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

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

Direct Measurements of DT Fuel Preheat from Hot Electrons in Direct-Drive Inertial Confinement Fusion.

Hot electrons generated by laser-plasma instabilities degrade the performance of laser-fusion implosions by preheating the DT fuel and reducing core compression. The hot-electron energy deposition in the DT fuel has been directly measured for the first time by comparing the hard x-ray signals between DT-layered and mass-equivalent ablator-only implosions. The electron energy deposition profile in the fuel is inferred through dedicated experiments using Cu-doped payloads of varying thickness. The measured preheat energy accurately explains the areal-density degradation observed in many OMEGA implosions. This technique can be used to assess the viability of the direct-drive approach to laser fusion with respect to the scaling of hot-electron preheat with laser energy.

Anomalous Absorption by the Two-Plasmon Decay Instability.

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.

Inferred UV fluence focal-spot profiles from soft x-ray pinhole-camera measurements on OMEGA.

A method was developed with laser-irradiated Au planar foils to characterize the focal spot of UV laser beams on a target at full energy from soft x-ray emission. A pinhole camera with a back-thinned charge-coupled device detector and filtration with thin Be and Al foil filters provides images of the x-ray emission at photon energies <2 keV. This method requires a careful measurement of the relationship between the applied UV fluence and the x-ray signal, which can be described by a power-law dependence. The measured exponent γ ∼ 2 provides a dynamic range of ∼25 for the inferred UV fluence. UV fluence profiles of selected beams were measured for 100-ps and 1-ns laser pulses and were compared to directly measured profiles from an UV equivalent-target-plane diagnostic. The inferred spot size and super-Gaussian order from the x-ray technique agree within several percent with the values measured with the direct UV measurements.

Theory and measurements of convective Raman side scatter in inertial confinement fusion experiments.

Raman side scatter, whereby scattered light is resonant while propagating perpendicularly to a density gradient in a plasma, was identified experimentally in planar-target experiments at the National Ignition Facility at intensities orders of magnitudes below the threshold for absolute instability. We have derived a new theoretical description of convective Raman side scatter below the absolute threshold, validated by numerical simulations. We show that inertial confinement fusion experiments at full ignition scale, i.e., with mm-scale spot sizes and density scale lengths, are prone to increased coupling losses from Raman side scatter as the instability can extend from the absolute regime near the quarter-critical density to the convective regime at lower electron densities.

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

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

First Observation of Cross-Beam Energy Transfer Mitigation for Direct-Drive Inertial Confinement Fusion Implosions Using Wavelength Detuning at the National Ignition Facility.

Cross-beam energy transfer (CBET) results from two-beam energy exchange via seeded stimulated Brillouin scattering, which detrimentally reduces ablation pressure and implosion velocity in direct-drive inertial confinement fusion. Mitigating CBET is demonstrated for the first time in inertial-confinement implosions at the National Ignition Facility by detuning the laser-source wavelengths (±2.3 Å UV) of the interacting beams. We show that, in polar direct-drive, wavelength detuning increases the equatorial region velocity experimentally by 16% and alters 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.

Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments.

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length (∼500 to 700 µm), electron temperature (∼3 to 5 keV), and laser intensity (6 to 16×10^{14} W/cm^{2}) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is ∼0.7% to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from ∼4×10^{14} to ∼6×10^{14} W/cm^{2}. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

Experimental Investigation of the Collective Raman Scattering of Multiple Laser Beams in Inhomogeneous Plasmas.

Experiments have been performed evidencing significant stimulated Raman sidescattering (SRS) at large angles from the density gradient. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating for the first time this multiple-beam collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. These findings obtained in plasmas conditions relevant of inertial confinement fusion experiments similarly apply to the more complex geometry of these experiments where anomalously large levels of SRS were measured.

Using Inertial Fusion Implosions to Measure the T+

Light nuclei were created during big-bang nucleosynthesis (BBN). Standard BBN theory, using rates inferred from accelerator-beam data, cannot explain high levels of ^{6}Li in low-metallicity stars. Using high-energy-density plasmas we measure the T(^{3}He,γ)^{6}Li reaction rate, a candidate for anomalously high ^{6}Li production; we find that the rate is too low to explain the observations, and different than values used in common BBN models. This is the first data directly relevant to BBN, and also the first use of laboratory plasmas, at comparable conditions to astrophysical systems, to address a problem in nuclear astrophysics.

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

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

Experimental Evidence of the Collective Brillouin Scattering of Multiple Laser Beams Sharing Acoustic Waves.

The indirect-drive scheme to inertial confinement fusion uses a large number of laser beams arranged in a symmetric angular distribution. Collective laser plasma instabilities can therefore develop that couple all the incident laser waves located in a cone to the daughter wave growing along the cone symmetry axis [D. F. DuBois et al., Phys. Fluids B 4, 241 (1992)]. With complementary diagnostics of Thomson scattering and of the scattered light, we demonstrate the occurrence of collective stimulated Brillouin sidescattering driving collective acoustic waves in indirect-drive experiments.

Measurements of the Conduction-Zone Length and Mass Ablation Rate in Cryogenic Direct-Drive Implosions on OMEGA.

Measurements of the conduction-zone length (110±20 µm at t=2.8 ns), the averaged mass ablation rate of the deuterated plastic (7.95±0.3 µg/ns), shell trajectory, and laser absorption are made in direct-drive cryogenic implosions and are used to quantify the electron thermal transport through the conduction zone. Hydrodynamic simulations that use nonlocal thermal transport and cross-beam energy transfer models reproduce these experimental observables. Hydrodynamic simulations that use a time-dependent flux-limited model reproduce the measured shell trajectory and the laser absorption but underestimate the mass ablation rate by ∼10% and the length of the conduction zone by nearly a factor of 2.

Gigabar spherical shock generation on the OMEGA laser.

This Letter presents the first experimental demonstration of the capability to launch shocks of several-hundred Mbar in spherical targets--a milestone for shock ignition [R. Betti et al., Phys. Rev. Lett. 98, 155001 (2007)]. Using the temporal delay between the launching of the strong shock at the outer surface of the spherical target and the time when the shock converges at the center, the shock-launching pressure can be inferred using radiation-hydrodynamic simulations. Peak ablation pressures exceeding 300 Mbar are inferred at absorbed laser intensities of ∼3×10(15) W/cm2. The shock strength is shown to be significantly enhanced by the coupling of suprathermal electrons with a total converted energy of up to 8% of the incident laser energy. At the end of the laser pulse, the shock pressure is estimated to exceed ∼1 Gbar because of convergence effects.

Exploration of the transition from the hydrodynamiclike to the strongly kinetic regime in shock-driven implosions.

Clear evidence of the transition from hydrodynamiclike to strongly kinetic shock-driven implosions is, for the first time, revealed and quantitatively assessed. Implosions with a range of initial equimolar D3He gas densities show that as the density is decreased, hydrodynamic simulations strongly diverge from and increasingly overpredict the observed nuclear yields, from a factor of ∼2 at 3.1 mg/cm3 to a factor of 100 at 0.14 mg/cm3. (The corresponding Knudsen number, the ratio of ion mean-free path to minimum shell radius, varied from 0.3 to 9; similarly, the ratio of fusion burn duration to ion diffusion time, another figure of merit of kinetic effects, varied from 0.3 to 14.) This result is shown to be unrelated to the effects of hydrodynamic mix. As a first step to garner insight into this transition, a reduced ion kinetic (RIK) model that includes gradient-diffusion and loss-term approximations to several transport processes was implemented within the framework of a one-dimensional radiation-transport code. After empirical calibration, the RIK simulations reproduce the observed yield trends, largely as a result of ion diffusion and the depletion of the reacting tail ions.

Nonuniformly driven two-plasmon-decay instability in direct-drive implosions.

Half-harmonic emission spectra and images taken during directly driven implosions show that the two-plasmon decay (TPD) instability is driven nonuniformly over the target surface and that multibeam effects dominate this instability. The images show a spatially limited extent of the TPD instability. A prominent spectral feature is used to determine the electron temperature in the corona. Near threshold the temperatures agree with one-dimensional hydrodynamic predictions but exceed them by ∼10% above the TPD threshold. Two-dimensional hydrodynamic simulations indicate that a significant part (∼20%) of the laser intensity must be locally absorbed by the TPD instability (i.e., by collisional damping of the electron plasma waves) to maintain these temperature islands.

Laser-beam zooming to mitigate crossed-beam energy losses in direct-drive implosions.

Spherically symmetric direct-drive-ignition designs driven by laser beams with a focal-spot size nearly equal to the target diameter suffer from energy losses due to crossed-beam energy transfer (CBET). Significant reduction of CBET and improvements in implosion hydrodynamic efficiency can be achieved by reducing the beam diameter. Narrow beams increase low-mode perturbations of the targets because of decreased illumination uniformity that degrades implosion performance. Initiating an implosion with nominal beams (equal in size to the target diameter) and reducing the beam diameter by ∼ 30%-40% after developing a sufficiently thick target corona, which smooths the perturbations, mitigate CBET while maintaining low-mode target uniformity in ignition designs with a fusion gain ≫ 1.

Experimental validation of the two-plasmon-decay common-wave process.

The energy in hot electrons produced by the two plasmon decay instability, in planar targets, is measured to be the same when driven by one or two laser beams and significantly reduced with four for a constant overlapped intensity on the OMEGA EP. This is caused by multiple beams sharing the same common electron-plasma wave. A model, consistent with the experimental results, predicts that multiple laser beams can only drive a resonant common two plasmon decay electron-plasma wave in the region of wave numbers bisecting the beams. In this region, the gain is proportional to the overlapped laser beam intensity.

Saturation of the two-plasmon decay instability in long-scale-length plasmas relevant to direct-drive inertial confinement fusion.

Measurements of the hot-electron generation by the two-plasmon-decay instability are made in plasmas relevant to direct-drive inertial confinement fusion. Density-scale lengths of 400 µm at n(cr)/4 in planar CH targets allowed the two-plasmon-decay instability to be driven to saturation for vacuum intensities above ~3.5×10(14) W cm(-2). In the saturated regime, ~1% of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to 90 keV as the laser beam intensity is increased from 2 to 7×10(14) W cm(-2). This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.

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

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