[en] In inertial confinement fusion experiments, a cold target material is accelerated by a hot, low-density plasma. The interface between the heavy and light materials is Rayleigh--Taylor (RT) unstable. To estimate the perturbation growth in accelerated targets, a postprocessor to the results of one-dimensional codes is developed. The postprocessor is based on the sharp-boundary model that takes into account time variation in the unperturbed state, mode interaction of neighboring interfaces in the target, effects of spherical convergence, and the mass ablation. The model reveals a new stabilizing effect of ablation for modes with wavelengths longer than the shell thickness. For such modes with γ_cl>V_a/d, the perturbation growth is reduced to η∼m(t)/m(0)e^{∫dt'γ_cl2-kV_blV_a/(2d)}, where γ_cl=kg is the classical RT growth rate of interface perturbations in the semi-infinite slab subject to gravitational field g, k is the wave number, d and m(t) are the slab thickness and mass, and V_a and V_bl are the ablation and blowoff velocities, respectively. The perturbation evolution calculated by using the developed postprocessor is shown to be in good agreement with the results of two-dimensional hydrodynamic simulations

[en] The evolution of shell modulations near peak compression of direct-drive spherical-target implosions has been measured using the 60-beam, 30-kJ UV OMEGA laser system. The spatial size and amplitude of shell-areal-density modulations decrease during the target compression, then increase during its decompression as expected. The shell uniformity at peak compression has been increased by reducing single-beam, laser-drive nonuniformity

[en] Initial designs for direct-drive targets for the National Ignition Facility had a one-dimensional (1-D) gain of 45. A pure cryogenic DT shell with the fuel set on a = 3 adiabat was used to provide adequate hydrodynamic stability. One-dimensional calculations indicate that a gain of over 80 may be achieved through the use of an outer layer of foam wetted by cryogenic deuterium and tritium (DT). The presence of higher-Z absorbers (C) in the foam increases the absorbed energy, allowing the target to contain more fuel

[en] Fast ignition with narrow-band coherent x-ray pulses has been revisited for cryogenic deuterium-tritium (DT) plasma conditions achieved on the OMEGA Laser System. In contrast to using hard-x-rays (hv = 3–6 keV) proposed in the original x-ray fast-ignition proposal, we find that soft-x-ray sources with hv ≈ 500 eV photons can be suitable for igniting the dense DT-plasmas achieved on OMEGA. Two-dimensional radiation–hydrodynamics simulations have identified the break-even conditions for realizing such a “hybrid” ignition scheme (direct-drive compression with soft-x-ray heating) with 50-μm-offset targets: ∼10 ps soft-x-ray pulse (hv ≈ 500 eV) with a total energy of 500–1000 J to be focused into a 10 μm spot-size. A variety of x-ray pulse parameters have also been investigated for optimization. It is noted that an order of magnitude increase in neutron yield has been predicted even with x-ray energy as low as ∼50 J. Scaling this idea to a 1 MJ large-scale target, a gain above ∼30 can be reached with the same soft-x-ray pulse at 1.65 kJ energy. Even though such energetic x-ray sources do not currently exist, we hope that the proposed ignition scheme may stimulate efforts on generating powerful soft-x-ray sources in the near future.

[en] A sharp boundary model describing the linear evolution of the R-T instability in accelerated planar foils and imploding spherical shells is derived for ablation fronts with large Froude numbers. The model consists of two coupled equations for the ablation front and the inner surface that can be solved for arbitrary time-dependent accelerations and ablation velocities. Applications of the model to study the ablation-front distortion and perturbation feedthrough of imploding cryogenic capsules have revealed that a properly selected modulation of the laser intensity could significantly reduce the growth of the R-T instability

[en] 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 (∼100 ps) and the first 200-300 ps 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 10¹⁴-10¹⁵ W/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 proposed

[en] Direct-drive, Rayleigh-Taylor (RT) growth experiments were performed using planar plastic targets on the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at laser intensities between ∼2x10¹⁴ and ∼1.5x10¹⁵ W/cm². The primary purpose of the experiments was to test fundamental physics in hydrocodes at the range of drive intensities relevant to ignition designs. The target acceleration was measured with a streak camera using side-on, x-ray radiography, while RT growth was measured with a framing camera using face-on radiography. In a laser-intensity range from 2 to 5x10¹⁴ W/cm², the measured RT growth agrees well with two-dimensional simulations, based on a local model of thermal-electron transport. The RT growth at drive intensities above ∼1.0x10¹⁵ W/cm² was strongly stabilized compared to the local model predictions. The experiments demonstrate that standard simulations, based on a local model of electron thermal transport, break down at peak intensities of ignition designs, although they work well at lower intensities. These results also imply that direct-drive ignition targets are significantly more stable than previously calculated using local electron-transport models at peak intensities of ignition designs. The preheating effects by nonlocal electron transport and hot electrons were identified as some of the stabilizing mechanisms

[en] Understanding and designing inertial confinement fusion (ICF) implosions through radiation-hydrodynamics simulations relies on the accurate knowledge of the equation of state (EOS) of the deuterium and tritium fuels. To minimize the drive energy for ignition, the imploding shell of DT fuel must be kept as cold as possible. Such low-adiabat ICF implosions can access to coupled and degenerate plasma conditions, in which the analytical EOS models become inaccurate due to many-body effects. Using the path-integral Monte Carlo (PIMC) simulations we have derived a first-principles EOS (FPEOS) table of deuterium that covers typical ICF fuel conditions at densities ranging from 0.002 to 1596 g/cm³ and temperatures of 1.35 eV to 5.5 keV. We report the internal energy and the pressure and discuss the structure of the plasma in terms of pair-correlation functions. When compared with the widely used SESAME table and the revised Kerley03 table, discrepancies in the internal energy and in the pressure are identified for moderately coupled and degenerate plasma conditions. In contrast to the SESAME table, the revised Kerley03 table is in better agreement with our FPEOS results over a wide range of densities and temperatures. Although subtle differences still exist for lower temperatures (T < 10 eV) and moderate densities (1 to 10 g/cm³), hydrodynamics simulations of cryogenic ICF implosions using the FPEOS table and the Kerley03 table have resulted in similar results for the peak density, areal density (ρR), and neutron yield, which differ significantly from the SESAME simulations.

[en] Direct-drive–ignition designs with plastic CH ablators create plasmas of long density scale lengths (L_n ≥ 500 μm) at the quarter-critical density (N_qc) region of the driving laser. The two-plasmon–decay (TPD) instability can exceed its threshold in such long-scale-length plasmas (LSPs). To investigate the scaling of TPD-induced hot electrons to laser intensity and plasma conditions, a series of planar experiments have been conducted at the Omega Laser Facility with 2-ns square pulses at the maximum laser energies available on OMEGA and OMEGA EP. Radiation–hydrodynamic simulations have been performed for these LSP experiments using the two-dimensional hydrocode draco. The simulated hydrodynamic evolution of such long-scale-length plasmas has been validated with the time-resolved full-aperture backscattering and Thomson-scattering measurements. draco simulations for CH ablator indicate that (1) ignition-relevant long-scale-length plasmas of L_n approaching ∼400 μm have been created; (2) the density scale length at N_qc scales as L_n(μm)≃(R_DPP×I^1/4/2); and (3) the electron temperature T_e at N_qc scales as T_e(keV)≃0.95×√(I), with the incident intensity (I) measured in 10¹⁴ W/cm² for plasmas created on both OMEGA and OMEGA EP configurations with different-sized (R_DPP) distributed phase plates. These intensity scalings are in good agreement with the self-similar model predictions. The measured conversion fraction of laser energy into hot electrons f_hot is found to have a similar behavior for both configurations: a rapid growth [f_hot≃f_c×(G_c/4)⁶ for G_c < 4] followed by a saturation of the form, f_hot≃f_c×(G_c/4)^1.2 for G_c ≥ 4, with the common wave gain is defined as G_c=3 × 10⁻²×I_qcL_nλ₀/T_e, where the laser intensity contributing to common-wave gain I_qc, L_n, T_e at N_qc, and the laser wavelength λ₀ are, respectively, measured in [10¹⁴ W/cm²], [μm], [keV], and [μm]. The saturation level f_c is observed to be f_c ≃ 10^–2 at around G_c ≃ 4. The hot-electron temperature scales roughly linear with G_c. Furthermore, to mitigate TPD instability in long-scale-length plasmas, different ablator materials such as saran and aluminum have been investigated on OMEGA EP. Hot-electron generation has been reduced by a factor of 3–10 for saran and aluminum plasmas, compared to the CH case at the same incident laser intensity. draco simulations suggest that saran might be a better ablator for direct-drive–ignition designs as it balances TPD mitigation with an acceptable hydro-efficiency.

[en] A technique to measure the mass ablation rate in direct-drive inertial confinement fusion implosions using a pinhole x-ray framing camera is presented. In target designs consisting of two layers of different materials, two x-ray self-emission peaks from the coronal plasma were measured once the laser burned through the higher-Z outer layer. The location of the inner peak is related to the position of the ablation front and the location of the outer peak corresponds to the position of the interface of the two layers in the plasma. The emergence of the second peak was used to measure the burnthrough time of the outer layer, giving the average mass ablation rate of the material and instantaneous mass remaining. By varying the thickness of the outer layer, the mass ablation rate can be obtained as a function of time. Simulations were used to validate the methods and verify that the measurement techniques are not sensitive to perturbation growth at the ablation surface