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Direct drive: Simulations and Results from the National Ignition Facility
Abstract
Direct-drive implosion physics is being investigated at the National Ignition Facility. The primary goal of the experiments is twofold: to validate modeling related to implosion velocity and to estimate the magnitude of hot-electron preheat. Implosion experiments indicate that the energetics is well-modeled when cross-beam energy transfer (CBET) is included in the simulation and an overall multiplier to the CBET gain factor is employed; time-resolvedscattered light and scattered-lightspectra display the correct trends. Trajectories from backlit images are well modeled, although those from measured self-emission images indicate increased shell thickness and reduced shell density relative to simulations. Sensitivity analyses indicate that the most likely cause for the density reduction is nonuniformity growth seeded by laser imprint and not laser-energy coupling. Hot-electron preheat is at tolerable levels in the ongoing experiments, although it is expected to increase after the mitigation of CBET. Future work will include continued model validation, imprint measurements, and mitigation of CBET and hot-electron preheat.
... Modeling and simulations of laser coupling, preheat, and laser imprint are tested at the MJ scale with NIF direct-drive experiments [20][21][22][23][24]. ...
... While these improvements are underway, the 100-Gbar Campaign has a parallel effort using a statistical approach to search for the optimum DT cryogenic implosion on OMEGA having the highest combination of areal density and primary neutron yield using the current capabilities [19]. The Megajoule Direct-Drive (MJDD) Campaign on the NIF investigates direct-drive physics at long scale lengths [20][21][22][23][24]. The behavior of laser-plasma interactions (LPI) in coronal plasmas characteristic of LDD ignition target designs is especially challenging to predict with certainty; therefore, experiments are conducted in plasmas having the relevant temperatures and scale lengths on the NIF for LPI, as well as for laser coupling and laser imprint. ...
... CBET reduces the coupling of laser energy to the imploding shell, and TPD and SRS are potential sources of suprathermal electrons that can cause the thermonuclear fuel to be preheated. The second element involves an investigation of LPI, energy coupling, and imprint mitigation at the MJ-scale on the NIF [20][21][22][23][24]. It includes planar, implosion, and cone-in-shell target platforms. ...
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 multi-year, systematic effort that is underway for the National Direct-Drive Inertial Confinement Fusion Program, including laser, target, and diagnostic improvements that are in progress, as well as recent results from the 100-Gbar Campaign on OMEGA and MJDD Campaign on NIF are presented.
... 7 The MJDD Campaign on the NIF investigates direct-drive physics at long density scale lengths and high temperatures characteristic of coronal plasmas of direct-drive ignition targets. 8,9 Currently only polar-direct-drive 10 implosions can be performed on the NIF with the current beam configuration [8][9][10] ; however, spherical direct drive is the ultimate goal of the National Direct-Drive Program. A reconfiguration study is underway to convert the NIF from the current polar arrangement of laser beams around the target chamber to a spherical distribution. ...
... 7 The MJDD Campaign on the NIF investigates direct-drive physics at long density scale lengths and high temperatures characteristic of coronal plasmas of direct-drive ignition targets. 8,9 Currently only polar-direct-drive 10 implosions can be performed on the NIF with the current beam configuration [8][9][10] ; however, spherical direct drive is the ultimate goal of the National Direct-Drive Program. A reconfiguration study is underway to convert the NIF from the current polar arrangement of laser beams around the target chamber to a spherical distribution. ...
... 11, 12, and 13) and simulations indicate that a reduction in P abl as large as 60% could occur in NIF-scale implosions. 9 In addition to CBET, hydrodynamic instabilities and low-mode drive asymmetries can reduce P hs and generate a mix of cold fuel/ablator into the hot spot, impacting the implosion performance rate. 15 Also, suprathermal electron generation by the two-plasmon-decay (TPD) instability and stimulated Raman scattering 16,17 (SRS), which can preheat the DT fuel, can raise α, and can lower the hot-spot pressure for a given P abl and v imp . ...
The goal of the National Direct-Drive Program is to demonstrate and understand the physics of laser direct drive (LDD). Efforts are underway on OMEGA for the 100-Gbar Campaign to demonstrate and understand the physics for hot-spot conditions and formation relevant for ignition at the 1-MJ scale, and on the National Ignition Facility to develop an understanding of the direct-drive physics at long scale lengths for the MJ Direct-Drive Campaign. The strategy of the National Direct-Drive Program is described; the requirements for the deuterium-tritium cryogenic fill-tube target being developed for OMEGA are presented; and preliminary LDD implosion measurements of hydrodynamic mixing seeded by laser imprint, the target-mounting stalk, and microscopic surface debris are reported.
... Measured ablation-front trajectories agree well with simulations that used a CBET multiplier of 2, which has been shown for similar NIF experiments that used CH shells. 39 To mitigate the effects of shell decompression on the ablation-front trajectories, the experiments were limited to early times. Large perturbations at the ablation front can expand the ablation-front surface away from the shell's center of mass. ...
... Large perturbations at the ablation front can expand the ablation-front surface away from the shell's center of mass. 39 In the OMEGA experiments, the 2-D SSD limits the imprint and perturbations were shown to have minimal impact on the trajectories. 35 On both facilities, the radiation from the Si layer reduced the Rayleigh-Taylor (RT) growth, but on the NIF, the RT growth caused by high levels of laser imprint occurred in spite of the smoothing effects, which mixed the Si and CH at the interface, reducing the contrast of the outer interface peak in the x-ray images. ...
The angularly resolved mass ablation rates and ablation-front trajectories for Si-coated CH targets were measured in direct-drive inertial confinement fusion experiments to quantify cross-beam energy transfer (CBET) while constraining the hydrodynamic coupling. A polar-direct-drive laser configuration, where the equatorial laser beams were dropped and the polar beams were repointed from a symmetric direct-drive configuration, was used to limit CBET at the pole while allowing it to persist at the equator. The combination of low- and high-CBET conditions observed in the same implosion allowed for the effects of CBET on the ablation rate and ablation pressure to be determined. Hydrodynamic simulations performed without CBET agreed with the measured ablation rate and ablation-front trajectory at the pole of the target, confirming that the CBET effects on the pole are small. The simulated mass ablation rates and ablation-front trajectories were in excellent agreement with the measurements at all angles when a CBET model based on Randall's equations [C. J. Randall et al., Phys. Fluids 24, 1474 (1981)] was included into the simulations with a multiplier on the CBET gain factor. These measurements were performed on OMEGA and at the National Ignition Facility to access a wide range of plasma conditions, laser intensities, and laser beam geometries. The presence of the CBET gain multiplier required to match the data in all of the configurations tested suggests that additional physics effects, such as intensity variations caused by diffraction, polarization effects, or shortcomings of extending the 1-D Randall model to 3-D, should be explored to explain the differences in observed and predicted drive.
... The convergence of direct-drive implosions on NIF is indeed presently restrained [71,72] by the laser-irradiation inhomogeneities imprinted on the target early in the laser pulse, with implementation of multiple-frequency optical smoothing [72,73] being considered to go beyond. Instead, FSBS (shown to still develop with LSSD applied) can provide this highest level of irradiation uniformity needed in the first hundreds of picoseconds, while the standard SSD smooths the laser irradiation of the target over the entire laser pulse. ...
Past experiments [S. Depierreux et al., Phys. Rev. Lett. 102, 195005 (2009)] have exhibited the plasma-induced incoherence (PII) process and the reduced imprint in the multikilojoule regime when a thin low-density foam is disposed in front of a solid target. Complementary experiments have been designed to analyze the mechanisms involved, the important parameters, and the role of the optical smoothing in the case of the laser megajoule. Forward stimulated Brillouin scattering (FSBS) is identified as the dominant mechanism governing the angular spray of the laser. FSBS also increases the laser bandwidth and imparts levels of temporal and spatial incoherencies beyond the present capacities of the optical smoothing of the megajoule laser facilities. Such a PII beam becomes suitable to achieve the high degree of irradiation uniformity required to experiment high-convergence efficient direct-drive inertial confinement fusion configurations at the megajoule scale which would otherwise require major changes in the laser chains. By reducing backscattering losses and/or allowing less optically applied smoothing, PII could relax the constraints imposed on the laser system and open the road to an increase in the energy coupled to the target in indirect-drive experiments.
... A. Implosion design D 2 -gas-filled CH shell implosions have been studied extensively on NIF in the PDD configuration 23 to understand laser energy coupling, implosion symmetry, and cross-beam energy transfer mitigation at ignition-relevant scales. [24][25][26] These implosions, driven by 192 beams of 351 nm light divided into 48 sets of four beams ("quads"), were adapted for studies of hot electron preheat. The capsules consisted of glow-discharge polymer (GDP) CH shells, some of which contained a Ge dopant at nominally $4% atomic concentration over an inner layer of the shell in order to diagnose hot electron energy deposition. ...
Hot electron preheat has been quantified in warm, directly driven inertial confinement fusion implosions on OMEGA and the National Ignition Facility (NIF), to support hydrodynamic scaling studies. These CH-shell experiments were designed to be hydrodynamically equivalent, spanning a factor of 40 in laser energy and a factor of 3.4 in spatial and temporal scales, while preserving the incident laser intensity of 1015 W/cm2. Experiments with similarly low levels of beam smoothing on OMEGA and NIF show a similar fraction (∼0.2%) of laser energy deposited as hot electron preheat in the unablated shell on both OMEGA and NIF and similar preheat per mass (∼2 kJ/mg), despite the NIF experiments generating a factor of three more hot electrons (∼1.5% of laser energy) than on OMEGA (∼0.5% of laser energy). This is plausibly explained by more absorption of hot electron energy in the ablated CH plasma on NIF due to larger areal density, as well as a smaller solid angle of the imploding shell as viewed from the hot electron generating region due to the hot electrons being produced at a larger standoff distance in lower-density regions by stimulated Raman scattering, in contrast to in higher-density regions by two-plasmon decay on OMEGA. The results indicate that for warm implosions at intensities of around 1015 W/cm2, hydrodynamic equivalence is not violated by hot electron preheat, though for cryogenic implosions, the reduced attenuation of hot electrons in deuterium–tritium plasma will have to be considered.
... The LPIs, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and two-plasmon decay (TPD), are among the major obstacles to achieving fusion ignition, as they can cause significant laser energy loss (Ping et al. 2019;, asymmetric and insufficient compression (Igumenshchev et al. 2012;Strozzi et al. 2017;Moody et al. 2012), and target preheating (Dewald et al. 2010;Smalyuk et al. 2008;Christopherson et al. 2021;Montgomery 2016). Cross-beam energy transfer (CBET) may significantly affect the implosion velocity and ablation pressure in ICF typically involving many laser beams (Radha et al. 2016), where one of the pump beams is a seed light to drive SBS via beating with the other one. Even though the nonlinear processes of LPIs and their mitigation have been investigated for five decades (Kaw 2017), these problems have not yet been completely understood and solved, partially due to the parameter sensitivity and complicated coupling mechanism at large temporal and spatial scales. ...
Laser plasma instabilities (LPIs) cause laser energy loss, asymmetric and insufficient compression, and target preheating, thus are assumed to be among the major concerns of inertial confinement fusion research. Mitigation of LPIs can enhance the laser–target coupling efficiency and optimize the target compression dynamics, which is critical for the realization of robust and high-efficiency fusion ignition. Broadband lasers with polychromatic components or random phases have been investigated for decades as an effective alternative to mitigate LPIs. Here, we present a brief overview on the progress of broadband LPIs, including the models of broadband lasers, the involved physics, the conditions for effective suppression of LPIs, and some schemes to produce broadband lasers.
... Understanding and quantifying preheat has been identified as a critical issue for the laser direct drive fusion program. [3][4][5][6][7][8] One of the leading sources of shell preheat in direct-drive implosions arises from hot electrons generated from laser-plasma instabilities (LPIs). 9 In direct drive experiments on OMEGA, 10 the primary instability is the two-plasmon-decay (TPD) instability which occurs when an incident electromagnetic wave decays into two electron plasma waves, each with half the original laser frequency. ...
Hot electrons generated from laser plasma instabilities degrade performance of direct drive implosions by preheating the deuterium and tritium (DT) fuel resulting in early decompression and lower areal densities at stagnation. A technique to quantify the hot electron preheat of the dense DT fuel and connect it to the degradation in areal density is described in detail. Hot electrons are measured primarily from the hard x-rays they emit as they slow down in the target. The DT preheat is inferred from a comparison of the hard x-ray signals between a DT-layered implosion and its mass equivalent ablator only implosion. The preheat energy spatial distribution within the imploding shell is inferred from experiments using high Z payloads of varying thicknesses. It is found that the electrons deposit their energy uniformly throughout the shell material. For typical direct-drive OMEGA implosions driven with an overlapped intensity of [Formula: see text], approximately [Formula: see text] of the laser energy is converted into preheat of the stagnated fuel which corresponds to areal density degradations of 10%–20%. The degradations in areal density explain some of the observed discrepancies between the simulated and measured areal densities.
... We use a laser ray trace method for depositing the energy in the capsule, which takes into account the 3D pointing geometry, but does not include the effects of cross-beam energy transfer or nonlocal electron thermal transport. Both of these effects are known to be important for modeling laser-matter interactions in direct drive implosions, [26,27,28,29] but we have nonetheless found that the salient features of our shots are modeled well using a more approximate treatment. Our models employ multi-group diffusion for the propagation of radiation, and we apply a flux limiter to the electron thermal conduction in the ablator during the laser pulse. ...
We examine the performance of pure boron, boron carbide, high density carbon, and boron nitride ablators in the polar direct drive exploding pusher (PDXP) platform. The platform uses the polar direct drive configuration at the National Ignition Facility to drive high ion temperatures in a room temperature capsule and has potential applications for plasma physics studies and as a neutron source. The higher tensile strength of these materials compared to plastic enables a thinner ablator to support higher gas pressures, which could help optimize its performance for plasma physics experiments, while ablators containing boron enable the possibility of collecting additional data to constrain models of the platform. Applying recently developed and experimentally validated equation of state models for the boron materials, we examine the performance of these materials as ablators in 2D simulations, with particular focus on changes to the ablator and gas areal density, as well as the predicted symmetry of the inherently 2D implosion.
... The OMEGA Facility at the Laboratory for Laser Energetics of the University of Rochester in the United States of America, with 60 beams of up to about 30 kJ and 30 TW of 0.35 μm laser light, is in a symmetric configuration optimized for direct-drive study [9][10][11][12][13]. In the direct-drive approach [14][15][16][17], symmetrically arranged laser beams directly illuminate a spherical target which contains a frozen shell of deuterium−tritium (DT) fuel. Under the laser illumination, the target shell is accelerated inward as the hot-plasma corona generated by the absorbed laser energy expands outward. ...
Direct-drive is one of the key approaches in the study of inertial confinement fusion, but the laser imprinting caused by laser intensity inhomogeneities is one of the main obstacles to achieving ignition in direct-drive. It has previously been demonstrated that a thin high-Z overcoat on the laser side of the target can significantly mitigate laser imprinting (S P Obenschain et al 2002 Phys. Plasmas 9 2234). In the current work, the 1D multi-group radiation hydrodynamic code RDMG, coupled with the detailed configuration accounting non-LTE atomic physics package MBDCA (RDMG−MBDCA) was used to study a Au-coated ignition target and its implosion performances under laser direct-drive, and a bare CH target was also simulated for comparison. Our study shows that the shell compressibility in the Au-coated target is enhanced with a smaller in-flight adiabat α if and a higher neutron yield Y id than in the bare CH target. This is because the Au coating helps to maintain a hotter CH plasma, which can ablate a wider electron conduction region with lower density leading to a weaker second shock, creating a more compressed shell and a higher yield than the bare CH target. We also compared the simulations from RDMG−MBDCA with those from RDMG−AA which is coupled with an averaged-atom (AA) non-LTE model. As a result, the shell from the AA model is less compressed with a higher α if and a lower Y id because the AA model gives a higher inward x-ray emission during the pre-pulse than the DCA model does, which therefore drives a stronger shock and leads to a higher fuel entropy.
... It is noted that an ad-hoc multiplier of f CBET ¼ 1.5 has been applied to the CBET gain in our current DRACO simulations to match the experimental trajectories. 73,74 Depending on the equation-of-state of CH used in simulations, this ad-hoc multiplier can vary from 1.5 (FPEOS) to 2.0 (SESAME) in order to minimize the trajectory differences between experiments and DRACO simulations for both OMEAG and NIF implosions. For radiationtransport modeling, the diffusion model was used with the 48-group astrophysics opacity tables, 75 while the firstprinciples opacity table 76 of D 2 is not essential for such warm target implosions. ...
Understanding the effects of laser imprint on target performance is critical to the success of
direct-drive inertial confinement fusion. Directly measuring the disruption caused by laser
imprints to the imploding shell and hot-spot formation, in comparison with multidimensional
radiation–hydrodynamic simulations, can provide a clear picture of how laser nonuniformities
cause target performance to degrade. With the recently developed x-ray self-emission imaging
technique and the state-of-the-art physics models recently implemented in the two-dimensional
hydrocode DRACO, a systematic study of laser-imprint effects on warm target implosions on
OMEGA has been performed using both experimental results and simulations. By varying the
laser-picket intensity, the imploding shells were set at different adiabats (from a¼2 to a¼6). As
the shell adiabats decreased, it was observed that (1) the measured shell thickness at the time the
hot spot lit up became larger than the uniform one-dimensional (1-D) predictions; (2) the hot-spot
core emitted earlier than the corresponding 1-D predictions; (3) the measured neutron yield first
increased then decreased as the shell adiabat a was reduced; and (4) the hot-spot size reduced as a
decreased for cases where SSD (smoothing by spectral dispersion) was on but became larger for
low-a shots in cases where SSD was off. Most of these experimental observations are well reproduced
by DRACO simulations with laser imprints including modes up to kmax¼200. These studies
identify the importance of laser imprint as the major source of degrading target performance for
OMEGA implosions of adiabat a�3. Mitigating laser imprints is required to improve low-a target
performance.
... In the US, direct-drive experiments have been conducted on the OMEGA 20 and Nike 21 lasers. Very recently, direct-drive experiments have been fielded on the NIF to study laser-to-target energy coupling, laser-drive asymmetries 89,90 and laser-plasma instabilities in long-scale-length plasmas 91 . Cryogenic implosions have been carried out on the OMEGA laser since 2000 92 . ...
The quest for controlled fusion energy has been ongoing for over a half century. The demonstration of ignition and energy gain from thermonuclear fuels in the laboratory has been a major goal of fusion research for decades. Thermonuclear ignition is widely considered a milestone in the development of fusion energy, as well as a major scientific achievement with important applications in national security and basic sciences. The US is arguably the world leader in the inertial confinement approach to fusion and has invested in large facilities to pursue it, with the objective of establishing the science related to the safety and reliability of the stockpile of nuclear weapons. Although significant progress has been made in recent years, major challenges still remain in the quest for thermonuclear ignition via laser fusion. Here, we review the current state of the art in inertial confinement fusion research and describe the underlying physical principles.
Precise modeling of shocks in inertial confinement fusion implosions is critical for obtaining the desired compression in experiments. Shock velocities and postshock conditions are determined by laser-energy deposition, heat conduction, and equations of state. This paper describes experiments at the National Ignition Facility (NIF) [E. M. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)] where multiple shocks are launched into a cone-in-shell target made of polystyrene, using laser-pulse shapes with two or three pickets and varying on-target intensities. Shocks are diagnosed using the velocity interferometric system for any reflector (VISAR) diagnostic [P. M. Celliers et al., Rev. Sci. Instrum. 75, 4916 (2004)]. Simulated and inferred shock velocities agree well for the range of intensities studied in this work. These directly-driven shock-timing experiments on the NIF provide a good measure of early-time laser-energy coupling. The validated models add to the credibility of direct-drive-ignition designs at the megajoule scale.
The recent achievement of fusion ignition with laser-driven technologies at the National Ignition Facility sets a historic accomplishment in fusion energy research. This accomplishment paves the way for using laser inertial fusion as a viable approach for future energy production. Europe has a unique opportunity to empower research in this field internationally, and the scientific community is eager to engage in this journey. We propose establishing a European programme on inertial-fusion energy with the mission to demonstrate laser-driven ignition in the direct-drive scheme and to develop pathway technologies for the commercial fusion reactor. The proposed roadmap is based on four complementary axes: (i) the physics of laser–plasma interaction and burning plasmas; (ii) high-energy high repetition rate laser technology; (iii) fusion reactor technology and materials; and (iv) reinforcement of the laser fusion community by international education and training programmes. We foresee collaboration with universities, research centres and industry and establishing joint activities with the private sector involved in laser fusion. This project aims to stimulate a broad range of high-profile industrial developments in laser, plasma and radiation technologies along with the expected high-level socio-economic impact.
A series of two-dimensional particle-in-cell simulations with speckled laser drivers was carried out to study hot electron generation in direct-drive inertial confinement fusion on OMEGA. Scaling laws were obtained for hot electron fraction and temperature as functions of laser/plasma conditions in the quarter-critical region. Using these scalings and conditions from hydro simulations, the temporal history of hot electron generation can be predicted. The scalings can be further improved to predict hard x-rays for a collection of OMEGA warm target implosions within experimental error bars. These scalings can be readily implemented into inertial confinement fusion design codes.
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.
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.
Shock ignition is a scheme for direct drive inertial confinement fusion that offers the potential for high gain with the current generation of laser facility; however, the benefits are thought to be dependent on the use of low adiabat implosions without laser–plasma instabilities reducing drive and generating hot electrons. A National Ignition Facility direct drive solid target experiment was used to calibrate a 3D Monte Carlo hot-electron model for 2D radiation-hydrodynamic simulations of a shock ignition implosion. The [Formula: see text] adiabat implosion was calculated to suffer a 35% peak areal density decrease when the hot electron population with temperature [Formula: see text] and energy [Formula: see text] was added to the simulation. Optimizing the pulse shape can recover [Formula: see text] of the peak areal density lost due to a change in shock timing. Despite the harmful impact of laser–plasma instabilities, the simulations indicate shock ignition as a viable method to improve performance and broaden the design space of near ignition high adiabat implosions.
Here, we present evidence, in the context of OMEGA cryogenic target implosions, that laser imprint, known to be capable of degrading laser-direct-drive target performance, plays a major role in generating fuel–ablator mix. OMEGA cryogenic target implosions show a performance boundary correlated with acceleration-phase shell stability; for sufficiently low adiabats (where the adiabat is the ratio of the pressure to the Fermi pressure) and high in-flight aspect ratios (IFAR's), the neutron-weighted shell areal density and neutron yield relative to the clean simulated values sharply decline. Direct evidence of Rayleigh–Taylor fuel–ablator mixing was previously obtained using a Si Heα backlighter driven by an ∼20-ps short pulse generated by OMEGA EP. The shadow cast by the shell shortly prior to stagnation, as diagnosed using backlit radiographs, shows a softening near the limb, which is evidence of an ablator–fuel mix region for a low-adiabat implosion (α ∼ 1.9, IFAR = 14) but not for a moderate adiabat implosion (α ∼ 2.5, IFAR = 10). We find good agreement between experimental and synthetic radiographs in simulations that model laser imprint and account for uncertainty in the initial ablator thickness. We further explore the role of other mechanisms such as classical instability growth at the fuel–ablator interface, species concentration diffusion, and long-wavelength drive and target asymmetries.
Cross-beam energy transfer (CBET) can significantly affect the energy coupling and symmetry of direct-drive implosions. We report on a series of direct-drive shots with 2.1 mm outer diameter capsules conducted on NIF for diagnostic development and calibration in which the wavelength separation ( Δ λ) between the inner and outer cone beams was varied. We observe a strong improvement in performance as Δ λ is applied, with the nuclear yield increasing by up to a factor of 4 ×. Other data including the nuclear bang time and implosion symmetry suggest that increasing Δ λ suppresses CBET and improves both the energy coupling and drive symmetry. These results provide a strong and important benchmark for CBET models applicable to direct-drive ignition designs.
Laser–plasma interaction instabilities can be detrimental for direct-drive inertial confinement fusion by generating high-energy electrons that preheat the target. An experimental platform has been developed and fielded on the National Ignition Facility to investigate hot-electron production from laser–plasma instabilities at direct-drive ignition-relevant conditions. The radiation-hydrodynamic code DRACO has been used to design planar-target experiments that generate plasma and interaction conditions comparable to direct-drive ignition designs: IL ∼ 10¹⁵ W/cm², Te > 3 keV, and density-gradient scale lengths of Ln ∼ 600 μm in the quarter-critical density region. The hot-electron properties were inferred by comparing the experimentally observed hard x-ray spectra to Monte Carlo simulations of hard x-ray emission from hot electrons depositing energy in the target. Hot-electron temperatures of ∼40 keV to 60 keV and the fraction of laser energy converted to hot electrons of ∼0.5% to 5% were inferred in plastic targets for laser intensities at the quarter-critical density surface of (∼4 to 14) × 10¹⁴ W/cm². The use of silicon ablators was found to mitigate the hot-electron preheat by increasing the threshold laser intensity for hot-electron generation from ∼3.5 × 10¹⁴ W/cm² in plastic to ∼6 × 10¹⁴ W/cm² in silicon. The overall hot-electron production is also reduced in silicon ablators when the intensity threshold is exceeded.
Radiation-hydrodynamic simulations of directly driven fusion experiments at the Omega Laser Facility predict absorption accurately when targets are driven at low overlapped laser intensity. Discrepancies appear at increased intensity, however, with higher-than-expected laser absorption on target. Strong correlations with signatures of the two-plasmon decay (TPD) instability—including half-harmonic and hard-x-ray emission—indicate that TPD is responsible for this anomalous absorption. Scattered light data suggest that up to ≈30% of the laser power reaching quarter-critical density can be absorbed locally when the TPD threshold is exceeded. A scaling of absorption versus TPD threshold parameter was empirically determined and validated using the laser–plasma simulation environment code.
Production of suprathermal electrons by stimulated Raman scattering (SRS) is a principal concern for contemporary direct-drive inertial confinement fusion experiments at the National Ignition Facility and similar systems since such electrons penetrate and preheat the target core, preventing efficient implosion. The higher temperatures and longer scale lengths in these experiments favor SRS over two-plasmon decay, which predominated in earlier experiments. In particular, current experiments are expected to exceed the threshold for absolute Raman side scatter, which would then dominate the interaction since it grows temporally until saturated by nonlinear mechanisms such as hot-electron production. Until recently, analyses of SRS side scatter have treated the case of a single laser beam incident on a plasma, but the direct-drive approach to laser fusion employs a multitude of beams to drive the implosion. Here, we present an analysis that can be applied to an arbitrary number of beams with varied angles of incidence and polarizations. In the case of a single beam, it allows a physically motivated derivation and verification of an analytic threshold formula. In the general case of multiple beams and arbitrary orientation and polarizations, the threshold is found by numerical integration of a set of first-order linear partial differential equations.
The numerical results for cryogenic direct drive targets of megajoule facilities with radiation in the second and third harmonics of a Nd laser are presented. The calculations were performed with the 1D radiation hydrodynamics code ERA with the laser light absorption model that takes into account stimulated Brillouin scattering (SBS), generation of fast electrons in the processes of two-plasmon decay (TPD), and stimulated Raman scattering (SRS). The verification of the developed models was carried out on the basis of the comparison with experiments performed at the OMEGA and NIF facilities. The ignition margin (WQ) of nonuniform fusion targets with an allowance for energy losses due to radiation transfer and heat conduction from the hot spot was the objective of the target optimization. The calculations showed that SBS and target heating by fast electrons generated in TPD and SRS fatally reduce WQ of targets with a CH ablator for the megajoule laser with wavelength λ = 0.53 µm. The possibilities of decreasing these effects by replacing a CH ablator with a glass ablator and reducing the laser intensity upon increasing the target aspect ratio are considered. However, in both cases, WQ remains substantially below unity for the laser with wavelength λ = 0.53 µm. The ignition margin increases by a factor of ∼2 upon transition from the second to the third harmonic of a Nd laser. A glass ablator almost eliminates fast electrons in calculation with the laser wavelength λ = 0.35 µm. In this case, if SBS is reduced by a factor of 3–4 via shifting the laser emission lines in the neighboring channels by Δμ ≈ 10–20 Å, the ignition margin WQ ∼ 2 and a fusion energy yield of ∼50 MJ are obtained in the 1D calculation for a laser energy of ∼2 MJ and the third harmonic of a Nd laser.
Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion (ICF) implosions. In direct-drive ICF, an important potential seed of such asymmetries is the capsule stalk mount, the impact of which has remained a contentious question. In this paper, we describe the results from an experiment on the OMEGA laser with intentional offsets at varying angles to the capsule stalk mount, which clearly demonstrates the impact of the stalk mount on implosion dynamics. The angle between stalk and offset is found to significantly impact observables. Specifically, a larger directional flow is observed in neutron spectrum measurements when the offset is toward rather than away from the stalk, while an offset at 42° to the stalk gives minimal directional flow but still generates a large flow field in the implosion. No significant directional flow is seen due to stalk only. Time-integrated x-ray images support these flow observations. A trend is also seen in implosion yield, with lower yield obtained for offsets with a smaller angle than with a larger angle toward the stalk. Radiation hydrodynamic simulations using 2D DRACO and 2D/3D Chimera not including the stalk mount and using 2D xRAGE including the stalk mount are brought to bear on the data. The yield trend, the minimal directional flow with stalk only, and the larger flow enhancement observed with the offset toward the stalk are all reproduced in the xRAGE simulations. The results strongly indicate that the stalk impact must be considered and mitigated to achieve high-performing implosions.
In order to accurately model implosion hydrodynamics in a radiation-hydrodynamics code, it is essential to include accurate accounting for energy deposition physics. In inertial confinement fusion (ICF), where capsules are driven by lasers or laser-driven x-rays, energy deposition profiles and energy transport have a strong impact on the development and evolution of capsule dynamics and hydrodynamic instabilities. Nevertheless, accurately modeling laser beam propagation in radiation-hydrodynamics codes presents unique challenges associated with disparate resolution requirements, the potential to seed spurious noise in highly unstable systems, and computational expense. We discuss a new method for coupling laser ray-tracing physics to a radiation hydrodynamics code, developed in the process of implementing the Mazinisin laser ray-trace into the xRAGE radiation hydrodynamics code. In contrast to previous approaches, in which laser ray-tracing is performed on the radiation-hydrodynamics mesh, our method involves a mesh generation and evolution strategy that addresses the unique requirements of the laser ray-trace in a separate mesh, enabling performance enhancements and strategies to reduce noise seeded by the discretization of beams into computational rays. In addition, we have employed several methods to ensure that spurious mesh imprinting is minimized. These involved optimizing the laser and radiation-hydrodynamics meshes as well as interpolation between them and requires the use of an exact initialization method for the radiation-hydrodynamics mesh. These techniques have enabled efficient computation of laser-driven implosions and other experiments with minimal introduction of spurious noise.
The use of transparent Al 2 O 3 /GaN/AlN/GaN structures as pyrometric sensors for measuring the parameters of high-intensity laser pulses is proposed. The peculiarities of the employment of such sensors in laser fusion facilities are analysed. Post-pulse distributions of the absorbed energy density are obtained for various parameters of both GaN layers. The local maxima of these distributions are minimised by varying the ratio of donor concentration and the ratio of their thicknesses under the condition of invariance of the total absorbed energy. The optimal structure configuration is established in terms of reducing the possible negative effect of laser impact on the pyroelectric coefficient stability.
Low-density foams of low-/mid-Z materials have been previously proposed to mitigate laser imprint for direct-drive inertial confinement fusion (ICF). For foam densities above the critical density of the drive laser, the mechanism of laser-imprint mitigation relies on the reduced growth rate of Rayleigh-Taylor instability because of the increased ablation velocity and density scale length at the ablation surface. Experimental demonstration of this concept has been limited so far to planar-target geometry. The impact of foams on spherical implosions has not yet been explored in experiments. To examine the viability of using an above-critical-density foam layer to mitigate laser-imprint effects in direct-drive ICF implosions on OMEGA, we have performed a series of 2-D DRACO simulations with state-of-the-art physics models, including nonlocal thermal transport, cross-beam energy transfer, and first-principles equation-of-state tables. The simulation results indicate that a 40-lm-thick CH or SiO 2 foam layer with a density of q ¼ 40 mg/cm 3 added to a D 2-filled polystyrene (CH) capsule can significantly improve the moderate-adiabat (a % 3) implosion performance. In comparison to the standard CH target implosion, an increase in neutron yield by a factor of 4 to 8 and the recovery of 1-D compression qR are predicted by DRACO simulations for a foam-target surface roughness of r rms 0.5 lm. These encouraging results could readily facilitate experimental demonstrations of laser-imprint mitigation with an above-critical-density foam layer. Published by AIP Publishing. https://doi.org/10.1063/1.5044609
High-intensity laser facilities, such as the National Ignition Facility (NIF), enable the experimental investigation of plasmas under extreme, high-energy-density conditions. Motivated by validating models for collisional heat-transfer processes in high-energy-density plasmas, we have developed an exploding pusher platform for use at the NIF in the polar-direct-drive configuration. The baseline design employs a 3 mm-diameter capsule, an 18 μm-thick CH ablator, and Ar-doped D2 gas to achieve several keV electron-ion temperature separations with relatively low convergence ratios. In an initial series of shots at the NIF—N160920–003, -005, and N160921–001—the ratio of the laser intensity at different polar angles was varied to optimize the symmetry of the implosion. Here we summarize experimental results from the shot series and present pre- and post-shot analysis. Although the polar-direct-drive configuration is inherently asymmetric, we successfully tuned a post-shot 1D model to a set of key implosion performance metrics. The post-shot model has proven effective for extrapolating capsule performance to higher incident laser drive. Overall, the simplicity of the platform and the efficacy of the post-shot 1D model make the polar-direct-drive exploding pusher platform attractive for a variety of applications beyond the originally targeted study of collisional processes in high-energy-density plasmas.
Two novel target designs are presented for using direct laser ablation (direct drive) at the National Ignition Facility to assemble and ignite cryogenic fuel using the existing indirect-drive beam configuration. These are the first ignition-relevant “polar” direct-drive target designs to include the physical effects of cross-beam energy transfer (CBET) between laser beams and nonlocal electron heat transport. A wavelength-detuning strategy is used to increase absorption and reduce scattered-light losses caused by CBET, allowing for ignition-relevant implosion velocities. Two designs are described: a moderate-adiabat sub-ignition alpha-burning design with a D–T neutron fusion yield of 1.2 × 10¹⁷ and a lower-adiabat ignition design with a gain of 27. Both designs have moderate in-flight aspect ratios, indicating acceptable levels of hydrodynamic instability during the implosion.
Cross-beam energy transfer (CBET) results from two-beam energy exchange via seeded stimulated Brillouin scattering, which detrimentally reduces laser-energy absorption for direct-drive inertial confinement fusion. Consequently, ablation pressure and implosion velocity suffer from the decreased absorption, reducing target performance in both symmetric and polar direct drive. Additionally, CBET alters the time-resolved scattered-light spectra and redistributes absorbed and scattered-light–changing shell morphology and low-mode drive symmetry. Mitigating CBET is demonstrated in inertial confinement implosions at the National Ignition Facility by detuning the laser-source wavelengths (±2.3 Å UV) of the interacting beams. In polar direct drive, wavelength detuning was shown to increase the equatorial region velocity experimentally by 16% and to alter the in-flight shell morphology. These experimental observations are consistent with design predictions of radiation–hydrodynamic simulations that indicate a 10% increase in the average ablation pressure. These results indicate that wavelength detuning successfully mitigates CBET. Simulations predict that optimized phase plates and wavelength-detuning CBET mitigation utilizing the three-legged beam layout of the OMEGA Laser System significantly increase absorption and achieve >100-Gbar hot-spot pressures in symmetric direct drive.
Polar-direct-drive exploding pushers are used as a high-yield, low-areal-density fusion product source at the National Ignition Facility with applications including diagnostic calibration, nuclear security, backlighting, electron-ion equilibration, and nucleosynthesis-relevant experiments. In this paper, two different paths to improving the performance of this platform are explored: (i) optimizing the laser drive, and (ii) optimizing the target. While the present study is specifically geared towards nucleosynthesis experiments, the results are generally applicable. Example data from T2/³He-gas-filled implosions with trace deuterium are used to show that yield and ion temperature (Tion) from 1.6 mm-outer-diameter thin-glass-shell capsule implosions are improved at a set laser energy by switching from a ramped to a square laser pulse shape, and that increased laser energy further improves yield and Tion, although by factors lower than predicted by 1 D simulations. Using data from D2/³He-gas-filled implosions, yield at a set Tion is experimentally verified to increase with capsule size. Uniform D³He-proton spectra from 3 mm-outer-diameter CH shell implosions demonstrate the utility of this platform for studying charged-particle-producing reactions relevant to stellar nucleosynthesis.
This paper describes the development of a platform to study astrophysically relevant nuclear reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition Facility laser facilities, with a particular focus on optimizing the implosions to study charged-particle-producing reactions. Primary requirements on the platform are high yield, for high statistics in the fusion product measurements, combined with low areal density, to allow the charged fusion products to escape. This is optimally achieved with direct-drive exploding pusher implosions using thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the implosions is also considered. The platform is initially employed to study the complementary T(t,2n)α, T(³He,np)α and ³He(³He,2p)α reactions. Proof-of-principle results from the first experiments demonstrating the ability to accurately measure the energy and yields of charged particles are presented. Lessons learned from these experiments will be used in studies of other reactions. The goals are to explore thermonuclear reaction rates and fundamental nuclear physics in stellar-like plasma environments, and to push this new frontier of nuclear astrophysics into unique regimes not reachable through existing platforms, with thermal ion velocity distributions, plasma screening, and low reactant energies.
Along with laser-indirect (X-ray)-drive and magnetic-drive target concepts, laser direct drive is a viable approach to achieving ignition and gain with inertial confinement fusion. In the United States, a national program has been established to demonstrate and understand the physics of laser direct drive. The program utilizes the Omega Laser Facility to conduct implosion and coupling physics at the nominally 30-kJ scale and laser–plasma interaction and coupling physics at the MJ scale at the National Ignition Facility. This article will discuss the motivation and challenges for laser direct drive and the broad-based program presently underway in the United States.
As hydrodynamics codes develop to increase understanding of three-dimensional (3-D) effects in inertial confinement fusion implosions, diagnostics must adapt to evaluate their predictive accuracy. A 3-D radiation postprocessor was developed to investigate the use of soft x-ray self-emission images of an imploding target to measure the size of nonuniformities on the target surface. Synthetic self-emission images calculated from 3-D simulations showed a narrow ring of emission outside the ablation surface of the target. Nonuniformities growing in directions perpendicular to the diagnostic axis were measured through angular variations in the radius of the steepest intensity gradient on the inside of the ring and through changes in the peak x-ray intensity in the ring as a function of angle. The technique was applied to an implosion to measure large 3-D nonuniformities resulting from two dropped laser beam quads at the National Ignition Facility.
A record fuel hot-spot pressure Phs=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, LM15119ER (2016)], similar to indirect-drive implosions [R. Betti et al., Phys. Rev. Lett. 114
Polar-direct-drive experiments conducted at the National Ignition Facility [E. I. Moses, Fusion Sci. Technol. 54, 361 (2008)] performed at laser irradiance between 1 and 2 × 10 15 W / cm 2 exhibit increased hard x-ray emission, decreased neutron yield, and reduced areal density as the irradiance is increased. Experimental x-ray images at the higher irradiances show x-ray emission at the equator, as well as degraded symmetry, that is not predicted in hydrodynamic simulations using flux-limited energy transport, but that appear when non-local electron transport together with a model to account for cross beam energy transfer (CBET) is utilized. The reduction in laser power for equatorial beams required in the simulations to reproduce the effects of CBET on the observed symmetry also reproduces the yield degradation consistent with experimental data.
An implicit, non-local thermal conduction algorithm based on the algorithm developed by Schurtz, Nicolai, and Busquet (SNB) [Schurtz et al., Phys. Plasmas 7, 4238 (2000)] for non-local electron transport is presented and has been implemented in the radiation-hydrodynamics code DRACO. To study the model's effect on DRACO's predictive capability, simulations of shot 60 303 from OMEGA are completed using the iSNB model, and the computed shock speed vs. time is compared to experiment. Temperature outputs from the iSNB model are compared with the non-local transport model of Goncharov et al. [Phys. Plasmas 13, 012702 (2006)]. Effects on adiabat are also examined in a polar drive surrogate simulation. Results show that the iSNB model is not only capable of flux-limitation but also preheat prediction while remaining numerically robust and sacrificing little computational speed. Additionally, the results provide strong incentive to further modify key parameters within the SNB theory, namely, the newly introduced non-local mean free path. This research was supported by the Laboratory for Laser Energetics of the University of Rochester.
Obtaining an accurate equation of state (EOS) of polystyrene (CH) is crucial to reliably design inertial
confinement fusion (ICF) capsules using CH/CH-based ablators. With first-principles calculations, we have
investigated the extended EOS of CH over a wide range of plasma conditions (ρ = 0.1 to 100 g/cm3 and T =
1000 to 4 000 000 K).When compared with the widely used SESAME-EOS table, the first-principles equation of
state (FPEOS) of CH has shown significant differences in the low-temperature regime, in which strong coupling
and electron degeneracy play an essential role in determining plasma properties. Hydrodynamic simulations of
cryogenic target implosions on OMEGA using the FPEOS table of CH have predicted ∼30% decrease in neutron
yield in comparison with the usual SESAME simulations. This is attributed to the ∼5% reduction in implosion
velocity that is caused by the ∼10% lower mass ablation rate of CH predicted by FPEOS. Simulations using
CH-FPEOS show better agreement with measurements of Hugoniot temperature and scattered light from ICF
implosions.
To support direct-drive inertial confinement fusion experiments at the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)] in its indirect-drive beam configuration, the polar-direct-drive (PDD) concept [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] has been proposed. Ignition in PDD geometry requires direct-drive–specific beam smoothing, phase plates, and repointing the NIF beams toward the equator to ensure symmetric target irradiation. First experiments to study the energetics and preheat in PDD implosions at the NIF have been performed. These experiments utilize the NIF in its current configuration, including beam geometry, phase plates, and beam smoothing. Room-temperature, 2.2-mm-diam plastic shells
filled with D2 gas were imploded with total drive energies ranging from ~500 to 750kJ with peak powers of 120 to 180 TW and peak on-target irradiances at the initial target radius from 8*10^14 to 1.2*10^15 W/cm2. Results from these initial experiments are presented, including measurements of shell trajectory, implosion symmetry, and the level of hot-electron preheat in plastic and Si ablators. Experiments are simulated with the 2-D hydrodynamics code DRACO including a full 3-D ray-trace to model oblique beams, and models for nonlocal electron transport and cross-beam energy transport (CBET). These simulations indicate that CBET affects the shell symmetry and
leads to a loss of energy imparted onto the shell, consistent with the experimental data.
Ongoing polar-direct-drive (PDD) implosions on the National Ignition Facility (NIF) [J. D. Lindl and E. I. Moses, Phys. Plasmas
18, 050901 (2011)] use existing NIF hardware, including indirect-drive phase plates. This limits the performance achievable in these implosions. Spot shapes are identified that significantly improve the uniformity of PDD NIF implosions; outer surface deviation is reduced by a factor of 7 at the end of the laser pulse and hot-spot distortion is reduced by a factor of 2 when the shell has converged by a factor of ∼10. As a result, the neutron yield increases by approximately a factor of 2. This set of laser spot shapes is a combination of circular and elliptical spots, along with elliptical spot shapes modulated by an additional higher-intensity ellipse offset from the center of the beam. This combination is motivated in this paper. It is also found that this improved implosion uniformity is obtained independent of the heat conduction model. This work indicates that significant improvement in performance can be obtained robustly with the proposed spot shapes.
Achieving symmetric capsule implosions with Polar Direct Drive [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004); R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005); F. J. Marshall et al., J. Phys. IV France 133, 153-157 (2006)] has been explored during recent Defect Induced Mix Experiment campaign on the Omega facility at the Laboratory for Laser Energetics. To minimize the implosion asymmetry due to laser drive, optimized laser cone powers, as well as improved beam pointings, were designed using 3D radiation-hydrodynamics code HYDRA [M. M. Marinak et al., Phys. Plasmas 3, 2070 (1996)]. Experimental back-lit radiographic and self-emission images revealed improved polar symmetry and increased neutron yield which were in good agreement with 2D HYDRA simulations. In particular, by reducing the energy in Omega's 21.4° polar rings by 16.75%, while increasing the energy in the 58.9° equatorial rings by 8.25% in such a way as to keep the overall energy to the target at 16 kJ, the second Legendre mode (P2) was reduced by a factor of 2, to less than 4% at bang time. At the same time the neutron yield increased by 62%. The polar symmetry was also improved relative to nominal DIME settings by a more radical repointing of OMEGA's 42.0° and 58.9° degree beams, to compensate for oblique incidence and reduced absorption at the equator, resulting in virtually no P2 around bang time and 33% more yield.
Reaching ignition in direct-drive (DD) inertial confinement fusion implosions requires achieving
central pressures in excess of 100 Gbar. The OMEGA laser system [T. R. Boehly et al., Opt.
Commun. 133, 495 (1997)] is used to study the physics of implosions that are hydrodynamically
equivalent to the ignition designs on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser
Focus World 30, 75 (1994)]. It is shown that the highest hot-spot pressures (up to 40 Gbar) are
achieved in target designs with a fuel adiabat of a ’ 4, an implosion velocity of 3.8�107 cm/s,
and a laser intensity of �1015 W/cm2. These moderate-adiabat implosions are well understood
using two-dimensional hydrocode simulations. The performance of lower-adiabat implosions is
significantly degraded relative to code predictions, a common feature between DD implosions on
OMEGA and indirect-drive cryogenic implosions on the NIF. Simplified theoretical models are
developed to gain physical understanding of the implosion dynamics that dictate the target
performance. These models indicate that degradations in the shell density and integrity (caused by
hydrodynamic instabilities during the target acceleration) coupled with hydrodynamics at
stagnation are the main failure mechanisms in low-adiabat designs. To demonstrate ignition
hydrodynamic equivalence in cryogenic implosions on OMEGA, the target-design robustness to
hydrodynamic instability growth must be improved by reducing laser-coupling losses caused by
cross beam energy transfer.
The success of direct-drive implosions depends critically on the ability to create high ablation pressures (∼100 Mbar) and accelerating the imploding shell to ignition-relevant velocities (>3.7×10^{7} cm/s) using direct laser illumination. This Letter reports on an experimental study of the conversion of absorbed laser energy into kinetic energy of the shell (rocket efficiency) where different ablators were used to vary the ratio of the atomic number to the atomic mass. The implosion velocity of Be shells is increased by 20% compared to C and CH shells in direct-drive implosions when a constant initial target mass is maintained. These measurements are consistent with the predicted increase in the rocket efficiency of 28% for Be and 5% for C compared to a CH ablator.
Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and sensitivity of the margin to characteristics of the experiment. The formalism is used to estimate probability of ignition. The ignition experiment will be preceded with an experimental campaign that determines features of the design that cannot be defined with simulations alone. The requirements for this campaign are summarized. Requirements are summarized for the laser and target fabrication.
Low-adiabat polar-drive (PD) [Skupsky et al., Phys. Plasmas 11, 2763 (2004)] implosion designs for the OMEGA [Boehly et al., Opt. Commun. 133, 495 (1997)] laser are described. These designs for cryogenic deuterium-tritium and warm plastic shells use a temporal laser pulse shape with three pickets followed by a main pulse [Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010)]. The designs are at two different on-target laser intensities, with different in-flight aspect ratios (IFARs). These designs permit studies of implosion energetics and target performance closer to ignition-relevant intensities ({approx}7 Multiplication-Sign 10{sup 14} W/cm{sup 2} at the quarter-critical surface, where nonlocal heat conduction and laser-plasma interactions can play an important role) but at lower values of IFAR {approx} 22 or at lower intensity ({approx}3 Multiplication-Sign 10{sup 14} W/cm{sup 2}) but at a higher IFAR (IFAR {approx} 32, where shell instability can play an important role). PD geometry requires repointing of laser beams to improve shell symmetry. The higher-intensity designs optimize target performance by repointing beams to a lesser extent, compensating for the reduced equatorial drive by increasing the energies of the repointed beams. They also use custom beam profiles that improve equatorial illumination at the expense of irradiation at higher latitudes. These latter designs will be studied when new phase plates for the OMEGA Laser System, corresponding to the custom beam profiles, are obtained.
Three research papers that address the approach taken by the National Ignition Campaign (NIC) to achieve ignition and thermonuclear burn in the laboratory and to develop a platform for ignition and high energy density (HED) applications are discussed. The first paper by Haan and team provides an overview of the ignition point design target and the methodology that has been developed to set specifications and assess the likelihood of success. This paper introduces a form of a generalized Lawson criterion (GLC) used to establish requirements on the laser, targets, and experiments such that the required imploded fuel conditions are achieved. The second paper by Landen and team addresses the experimental approach adopted to optimize the principal implosion input variables in the presence of physics uncertainties. The third paper by Edwards and team discusses the use of cryogenically layered targets to assess the progress toward achieving the compressed fuel conditions required for ignition.
The experimental and theoretical aspects of electron thermal transport in direct-drive laser-fusion are reviewed. The Fokker–Planck equation and the flux-limited diffusion model, which is widely used in laser-fusion simulation codes, are described. After a discussion on the limitation of planar-target transport experiments, results of spherical experiments are surveyed. Solutions of the Fokker–Planck equations for cathode problems and for cases with stationary and moving ion density profiles are presented. Limitations of the flux-limited diffusion model are discussed in light of the Fokker–Planck results. Comparisons between experimental and theoretical results lead to the conclusion that the modeling of electron thermal transport in uniform spherical geometry does not require the existence of magnetic fields or anomalous plasma effects.
Warm deuterium-gas-filled plastic shells are imploded by direct irradiation from the OMEGA laser T. R. Boehly et al., Opt. Commun. 133, 495 1997. The pulse shapes contain three pickets that precede a sharp rise to a constant laser intensity at 4.5 10 14 W / cm 2 . The in-flight-aspect-ratio IFAR, a crucial measure of shell instability to nonuniformity growth, is varied in these implosions by changing picket energies and the timing among the pickets. Simulations that include cross-beam energy transfer in addition to inverse bremsstrahlung for the laser-energy deposition models show better agreement with measurements of the neutron bang time and temporally resolved scattered light and therefore more correctly model the shell kinetic energy. It is also shown that target performance improves significantly as IFAR is reduced. Nearly twice the neutron yield is measured for IFAR 31 compared to IFAR 60. The ratio of the measured to simulated neutron yield and areal density increases significantly with decreasing IFAR. These implosions unambiguously link target performance to in-flight shell instability attributable to short-wavelength growth and indicate that IFAR 40 is required to achieve adequate compression at this intensity. © 2011 American Institute of Physics.
Nonuniformities seeded by both long- and short-wavelength laser perturbations can grow via Rayleigh-
Taylor (RT) instability in direct-drive inertial confinement fusion, leading to performance reduction in
low-adiabat implosions. To mitigate the effect of laser imprinting on target performance, spherical RT
experiments have been performed on OMEGA using Si- or Ge-doped plastic targets in a cone-in-shell
configuration. Compared to a pure plastic target, radiation preheating from these high-Z dopants (Si=Ge)
increases the ablation velocity and the standoff distance between the ablation front and laser-deposition
region, thereby reducing both the imprinting efficiency and the RT growth rate. Experiments showed a
factor of 2–3 reduction in the laser-imprinting efficiency and a reduced RT growth rate, leading to
significant (3–5 times) reduction in the �rms of shell �R modulation for Si- or Ge-doped targets. These
features are reproduced by radiation-hydrodynamics simulations using the two-dimensional hydrocode
DRACO.
Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [
T. R. Boehly et al., Opt. Commun. 133, 495 (1997)
] are investigated using the multidimensional hydrodynamic code, DRACO [
D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)
]. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength (ℓ<10) modes due to surface roughness and beam imbalance and the intermediate modes (20 ⩽ ℓ ⩽ 50) due to single-beam nonuniformities. The neutron-production rate for these thick shells truncates relative to one-dimensional (1D) predictions. The yield degradation in the thin shells is mainly due to shell breakup at short wavelengths (λ ∼ Δ, where Δ is the in-flight shell thickness). The neutron-rate curves for the thinner shells have significantly lower amplitudes and a fall-off that is less steep than 1D rates. DRACO simulation results are consistent with experimental observations.
A technique to measure the shell trajectory in direct-drive inertial confinement fusion implosions is presented. The x-ray self emission of the target is measured with an x-ray framing camera. Optimized filtering limits the x-ray emission from the corona plasma, isolating a sharp intensity gradient very near the ablation surface. This enables one to measure the radius of the imploding shell with an accuracy better than 1 mu m and to determine a 200-ps average velocity to better than 2%. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4732179]
In preparation for the start of NIF ignition experiments, we have designed a porfolio of targets that span the temperature range that is consistent with initial NIF operations: 300 eV, 285 eV, and 270 eV. Because these targets are quite complicated, we have developed a plan for choosing the optimum hohlraum for the first ignition attempt that is based on this portfolio of designs coupled with early NIF experiements using 96 beams. These early experiments will measure the laser plasma instabilities of the candidate designs and will demonstrate our ability to tune symmetry in these designs. These experimental results, coupled with the theory and simulations that went into the designs, will allow us to choose the optimal hohlraum for the first NIF ignition attempt.
Hydrogen may be compressed to more than 10,000 times liquid density by an implosion system energized by a high energy laser. This scheme makes possible efficient thermonuclear burn of small pellets of heavy hydrogen isotopes, and makes feasible fusion power reactors using practical lasers.
Backscattered light via laser-plasma instabilities has been measured in early NIF hohlraum experiments on two beam quads using a suite of detectors. A full aperture backscatter system and near backscatter imager (NBI) instrument separately measure the stimulated Brillouin and stimulated Raman scattered light. Both instruments work in conjunction to determine the total backscattered power to an accuracy of ∼15%. In order to achieve the power accuracy we have added time-resolution to the NBI for the first time. This capability provides a temporally resolved spatial image of the backscatter which can be viewed as a movie.
The fuel layer density of an imploding laser-driven spherical shell is inferred from framed x-ray radiographs. The density distribution is determined by using Abel inversion to compute the radial distribution of the opacity kappa from the observed optical depth tau. With the additional assumption of the mass of the remaining fuel, the absolute density distribution is determined. This is demonstrated on the OMEGA laser system with two x-ray backlighters of different mean energies that lead to the same inferred density distribution independent of backlighter energy.
The success of ignition target designs in inertial confinement fusion (ICF) experiments critically depends on the ability to maintain the main fuel entropy at a low level while accelerating the shell to ignition-relevant velocities of V imp > 3 ×107 cm/s. The University of Rochester’s Laboratory for Laser Energetics has been implodingcryogenic deuterium and deuterium–tritium targets on the Omega Laser System for over a decade. Fuel entropy is inferred in these experiment by measuring fuel areal density near peak compression. Measured areal densities up to ⟨ρR⟩n ∼ 300 mg/cm2 (larger than 85 % of predicted values) are demonstrated in the cryogenic implosion with V imp approaching 3 ×107 cm/s and peak laser intensities of 8 ×1014 W/cm2. Scaled to the laser energies available at the National Ignition Facility, implosions, hydrodynamically equivalent to theseOmega designs, are predicted to achieve ⟨ρR⟩n > 1. 2 g/cm2, sufficient for ignition demonstration in direct-drive ICF experiments.
In laser-driven inertial confinement fusion, hot electrons can preheat the fuel and prevent fusion-pellet compression to ignition conditions. Measuring the hot-electron population is key to designing an optimized ignition platform. The hot electrons in these high-intensity, laser-driven experiments, created via laser-plasma interactions, can be inferred from the bremsstrahlung generated by hot electrons interacting with the target. At the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)], the filter-fluorescer x-ray (FFLEX) diagnostic-a multichannel, hard x-ray spectrometer operating in the 20-500 keV range-has been upgraded to provide fully time-resolved, absolute measurements of the bremsstrahlung spectrum with ∼300 ps resolution. Initial time-resolved data exhibited significant background and low signal-to-noise ratio, leading to a redesign of the FFLEX housing and enhanced shielding around the detector. The FFLEX x-ray sensitivity was characterized with an absolutely calibrated, energy-dispersive high-purity germanium detector using the high-energy x-ray source at NSTec Livermore Operations over a range of K-shell fluorescence energies up to 111 keV (U Kβ). The detectors impulse response function was measured in situ on NIF short-pulse (∼90 ps) experiments, and in off-line tests.
The National Ignition Facility (NIF) utilizes several different
pixelated sensor technologies for various measurement systems that
include alignment cameras, laser energy sensors, and high-speed framing
cameras. These systems remain in the facility where they are exposed to
14MeV neutrons during a NIF shot. The image quality of the sensors
degrades as a function of radiation-induced damage. This article reports
on a figure-of-merit technique that aids in the tracking of the
performance of pixelated sensors when exposed to neutron radiation from
NIF. The sensor dark current growth can be displayed over time in a 2D
visual representation for tracking radiation induced damage. Predictions
of increased noise as a function of neutron fluence for future NIF shots
allow simulation of reduced performance for each of the individual
camera applications. This predicted longevity allows for proper
management of the camera systems.
In recent experiments using directly driven spherical targets on the
OMEGA laser system, the energy in fast electrons was found to reach ~1%
of the laser energy at an irradiance of ~1.1 × 1015 W/cm2. The
fraction of these fast electrons absorbed in the compressed fuel shell
depends on their angular divergence. This paper describes measurements
of this divergence deduced from a series of shots where Mo-coated shells
of increasing diameter (D) were suspended within an outer CH shell. The
intensity of the Mo-Kα line and the hard x-ray radiation were
found to increase approximately as ~D2, indicating wide divergence of
the fast electrons. Alternative interpretations of these results
(electron scattering, radiation excitation of Kα, and an electric
field due to return current) are shown to be unimportant.
The linear stability analysis of accelerated ablation fronts is carried out self‐consistently by retaining the effect of finite thermal conductivity. Its temperature dependence along with the density gradient scale length are adjusted to fit the density profiles obtained in the one‐dimensional simulations. The effects of diffusive radiation transport are included through the nonlinear thermal conductivity (κ∼Tν). The growth rate is derived by using a boundary layer analysis for Fr≫1 (Fr is the Froude number) and a WKB approximation for Fr≪1. The self‐consistent Atwood number depends on the mode wavelength and the power law index for thermal conduction. The analytic growth rate and cutoff wave number are in good agreement with the numerical solutions for arbitrary ν≳1.
Direct-drive-implosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have showed discrepancies between simulations of the scattered (non-absorbed) light levels and measured ones that indicate the presence of a mechanism that reduces laser coupling efficiency by 10%-20%. This appears to be due to crossed-beam energy transfer (CBET) that involves electromagnetic-seeded, low-gain stimulated Brillouin scattering. CBET scatters energy from the central portion of the incoming light beam to outgoing light, reducing the laser absorption and hydrodynamic efficiency of implosions. One-dimensional hydrodynamic simulations including CBET show good agreement with all observables in implosion experiments on OMEGA. Three strategies to mitigate CBET and improve laser coupling are considered: the use of narrow beams, multicolor lasers, and higher-Z ablators. Experiments on OMEGA using narrow beams have demonstrated improvements in implosion performance.
The two-plasmon instability in warm inhomogeneous plasma for a normally incident pump is investigated. The complex eigenfrequencies of the absolute instability are derived by reducing the linearized fluid equation to a Schroedinger equation in wavenumber space. These eigenvalues are produced by several methods, such as by combining a perturbation expansion in powers of the reciprocal scale length with WKB theory. Solutions for the resulting algebraic equations are obtained by the use of three analytical approximations and by direct numerical solution. Other methods for obtaining these solutions include the analysis of the Schroedinger equation using an interactive WKB computer code, and the use of a shooting code. These different methods are all employed and compared for threshold curves and growth rates above threshold. Also obtained are several eigenfunction forms. The perturbation method is found to be in good agreement above threshold with more exact calculations. The experimental implications of these findings are examined.
The linear stability of an ablating plasma is investigated as an eigenvalue problem by assuming the plasma to be at the stationary state. For various structures of the ablating plasma, the growth rate is found to be expressed well in the form Γ=α(kg)1/2 −βkVa, where α=0.9, β&bartil;3–4, and Va is the flow velocity across the ablation front, and is found to agree well with recent two-dimensional simulations in a classical transport regime. Short-wavelength lasers inducing enhanced mass ablation are suggested to be advantageous to stable implosion because of the ablative stabilization.
It is shown that, when two superposed fluids of different densities are accelerated in a direction perpendicular to their interface, this surface is stable or unstable according to whether the acceleration is directed from the heavier to the lighter fluid or vice versa. The relationship between the rate of development of the instability and the length of wave-like disturbances, the acceleration and the densities is found, and similar calculations are made for the case when a sheet of liquid of uniform depth is accelerated.
Smoothing, caused by the small-spatial-scale B integral, was measured on the OMEGA laser (a high-power, solid-state laser used for inertial confinement fusion research) without applied bandwidth. The intrinsic nonuniformity of laser irradiation [i.e., irradiation without smoothing by spectral dispersion] was determined from fluence distributions in equivalent-target-plane images of beams with phase plates. These data are compared with simulations that include both small-spatial-scale and whole-beam B integrals. The nonuniformity decreases with increasing average intensity. High-intensity beams can acquire bandwidth as a result of the intensity-dependent phase accumulated in the laser chain. The far-field speckle pattern produced by a phase plate can shift as the near-field phase front changes, which decreases the nonuniformity. The far-field power spectrum is affected mainly in the high spatial frequencies, where it is not expected to mitigate hydrodynamic instabilities.
The Rayleigh-Taylor instability in laser-driven spherical implosions can be stabilized by convective flow and by the "fire-polishing" effect, but the size of the stabilization effect depends on details of the thermal conductivity near the ablation surface.
The noise spectrum from which stimulated Brillouin scatter grows has two sources in laser fusion plasmas: a broadband source due to ion-acoustic fluctuations, and a line source, usually much larger, which is the nonabsorbed light returning from the plasma critical surface. A theoretical description of stimulated Brillouin backscatter when the fluctuation source may be neglected and the scatter grows exclusively from the nonabsorbed light is given. Gradients of background density, velocity, and temperature are allowed. Theoretical predictions are compared to numerical simulations of scatter for parameters of recent experiments. It is found that stimulated Brillouin scatter can be greatly enhanced by the presence of a critical surface and that it can become an important part of the total energy balance.
It is shown by comparison with calculations that anomalies in the results of intense laser irradiation of solid targets, including two-humped ion distributions, indicate a reduction of electron thermal conduction to considerably below classical values. This reduction is interpreted as a flux limit and appears to be sufficiently restrictive to modify significantly the design of laser fusion targets.
The effect of medium-Z doping of plastic ablators on laser imprinting and Rayleigh–Taylor (RT)
instability growth was studied using spherical direct-drive implosions on the OMEGA Laser
System [T. R. Boehly et al., Opt. Commun. 133, 495 (1977)]. The targets were spherical plastic
(CH) shells, with an outer diameter of 860 lm and a thickness of 22 lm, volume doped with a
varied concentration of Si (4.3% and 7.4%) and Ge (3.9%). The targets were imploded with 48
beams with a low-adiabat, triple-picket laser shape pulse with a peak intensity of 4 � 1014W=cm2,
and a pulse duration of 2.5 ns. The shells were x-ray radiographed through a 400 -lm opening in
the side of the target. The results show that volumetric impurity doping strongly reduces the shell
density modulation and the instability growth rate. The amplitude of the initial imprint is reduced
by a factor of 2.560.5 for CH[4.3% Si] targets and by a factor of 360.5 for CH[7.4% Si] and
CH[3.9% Ge] targets. At the end of the acceleration phase, the reduction factor becomes 360.5
and 560.5, correspondingly. The RT instability growth rate in doped targets is reduced by a factor
of 1.5 compared to undoped ones. Simulations using the two-dimensional, radiationhydrodynamics
code DRACO show good agreement with the measurements.
Relations between stagnation and in-flight phases are derived both analytically and numerically, for hydrodynamic variables relevant to direct-drive inertial confinement fusion implosions. Scaling laws are derived for the stagnation values of the shell density and areal density and for the hot-spot pressure, temperature, and areal density. A simple formula is also derived for the thermonuclear energy gain and in-flight aspect ratio. Implosions of cryogenic deuterium-tritium capsules driven by UV laser energies ranging from 25 kJ to 2 MJ are simulated with a one-dimensional hydrodynamics code to generate the implosion database used in the scaling law derivation. These scaling laws provide guidelines for optimized fuel assembly and laser pulse design for direct-drive fast ignition and conventional inertial confinement fusion.
A full aperture backscatter station (FABS) target diagnostic has been activated on the first four beams of the National Ignition Facility. FABS measures both stimulated Brillouin scattering and stimulated Raman scattering with a suite of measurement instruments. Digital cameras and spectrometers record spectrally resolved energy for both P and S polarized light. Streaked spectrometers measure the spectral and temporal behavior of the backscattered light. Calorimeters and fast photodetectors measure the integrated energy and temporal behavior of the light, respectively. This article provides an overview of the FABS measurement system and detailed descriptions of the diagnostic instruments and the optical path.
Three recent developments in direct-drive target design have enhanced the possibility of achieving high target gain on the National Ignition Facility (NIF): (1) Laser absorption was increased by almost 50% using wetted-foam targets. (2) Adiabat shaping significantly increased the hydrodynamic stability of the target during the acceleration phase of the implosion without sacrificing target gain. (3) Techniques to reduce laser imprint using pulse shaping and radiation preheat were developed. These design features can be employed for direct-drive-ignition experiments while the NIF is in the x-ray-drive configuration. This involves repointing some of the beams toward the equator of the target to improve uniformity of target drive. This approach, known as polar direct drive (PDD), will enhance the capability of the NIF to explore ignition conditions. PDD will couple more energy to the fuel than x-ray drive. The compressed fuel core can be more easily accessed for high-ρR diagnostic development and for fast-ignitor studies. Polar direct drive is examined in this manuscript using two-dimensional hydrodynamic simulations to determine the level of target performance that can be achieved. © 2004 American Institute of Physics.
Excessive increase in the shell entropy and degradation from spherical symmetry in inertial confinement fusion implosions limit shell compression and could impede ignition. The entropy is controlled by accurately timing shock waves launched into the shell at an early stage of an implosion. The seeding of the Rayleigh-Taylor instability, the main source of the asymmetry growth, is also set at early times during the shock transit across the shell. In this paper we model the shock timing and early perturbation growth of directly driven targets measured on the OMEGA laser system [
T. R. Boehly et al., Opt. Commun. 133, 495 (1997)
]. By analyzing the distortion evolution, it is shown that one of the main parameters characterizing the growth is the size of the conduction zone Dc, defined as a distance between the ablation front and the laser deposition region. Modes with kDc>1 are stable and experience oscillatory behavior [
V. N. Goncharov, Phys. Rev. Lett. 82, 2091 (1999)
]. The model shows that the main stabilizing mechanism is the dynamic overpressure due to modulations in the blow-off velocity inside the conduction zone. The long wavelengths with kDc<1 experience growth because of coupled Richtmyer-Meshkov-like and Landau-Darrieus instabilities [
L. D. Landau and E. M. Lifshitz, Fluid Mechanics (Pergamon, New York, 1982)
]. To match the simulation results with both the shock timing and perturbation growth measurements a new nonlocal thermal transport model is developed and used in hydrocodes.
Polar direct drive (PDD)
[S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)]
will allow direct-drive ignition experiments on the National Ignition Facility (NIF)
[J. Paisner et al., Laser Focus World 30, 75 (1994)]
as it is configured for x-ray drive. Optimal drive uniformity is obtained via a combination of beam repointing, pulse shapes, spot shapes, and∕or target design. This article describes progress in the development of standard and “Saturn”
[R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 0952002 (2005)]
PDD target designs. Initial evaluation of experiments on the OMEGA Laser System
[T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)]
and simulations were carried out with the two-dimensional hydrodynamics code SAGE
[R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005)]
. This article adds to this body of work by including fusion particle production and transport as well as radiation transport within the two-dimensional DRACO
[P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)]
hydrodynamics simulations used to model experiments. Forty OMEGA beams arranged in six rings to emulate the NIF x-ray-drive configuration are used to perform direct-drive implosions of CH shells filled with D2 gas. Target performance was diagnosed with framed x-ray backlighting and by the measured fusion yield. Saturn target experiments have resulted in ∼ 75% of the yield from energy-equivalent, symmetrically irradiated implosions. The results of the two-dimensional PDD simulations performed with DRACO are in good agreement with experimental x-ray radiographs. DRACO is being used to further optimize standard PDD designs. In addition, DRACO simulations of NIF-scale PDD designs show ignition with a gain of 20 and the development of a 40 μm radius, 10 keV region with a neutron-averaged ρr of 1270 mg/cm2 near stagnation.
The two-plasmon-decay (TPD) instability in direct-drive irradiation OMEGA [
J. M. Soures, R. L. McCrory, C. P. Verdon, et al., Phys. Plasmas 3, 2108 (1996)
] experiments is seen in the half-integer harmonic emission. Experimental time-resolved ω/2 and 3ω/2 spectra indicate that the linear theory for the absolute TPD instability reasonably predicts TPD thresholds. The plasma wave spectra do not, however, agree at all with the predictions of the linear theory. This is most likely a consequence of the nonlinear evolution of this instability once it is above threshold. This is demonstrated with spectral data obtained from spherical implosion experiments as well as planar target experiments. In the latter, Thomson scattering shows the importance of the Landau cutoff. For the TPD instability, the Landau cutoff is found to be respected in all spherical and planar target experiments. In addition, the maximum plasma wave amplitudes appear to occur near the Landau cutoff.
The minimum energy needed to ignite an inertial confinement fusion capsule is of considerable interest in the optimization of an inertial fusion driver. Recent computational work investigating this minimum energy has found that it depends on the capsule implosion history, in particular, on the capsule drive pressure. This dependence is examined using a series of LASNEX simulations to find ignited capsules which have different values of the implosion velocity, fuel adiabat and drive pressure. It is found that the main effect of varying the drive pressure is to alter the stagnation of the capsule, changing its stagnation adiabat, which, in turn, affects the energy required for ignition. To account for this effect a generalized scaling law has been devised for the ignition energy, Eign αif1.88±0.05v-5.89±0.12P-0.77±0.03. This generalized scaling law agrees with the results of previous work in the appropriate limits.
OMEGA is a 60-terawatt, 60-beam, frequency-tripled Nd:glass laser system designed to perform precision direct-drive inertial-confinement-fusion (ICF) experiments. The upgrade to the system, completed in April 1995, met or surpassed all technical requirements. The acceptance tests demonstrated exceptional performance throughout the system: high driver stability (< 2% variations), precise control of the beam profiles and amplifier gains, 75% frequency-conversion efficiency, beam energy balance less than 8% and stable on-target irradiation of up to 37 kJ UV. We present these results and show that the system performance is well modeled by our propagation and frequency-conversion codes.
SPECT3D is a multi-dimensional collisional-radiative code used to post-process the output from radiation-hydrodynamics (RH) and particle-in-cell (PIC) codes to generate diagnostic signatures (e.g. images, spectra) that can be compared directly with experimental measurements. This ability to post-process simulation code output plays a pivotal role in assessing the reliability of RH and PIC simulation codes and their physics models. SPECT3D has the capability to operate on plasmas in 1D, 2D, and 3D geometries. It computes a variety of diagnostic signatures that can be compared with experimental measurements, including: time-resolved and time-integrated spectra, space-resolved spectra and streaked spectra; filtered and monochromatic images; and X-ray diode signals. Simulated images and spectra can include the effects of backlighters, as well as the effects of instrumental broadening and time-gating. SPECT3D also includes a drilldown capability that shows where frequency-dependent radiation is emitted and absorbed as it propagates through the plasma towards the detector, thereby providing insights on where the radiation seen by a detector originates within the plasma.
A multidimensional measurable criterion for central ignition of inertial-confinement-fusion capsules is derived. The criterion accounts for the effects of implosion nonuniformities and depends on three measurable parameters: the neutron-averaged total areal density (rhoR(n)(tot)), the ion temperature (T(n)), and the yield over clean (YOC=ratio of the measured neutron yield to the predicted one-dimensional yield). The YOC measures the implosion uniformity. The criterion can be approximated by chi=(rhoR(n)(tot))(0.8) x (T(n)/4.7)(1.7)YOC(mu)>1 (where rhoR is in g cm(-2), T in keV, and mu approximately 0.4-0.5) and can be used to assess the performance of cryogenic implosions on the NIF and OMEGA. Cryogenic implosions on OMEGA have achieved chi approximately 0.02-0.03.
In this paper we present measurements of thermal transport in spherical geometry made with 24 uv (351-nm) beams from the OMEGA laser system of the Laboratory for Laser Energetics of the University of Rochester. The measurements, using time-resolved x-ray spectroscopy on solid glass targets coated with varying thicknesses of plastic, provide absolute measurements of the onset times of the resonance x-ray lines of silicon. From these measurements, the scaling of the instantaneous mass-ablation rate with absorbed intensity is obtained for times before and after the peak of the laser pulse. The observed large burnthrough depths and early onsets of the x-ray lines are explained by carrying out detailed hydrodynamic-code simulations for the range of the estimated laser-intensity distribution on target which is obtained from the superposition of the equivalent target-plane intensity distribution of a single beam. We find that neglecting the effects of laser illumination nonuniformities can lead to erroneous conclusions about the heat transport. We conclude from the analysis that the experimental results can be explained by the presence of significant energy at intensities three times the nominal intensity, contained in hot spots of size less than 20 mum. This is almost twice as much as the maximum intensity obtained from the superposition of the single-beam distribution.
New direct-drive spherical implosion experiments with deuterium filled plastic shells have demonstrated significant and absolute ($2\text{\hskip-0.22em}\ifmmode\times\else\texttimes\fi{}$) improvements in neutron yield when the shells are coated with a very thin layer ($$\sim${}200\char21{}400\text{ }\text{ }\AA{}$) of high-$Z$ material such as palladium. This improvement is interpreted as resulting from increased stability of the imploding shell. These results provide for a possible path to control laser imprint and stability in laser-fusion-energy target designs.
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