Session CO4: Direct and Indirect Drive, Shock and Fast Ignition

Revolver Designs for the National Ignition Facility Using Current and Optimized Phase Plates

The three-shell Revolver target[1] proposed for direct-drive ignition at the National Ignition Facility has a diameter (~6 mm) much larger than that of conventional designs. Simulations carried out using the 2-D hydrodynamics code SAGE[2] illustrate the advantages and challenges of the design. The absorption is almost 100%, the absorption occurs at a significant distance from the ablation surface, and the average intensity reaching the quarter-critical surface is low (just over 1 x 1014 W/cm2). However, uniformity presents a challenge, especially if the current phase plates are used. An optimized design, taking into account the full 3-D energy-deposition pattern, results in an implosion velocity nonuniformity of ~2.5% rms with the nonuniformity modes dominated by the small beam sizes. This can be improved to ~1.2% using custom phase plates with larger beam spots. Further improvement should be realized through thermal smoothing of azimuthal variations, not accounted for in the simulations.

Numerical Investigation of Laser-Imprint Mitigation in Revolver Ignition Designs

The three-shell Revolver target[1] proposed for direct-drive ignition at the National Ignition Facility (NIF) utilizes a large 6-mm beryllium shell as an ablator. This ablator will be subjected to significant levels of laser imprint arising directly from the NIF laser beams illuminating the target without sufficient laser smoothing. Initial studies[2] have indicated that such imprint can severely distort the incoming shell and lead to ignition failure. Two-dimensional simulation results, employing nonlocal electron thermal transport and cross‑beam energy transport, will be presented investigating the implementation of several laser-smoothing techniques, as well as tamper layers within the target, to evaluate their efficacy in limiting the ensuing perturbation growth.

Laser–Plasma Interaction Experiments at Direct-Drive Ignition-Relevant Scale Lengths at the National Ignition Facility

Experiments at the National Ignition Facility have probed laser-plasma interactions and hot-electron production at plasma conditions relevant to direct-drive ignition, with predicted density scale lengths of L_n ~500-700 μm, electron temperatures of T_e ~4-5 keV, and overlapped laser intensities of I ~6-15 × 10¹⁴ W/cm². The fraction of laser energy converted to hot electrons is 1-3%, while the hot-electron temperature is 45-60 keV. Only a sharp red-shifted feature is observed around ω/2, along with significant stimulated Raman scattering (SRS), including sidescattering, at lower densities, suggesting that SRS dominates hot-electron production, unlike in shorter-scale-length plasmas on OMEGA that are dominated by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. Subsequent measurements of 3ω/2 emission have revealed evidence of TPD; upcoming experiments will identify dominant plasma waves. The coupling of hot electrons to an implosion will be measured in new experiments, which will determine the need for preheat mitigation strategies for direct-drive ignition.   

Implementation of the Low-Noise, 3-D Ray-Trace, Inverse-Projection Method in the Radiation–Hydrodynamics Code HYDRA

Hydrodynamic simulations of direct-drive inertial confinement fusion (ICF) targets require low-noise laser-energy deposition to achieve high-fidelity implosions. Directly driven ICF targets are especially susceptible to laser-deposition noise because of the proximity of the coronal absorption profile to the ablation surface as well as the high laser energy coupling efficiency. Using the inverse-projection method, judicious selection of the simulated laser rays’ origination points dramatically reduces the deposition noise in comparison to random spatial distributions coupled with temporal averaging. The significantly lower ray number density required using the inverse-projection method also improves computational efficiency. The inverse-projection method, originally developed as one part of the Mazinisin 3-D laser ray-trace in the 2-D radiation-hydro code DRACO, has been was extended to the 3-D radiation–hydro code HYDRA. The initial results of 3-D polar-direct-drive implosion simulations demonstrating the improved noise characteristics will be shown as part of the introduction of the inverse-projection method into HYDRA.

Cross-Beam Energy Transfer Platform Development on OMEGA

A cross-beam energy transfer (CBET) platform was activated on OMEGA. A gas jet was heated by ten 500-ps, 351-nm beams that used phase plates to generate a ~2-mm-diam spherical plasma. Optical Thomson scattering measured a peak temperature of ~700 eV over a uniform ~5 x 10¹⁹ cm–3 electron density plateau of ~1 mm. These results are in good agreement with radiation–hydrodynamics simulations performed by HYDRA. To study CBET, a wavelength‑tunable UV beam (Δλ_3w ~ 3.5 nm) and a 351-nm beam act as the probe and pump beams, respectively. The frequency of the probe was tuned so that the beat frequency of the crossed beams was resonant with ion-acoustic waves in the plasma, causing Brillouin scattering, which transferred energy between the beams. The results will be compared to LPSE simulations to understand limitations of linear CBET modeling.    

Density Measurements of the Inner Shell Release

In inertial confinement fusion implosions, the release of plasma off the inner surface of the target shell after the shock breakout is an important parameter to the performance of the design. If the release has a higher density or longer scale length than that predicted by hydrodynamic simulations, the mass increase in the hot spot can decrease its compressibility and reduce performance compared to what is expected from the simulations. Experiments on OMEGA EP at the Laboratory for Laser Energetics were performed to measure the plasma expanding on the back side of a CH shell driven by two UV laser beams with a total of 6 kJ of energy in a 5-ns pulse focused to a 750-mm spot. The peak position of the driven shell was tracked using x-ray radiography streaked over 4 ns. The low-density plasma expanding off the undriven side of the shell after the shock breaks through was measured using the 4ω interferometer and angular filter refractometer. The experimental data will be presented and compared to the hydrodynamic simulations using 2-D radiation–hydrodynamic code DRACO.