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Neutron yields of direct-drive, low-adiabat (α≈2 to 3) cryogenic D2 target implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have been systematically investigated using the two-dimensional (2D) radiation hydrodynamics code DRACO [P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)]. Low-mode (l≤12) perturbations, including initial target offset, ice-layer roughness, and laser-beam power imbalance, were found to be the primary source of yield reduction for thin-shell (5 μm), low-α, cryogenic targets. The 2D simulations of thin-shell implosions track experimental measurements for different target conditions and peak laser intensities ranging from 2.5×1014–6×1014 W/cm2. Simulations indicate that the fusion yield is sensitive to the relative phases between the target offset and the ice-layer perturbations. The results provide a reasonable good guide to understanding the yield degradation in direct-drive, low-adiabat, cryogenic, thin-shell-target implosions. Thick-shell (...

The effects of two types of target mounts, stalks and spider silks, on the implosion of both room-temperature D2-gas-filled shells and cryogenic D2-ice-filled shells have been studied both experimentally and by means of two-dimensional simulations. The simulations indicate that the hydrodynamic effect of the expanding plasma created by the ablation of material from the target mounts and refraction of laser light by this plasma induce perturbations in the imploding shell that are damaging to the implosion. The spider silks are the more-damaging type of mount since the silks (typically four) are arrayed over the target surface, whereas the stalk (typically one) meets the target at a single point. Stalks are therefore preferred over silks as a target mount. The scale and magnitude of the perturbations induced by the spider silks have been verified by planar-target experiments performed on the OMEGA laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1995)]. The perturbations predicted by simulations to arise from stalks qualitatively agree with the results of implosion experiments using Ti-doped plastic shells.

The success of direct-drive-ignition target designs depends on two issues: the ability to maintain the main fuel entropy at a low level and the control of the nonuniformity growth during the implosion. Modelling the ICF experiments requires an accurate account for all sources of shell heating, including the shock heating, radiation and suprathermal electrons preheat, and small-scale perturbation growth. To increase calculation accuracy, a new heat-transport model has been developed and implemented in the 1-D hydrocode LILAC. This model includes both the effect of the resonance absorption and the nonlocal thermal transport. The OMEGA experiments designed with the help of the new model have achieved high-areal-density (ρR > 200 mg/cm2) fuel assembly in the low-adiabat cryogenic shell implosions.

The success of direct-drive-ignition target designs depends on two issues: the ability to maintain the main fuel adiabat at a low level and the control of the nonuniformity growth during the implosion. A series of experiments was performed on the OMEGA Laser System [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to study the physics of low-adiabat, high-compression cryogenic fuel assembly. Modeling these experiments requires an accurate account for all sources of shell heating, including shock heating and suprathermal electron preheat. To increase calculation accuracy, a nonlocal heat-transport model was implemented in the 1D hydrocode. High-areal-density cryogenic fuel assembly with ρR>200mg∕cm2 [T. C. Sangster, V. N. Goncharov, P. B. Radha et al., “High-areal-density fuel assembly in direct-drive cryogenic implosions,” Phys. Rev. Lett. (submitted)] has been achieved on OMEGA in designs where the shock timing was optimized using the nonlocal treatment of the heat conductio...

Time-dependent and time-integrated absorption fractions are inferred from scattered-light measurements in room-temperature and cryogenic direct-drive-implosion experiments on OMEGA. The measurements agree reasonably well with hydrodynamic simulations that include nonlocal electron-heat transport. Discrepancies in the time-resolved scattered-light spectra between simulations and experiments remain for complex laser pulse shapes, indicating beam-to-beam energy transfer and commensurate coupling losses. Time-resolved scattered-light spectra near ω∕2 and 3ω∕2 as well as time-resolved hard-x-ray measurements indicate the presence of a strongly driven two-plasmon-decay (TPD) instability at high intensities that may influence the observed laser light absorption. Experiments indicate that energetic electron production due to the TPD instability can be mitigated with high-Z-doped plastic shells.

The University of Rochester's Laboratory for Laser Energetics has been imploding thick cryogenic targets for six years. Improvements in the Cryogenic Target Handling System and the ability to accurately design laser pulse shapes that properly time shocks and minimize electron preheat, produced high fuel areal densities in deuterium cryogenic targets (202±7 mg/cm2). The areal density was inferred from the energy loss of secondary protons in the fuel (D2) shell. Targets were driven on a low final adiabat (α = 2) employing techniques to radially grade the adiabat (the highest adiabat at the ablation surface). The ice layer meets the target-design toughness specification for DT ice of 1-μm rms (all modes), while D2 ice layers average 3.0-μm-rms roughness. The implosion experiments and the improvements in the quality and understanding of cryogenic targets are presented.

Resonance absorption enhances the early time laser absorption in direct-drive inertial confinement fusion implosions, affecting the performance of imploding capsules. In this paper, resonance absorption is studied both theoretically and experimentally for a λ=351-nm laser. Simulations demonstrate an important contribution of the resonance absorption during both the short laser picket (∼100ps) and the first 200–300ps in the long laser pulse. It is shown that for the conditions relevant to the direct-drive implosions on the OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], the early time enhancement of laser absorption can be up to 20% for drive intensities of 1014–1015W∕cm2. Planar reflection light experiments on OMEGA were conducted to validate the theoretical results. There is a generally good agreement between simulation and experimental results. As an additional diagnostic of resonance absorption, shock-timing experiments employing OMEGA drive beams of different polarization are p...

Direct-drive laser absorption, mass ablation rate, and shock heating are experimentally studied on the OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] to validate hydrodynamics simulations. High-gain, direct-drive inertial confinement fusion target implosions require accurate predictions of the shell adiabat α (entropy), defined as the pressure in the main fuel layer to the Fermi-degenerate pressure, and the implosion velocity of the shell. The laser pulse shape determines the shell adiabat and the hydrodynamic efficiency determines the implosion velocity. A comprehensive set of measurements tracking the flow of energy from the laser to the target was conducted. Time-resolved measurements of laser absorption in the corona are performed on spherical implosion experiments. The mass ablation rate is inferred from time-resolved Ti K-shell spectroscopic measurements of nonaccelerating, solid CH spherical targets with a buried tracer layer of Ti. Shock heating is diagnosed in planar-CH-fo...

The timing of multiple shock waves is crucial to the performance of inertial confinement fusion ignition targets. Presented are measurements of velocities and optical self-emission from shock waves in polystyrene targets driven by two 90-ps pulses separated by 1.5–2ns. These pulses drive two shock waves that coalesce in the target, and the resultant velocity histories, coalescence times, and transit times are unambiguously observed in both velocity interferometry and self-emission data. These results are in good agreement with one-dimensional hydrodynamics code predictions.

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)] hydr...

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11 2763 (2004)] shows promise for achieving direct-drive ignition while the National Ignition Facility (NIF) [E. M. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41 B39 (1999)] is initially configured for indirect drive. Experiments have been carried out using 40 repointed beams of the 60-beam OMEGA laser system [T. R. Boehly et al., Rev. Sci. Instrum. 66 508 (1995)] to approximate the NIF PDD configuration. Backlit x-ray framing-camera images of D2-filled spherical CH capsules show a characteristic nonuniformity pattern that is in close agreement with predictions. Saturn targets (similar capsules surrounded by a plastic ring) increase the drive on the equator, suggesting that highly symmetric PDD implosions may be possible with appropriate tuning. Two-dimensional (2D) simulations reproduced the approximately threefold reduction in yield found for the non-Saturn PDD capsules. Preliminary simulations for a NIF Saturn design predict a high ...

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