct-drive implosions on OMEGA. The yield is found to be reduced by the ratio of laser beam to target radius, the asymmetry in inferred ion temperatures from the ℓ=1 mode, the time span over which tritium fuel has decayed, and parameters related to the implosion hydrodynamic stability. When adjusted for tritium decay and ℓ=1 mode, the highest yield in OMEGA cryogenic implosions is predicted to exceed 2×10^{14} fusion reactions.

Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing

Laser-direct-drive symmetric implosions on OMEGA illuminate a target with 60 laser beams and are designed to produce spherical implosions. Each beam is smoothed using orthogonal polarizations obtained by passing the laser beams through distributed polarization rotators (DPRs). Observations of light scattered from OMEGA implosions do not show the expected symmetry and have much larger variation than standard predictions. For the first time, we have quantified the scattered-light nonuniformity from individual beams and identified the DPRs as the source of the enhanced nonuniformity. An instrument was invented that isolated and measured the variation in the intensity and polarization of the light scattered from each OMEGA beam. The asymmetric intensity and polarization measurements are explained when the on-target offsets between the two orthogonal polarizations produced by the DPRs are modeled using a 3D cross-beam energy transfer (CBET) code that tracks the polarizations of each beam. The time-integrated nonuniformity in laser absorption and scattered light due to CBET and the DPR polarization offsets during high-performance OMEGA implosions is predicted to be significant and dominated by low spherical harmonic mode numbers. The nonuniformity is predicted to be greatly reduced by replacing the DPRs with new optics that create smaller offsets.

The Scattered Light Time-history Diagnostic suite at the National Ignition Facility

The Scattered Light Time-history Diagnostic (SLTD) is being implemented at the National Ignition Facility (NIF) to greatly expand the angular coverage of absolute scattered-light measurements for direct- and indirect-drive inertial confinement fusion (ICF) experiments. The SLTD array will ultimately consist of 15 units mounted at a variety of polar and azimuthal angles on the NIF target chamber, complementing the existing NIF backscatter suite. Each SLTD unit collects and diffuses scattered light onto a set of three optical fibers, which transport the light to filtered photodiodes to measure scattered light in different wavelength bands: stimulated Brillouin scattering (350 nm-352 nm), stimulated Raman scattering (430 nm-760 nm), and ω/2 (695 nm-745 nm). SLTD measures scattered light with a time resolution of ∼1 ns and a signal-to-noise ratio of up to 500. Currently, six units are operational and recording data. Measurements of the angular dependence of scattered light will strongly constrain models of laser energy coupling in ICF experiments and allow for a more robust inference of the total laser energy coupled to implosions.

Direct-drive laser fusion: status, plans and future

Laser-direct drive (LDD), along with laser indirect (X-ray) drive (LID) and magnetic drive with pulsed power, is one of the three viable inertial confinement fusion approaches to achieving fusion ignition and gain in the laboratory. The LDD programme is primarily being executed at both the Omega Laser Facility at the Laboratory for Laser Energetics and at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. LDD research at Omega includes cryogenic implosions, fundamental physics including material properties, hydrodynamics and laser-plasma interaction physics. LDD research on the NIF is focused on energy coupling and laser-plasma interactions physics at ignition-scale plasmas. Limited implosions on the NIF in the 'polar-drive' configuration, where the irradiation geometry is configured for LID, are also a feature of LDD research. The ability to conduct research over a large range of energy, power and scale size using both Omega and the NIF is a major positive aspect of LDD research that reduces the risk in scaling from OMEGA to megajoule-class lasers. The paper will summarize the present status of LDD research and plans for the future with the goal of ultimately achieving a burning plasma in the laboratory. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Novel Hot-Spot Ignition Designs for Inertial Confinement Fusion with Liquid-Deuterium-Tritium Spheres

A new class of ignition designs is proposed for inertial confinement fusion experiments. These designs are based on the hot-spot ignition approach, but instead of a conventional target that is comprised of a spherical shell with a thin frozen deuterium-tritium (DT) layer, a liquid DT sphere inside a wetted-foam shell is used, and the lower-density central region and higher-density shell are created dynamically by appropriately shaping the laser pulse. These offer several advantages, including simplicity in target production (suitable for mass production for inertial fusion energy), absence of the fill tube (leading to a more-symmetric implosion), and lower sensitivity to both laser imprint and physics uncertainty in shock interaction with the ice-vapor interface. The design evolution starts by launching an ∼1-Mbar shock into a DT sphere. After bouncing from the center, the reflected shock reaches the outer surface of the sphere and the shocked material starts to expand outward. Supporting ablation pressure ultimately stops such expansion and subsequently launches a shock toward the target center, compressing the ablator and fuel, and forming a shell. The shell is then accelerated and fuel is compressed by appropriately shaping the drive laser pulse, forming a hot spot using the conventional or shock ignition approaches. This Letter demonstrates the feasibility of the new concept using hydrodynamic simulations and discusses the advantages and disadvantages of the concept compared with more-traditional inertial confinement fusion designs.

Implementing a microphysics model in hydrodynamic simulations to study the initial plasma formation in dielectric ablator materials for direct-drive implosions

A microphysics model to describe the photoionization and impact ionization processes in dielectric ablator materials like plastic has been implemented into the one-dimensional hydrodynamic code LILAC for planar and spherical targets. At present, the initial plasma formation during the early stages of a laser drive are modeled in an ad hoc manner, until the formation of a critical surface. Implementation of the physics-based models predict higher values of electron temperature and pressure than the ad hoc model. Moreover, the numerical predictions are consistent with previous experimental observations of the shinethrough mechanism in plastic ablators. For planar targets, a decompression of the rear end of the target was observed that is similar to recent experiments. An application of this model is to understand the laser-imprint mechanism that is caused by nonuniform laser irradiation due to single beam speckle.

Modeling the solid-to-plasma transition for laser imprinting in direct-drive inertial confinement fusion

Laser imprinting possesses a potential danger for low-adiabat and high-convergence implosions in direct-drive inertial confinement fusion (ICF). Within certain direct-drive ICF schemes, a laser picket (prepulse) is used to condition the target to increase the interaction efficiency with the main pulse. Whereas initially the target is in a solid state (of ablators such as polystyrene) with specific electronic and optical properties, the current state-of-the-art hydrocodes assume an initial plasma state, which ignores the detailed plasma formation process. To overcome this strong assumption, a model describing the solid-to-plasma transition, eventually aiming at being implemented in hydrocodes, is developed. It describes the evolution of main physical quantities of interest, including the free electron density, collision frequency, absorbed laser energy, temperatures, and pressure, during the first stage of the laser-matter interaction. The results show that a time about 100 ps is required for the matter to undergo the phase transition, the initial solid state thus having a notable impact on the subsequent plasma dynamics. The nonlinear absorption processes (associated to the solid state) are also shown to have an influence on the thermodynamic quantities after the phase transition, leading to target deformations depending on the initial solid state. The negative consequences for the ICF schemes consist in shearing of the ablator and possibly preliminary heating of the deuterium-tritium fuel.

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.

Measurement of the shell decompression in direct-drive inertial-confinement-fusion implosions

A series of direct-drive implosions performed on OMEGA were used to isolate the effect of an adiabat on the in-flight shell thickness. The maximum in-flight shell thickness was measured to decrease from 75±2 to 60±2μm when the adiabat of the shell was reduced from 6 to 4.5, but when decreasing the adiabat further (1.8), the shell thickness increased to 75±2μm due to the growth of the Rayleigh-Taylor instability. Hydrodynamic simulations suggest that a laser imprint is the dominant seed for these nonuniformities.

First Measurements of Deuterium-Tritium and Deuterium-Deuterium Fusion Reaction Yields in Ignition-Scalable Direct-Drive Implosions

The deuterium-tritium (D-T) and deuterium-deuterium neutron yield ratio in cryogenic inertial confinement fusion (ICF) experiments is used to examine multifluid effects, traditionally not included in ICF modeling. This ratio has been measured for ignition-scalable direct-drive cryogenic DT implosions at the Omega Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)OPCOB80030-401810.1016/S0030-4018(96)00325-2] using a high-dynamic-range neutron time-of-flight spectrometer. The experimentally inferred yield ratio is consistent with both the calculated values of the nuclear reaction rates and the measured preshot target-fuel composition. These observations indicate that the physical mechanisms that have been proposed to alter the fuel composition, such as species separation of the hydrogen isotopes [D. T. Casey et al., Phys. Rev. Lett. 108, 075002 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.075002], are not significant during the period of peak neutron production in ignition-scalable cryogenic direct-drive DT implosions.

High-dynamic-range neutron time-of-flight detector used to infer the D(t,n)4He and D(d,n)3He reaction yield and ion temperature on OMEGA

Upgraded microchannel-plate-based photomultiplier tubes (MCP-PMT's) with increased stability to signal-shape linearity have been implemented on the 13.4-m neutron time-of-flight (nTOF) detector at the Omega Laser Facility. This diagnostic uses oxygenated xylene doped with diphenyloxazole C₁₅H₁₁NO + p-bis-(o-methylstyryl)-benzene (PPO + bis-MSB) wavelength shifting dyes and is coupled through four viewing ports to fast-gating MCP-PMT's, each with a different gain to allow one to measure the light output over a dynamic range of 1 × 10⁶. With these enhancements, the 13.4-m nTOF can measure the D(t,n)⁴He and D(d,n)³He reaction yields and average ion temperatures in a single line of sight. Once calibrated for absolute neutron sensitivity, the nTOF detectors can be used to measure the neutron yield from 1 × 10⁹ to 1 × 10¹⁴ and the ion temperature with an accuracy approaching 5% for both the D(t,n)⁴He and D(d,n)³He reactions.

Demonstration of Fuel Hot-Spot Pressure in Excess of 50 Gbar for Direct-Drive, Layered Deuterium-Tritium Implosions on OMEGA

A record fuel hot-spot pressure P_{hs}=56±7  Gbar was inferred from x-ray and nuclear diagnostics for direct-drive inertial confinement fusion cryogenic, layered deuterium-tritium implosions on the 60-beam, 30-kJ, 351-nm OMEGA Laser System. When hydrodynamically scaled to the energy of the National Ignition Facility, these implosions achieved a Lawson parameter ∼60% of the value required for ignition [A. Bose et al., Phys. Rev. E 93, 011201(R) (2016)], similar to indirect-drive implosions [R. Betti et al., Phys. Rev. Lett. 114, 255003 (2015)], and nearly half of the direct-drive ignition-threshold pressure. Relative to symmetric, one-dimensional simulations, the inferred hot-spot pressure is approximately 40% lower. Three-dimensional simulations suggest that low-mode distortion of the hot spot seeded by laser-drive nonuniformity and target-positioning error reduces target performance.

Masked-backlighter technique used to simultaneously image x-ray absorption and x-ray emission from an inertial confinement fusion plasma

A method to simultaneously image both the absorption and the self-emission of an imploding inertial confinement fusion plasma has been demonstrated on the OMEGA Laser System. The technique involves the use of a high-Z backlighter, half of which is covered with a low-Z material, and a high-speed x-ray framing camera aligned to capture images backlit by this masked backlighter. Two strips of the four-strip framing camera record images backlit by the high-Z portion of the backlighter, while the other two strips record images aligned with the low-Z portion of the backlighter. The emission from the low-Z material is effectively eliminated by a high-Z filter positioned in front of the framing camera, limiting the detected backlighter emission to that of the principal emission line of the high-Z material. As a result, half of the images are of self-emission from the plasma and the other half are of self-emission plus the backlighter. The advantage of this technique is that the self-emission simultaneous with backlighter absorption is independently measured from a nearby direction. The absorption occurs only in the high-Z backlit frames and is either spatially separated from the emission or the self-emission is suppressed by filtering, or by using a backlighter much brighter than the self-emission, or by subtraction. The masked-backlighter technique has been used on the OMEGA Laser System to simultaneously measure the emission profiles and the absorption profiles of polar-driven implosions.

Comparison between x-ray scattering and velocity-interferometry measurements from shocked liquid deuterium

The equation of state of light elements is essential to understand the structure of Jovian planets and inertial confinement fusion research. The Omega laser was used to drive a planar shock wave in the cryogenically cooled deuterium, creating warm dense matter conditions. X-ray scattering was used to determine the spectrum near the boundary of the collective and noncollective scattering regimes using a narrow band x-ray source in backscattering geometry. Our scattering spectra are thus sensitive to the individual electron motion as well as the collective plasma behavior and provide a measurement of the electron density, temperature, and ionization state. Our data are consistent with velocity-interferometry measurements previously taken on the same shocked deuterium conditions and presented by K. Falk et al. [High Energy Density Phys. 8, 76 (2012)]. This work presents a comparison of the two diagnostic systems and offers a detailed discussion of challenges encountered.

Inelastic x-ray scattering from shocked liquid deuterium

The Fermi-degenerate plasma conditions created in liquid deuterium by a laser-ablation-driven shock wave were probed with noncollective, spectrally resolved, inelastic x-ray Thomson scattering employing Cl Ly(α) line emission at 2.96 keV. These first x-ray Thomson scattering measurements of the microscopic properties of shocked deuterium show an inferred spatially averaged electron temperature of 8±5  eV, an electron density of 2.2(±0.5)×10(23)  cm(-3), and an ionization of 0.8 (-0.25, +0.15). Two-dimensional hydrodynamic simulations using equation-of-state models suited for the extreme parameters occurring in inertial confinement fusion research and planetary interiors are consistent with the experimental results.

High-resolution spectroscopy used to measure inertial confinement fusion neutron spectra on Omega (invited)

The areal density (ρR) of cryogenic DT implosions on Omega is inferred by measuring the spectrum of neutrons that elastically scatter off the dense deuterium (D) and tritium (T) fuel. Neutron time-of-flight (nTOF) techniques are used to measure the energy spectrum with high resolution. High signal-to-background data has been recorded on cryogenic DT implosions using a well-collimated 13.4-m line of sight and an nTOF detector with an advanced liquid scintillator compound. An innovative method to analyze the elastically scattered neutron spectra was developed using well-known cross sections of the DT nuclear reactions. The estimated areal densities are consistent with alternative ρR measurements and 1-D simulations.

Measurements of the T(t,2n)4He neutron spectrum at low reactant energies from inertial confinement implosions

Measurements of the neutron spectrum from the T(t,2n)4He (tt) reaction have been conducted using inertial confinement fusion implosions at the OMEGA laser facility. In these experiments, deuterium-tritium (DT) gas-filled capsules were imploded to study the tt reaction in thermonuclear plasmas at low reactant center-of-mass (c.m.) energies. In contrast to accelerator experiments at higher c.m. energies (above 100 keV), these results indicate a negligible n + 5He reaction channel at a c.m. energy of 23 keV.

Increasing hydrodynamic efficiency by reducing cross-beam energy transfer in direct-drive-implosion experiments

A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by ∼35%, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by 20%. Over this range, the absorption is measured to increase by 15%, resulting in a 17% increase in the implosion velocity and a 10% earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed 8  μm and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models.

Evidence for stratification of deuterium-tritium fuel in inertial confinement fusion implosions

Measurements of the D(d,p)T (dd) and T(t,2n)(4)He (tt) reaction yields have been compared with those of the D(t,n)(4)He (dt) reaction yield, using deuterium-tritium gas-filled inertial confinement fusion capsule implosions. In these experiments, carried out on the OMEGA laser, absolute spectral measurements of dd protons and tt neutrons were obtained. From these measurements, it was concluded that the dd yield is anomalously low and the tt yield is anomalously high relative to the dt yield, an observation that we conjecture to be caused by a stratification of the fuel in the implosion core. This effect may be present in ignition experiments planned on the National Ignition Facility.

Demonstration of the highest deuterium-tritium areal density using multiple-picket cryogenic designs on OMEGA

The performance of triple-picket deuterium-tritium cryogenic target designs on the OMEGA Laser System [T. R. Boehly, Opt. Commun. 133, 495 (1997)] is reported. These designs facilitate control of shock heating in low-adiabat inertial confinement fusion targets. Areal densities up to 300 mg/cm2 (the highest ever measured in cryogenic deuterium-tritium implosions) are inferred in the experiments with an implosion velocity approximately 3x10(7) cm/s driven at peak laser intensities of 8x10(14) W/cm2. Extension of these designs to ignition on the National Ignition Facility [J. A. Paisner, Laser Focus World 30, 75 (1994)] is presented.

Implosion experiments using glass ablators for direct-drive inertial confinement fusion

Direct-drive implosions with 20-microm-thick glass shells were conducted on the Omega Laser Facility to test the performance of high-Z glass ablators for direct-drive, inertial confinement fusion. The x-ray signal caused by hot electrons generated by two-plasmon-decay instability was reduced by more than approximately 40x and hot-electron temperature by approximately 2x in the glass compared to plastic ablators at ignition-relevant drive intensities of approximately 1x10(15) W/cm2, suggesting reduced target preheat. The measured absorption and compression were close to 1D predictions. The measured soft x-ray production in the spectral range of approximately 2 to 4 keV was approximately 2x to 3x lower than 1D predictions, indicating that the shell preheat caused by soft x-rays is less than predicted. A direct-drive-ignition design based on glass ablators is introduced.

Studies of plastic-ablator compressibility for direct-drive inertial confinement fusion on OMEGA

The compression of planar plastic targets was studied with x-ray radiography in the range of laser intensities of I approximately 0.5 to 1.5x10(15) W/cm2 using square (low-compression) and shaped (high-compression) pulses. Two-dimensional simulations with the radiative hydrocode DRACO show good agreement with measurements at laser intensities up to I approximately 10(15) W/cm2. These results provide the first experimental evidence for low-entropy, adiabatic compression of plastic shells in the laser intensity regime relevant to direct-drive inertial confinement fusion. A density reduction near the end of the drive at a high intensity of I approximately 1.5x10(15) W/cm2 has been correlated with the hard x-ray signal caused by hot electrons from two-plasmon-decay instability.

Role of hot-electron preheating in the compression of direct-drive imploding targets with cryogenic D2 ablators

The compression of direct-drive, spherical implosions is studied using cryogenic D2 targets on the 60-beam, 351-nm OMEGA laser with intensities ranging from approximately 3x10(14) to approximately 1x10(15) W/cm2. The hard-x-ray signal from hot electrons generated by laser-plasma instabilities increases with laser intensity, while the areal density decreases. Mitigating hot-electron production, by reducing the laser intensity to approximately 3x10(14) W/cm2, results in areal density of the order of approximately 140 mg/cm2, in good agreement with 1D simulations. These results will be considered in future direct-drive-ignition designs.

High-areal-density fuel assembly in direct-drive cryogenic implosions

The first observation of ignition-relevant areal-density deuterium from implosions of capsules with cryogenic fuel layers at ignition-relevant adiabats is reported. The experiments were performed on the 60-beam, 30-kJUV OMEGA Laser System [T. R. Boehly, Opt. Commun. 133, 495 (1997)10.1016/S0030-4018(96)00325-2]. Neutron-averaged areal densities of 202+/-7 mg/cm2 and 182+/-7 mg/cm2 (corresponding to estimated peak fuel densities in excess of 100 g/cm3) were inferred using an 18-kJ direct-drive pulse designed to put the converging fuel on an adiabat of 2.5. These areal densities are in good agreement with the predictions of hydrodynamic simulations indicating that the fuel adiabat can be accurately controlled under ignition-relevant conditions.

High-rhoR implosions for fast-ignition fuel assembly

Thick, 40 microm plastic shells filled with 25-35 atm of D2 or D3He were imploded on a low-adiabat (alpha approximately 1.3) and with a low-implosion velocity ( approximately 2 x 10(70 cm/s) on the OMEGA laser to generate massive cores of compressed plasma with high areal densities optimal for fast ignition. The targets are driven by 20-kJ relaxation adiabat-shaping laser pulses to keep the inner portion of the shell nearly Fermi degenerate. The measured kinetic energy downshift of proton spectra is in good agreement with the theoretical predictions yielding burn-averaged areal densities of 0.130+/-0.017 g/cm2 and peak rhoR during the burn of about 0.24+/-0.018 g/cm2, the largest rhoR measured on OMEGA to date. The same implosions with empty plastic shells are expected to reach 1.3 g/cm2 across the core (i.e., 2rhoR) enough to stop fast electrons with energies up to 4.5 MeV typical of fast ignition scenarios.

Effects of nonuniform illumination on implosion asymmetry in direct-drive inertial confinement fusion

Target areal density (rhoR) asymmetries in OMEGA direct-drive spherical implosions are studied. The rms variation <deltarhoR>/<rhoR> for low-mode-number structure is approximately proportional to the rms variation of on-target laser intensity <deltaI>/<I> with an amplification factor of approximately 1/2(C(r)-1), where C(r) is the capsule convergence ratio. This result has critical implications for future work on the National Ignition Facility as well as OMEGA.

Dependence of shell mix on feedthrough in direct drive inertial confinement fusion

The mixing of cold, high-density shell plasma with the low-density, hot spot plasma by the Rayleigh-Taylor instability in inertial confinement fusion is experimentally shown to correlate with the calculated perturbation feedthrough from the ablation surface to the inner shell surface. A fourfold decrease in the density of shell material in the mix region of direct drive implosions of gas filled spherical plastic shells having predicted convergence ratios approximately 15 was observed when laser imprint levels were reduced and the initial shell was thicker, corresponding to a reduction in the feedthrough rms level by a factor of 6. Shell mix is also shown to limit the spherical compression of the implosion.

Time-resolved areal-density measurements with proton spectroscopy in spherical implosions

The temporal history of the target areal-density near peak compression of direct-drive spherical target implosions has been inferred with 14.7-MeV deuterium-helium-3 D3He proton spectroscopy of the 60-beam, 30-kJ UV OMEGA laser system. The target areal-density grows by a factor of approximately 8 during the time of neutron-production ( approximately 400 ps) before reaching 123+/-16 mg/cm(2) at peak compression in the implosion of a 950-micrometer-diam, 20-micrometer-thick plastic CH capsule filled with 4 atm of D3He fuel.

Measuring implosion dynamics through rhoR evolution in inertial-confinement fusion experiments

The areal density (rhoR) of D3He filled plastic capsules imploded at OMEGA has been measured at shock coalescence (1.7 ns) and, 400 ps later, during compressive burn, through the energy downshift of 14.7-MeV D3He protons. In this time interval, the azimuthally averaged rhoR changes from 13+/-2.5 to 70+/-8 mg/cm(2). The experiments demonstrate that fuel-shell mix is absent in the central regions at shock coalescence, and that the shell has no holes during compressive burn. We conjecture that rhoR asymmetries measured during compressive burn may be seeded by the time of shock coalescence.

Effects of fuel-shell mix upon direct-drive, spherical implosions on OMEGA

Fuel-shell mix and implosion performance are studied for many capsule types in direct-drive experiments at OMEGA. The amount of mixing and the size of the mix region are inferred from charged-particle spectrometry data and confirmed with an experimentally constrained model. Measured yields and convergence ratios CR fall short of one-dimensional predictions, especially for low capsule fill pressures. CR is approximately 11 for pressures from 3 to 15 atm, in contrast to predictions of approximately 25 for 3 atm and approximately 12 for 15 atm. The performance shortfalls are likely to be caused by fuel-shell mix.

Shell mix in the compressed core of spherical implosions

The Rayleigh-Taylor instability in its highly nonlinear, turbulent stage causes atomic-scale mixing of the shell material with the fuel in the compressed core of inertial-confinement fusion targets. The density of shell material mixed into the outer core of direct-drive plastic-shell spherical-target implosions on the 60-beam, OMEGA laser system is estimated to be 3.4(+/-1.2) g/cm(3) from time-resolved x-ray spectroscopy, charged-particle spectroscopy, and core x-ray images. The estimated fuel density, 3.6(+/-1) g/cm(3), accounts for only approximately 50% of the neutron-burn-averaged electron density, n(e)=2.2(+/-0.4)x10(24) cm(-3).