LLE Review 130

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Highlights

This volume of LLE Review, covering January–March 2012, features “OMEGA Polar-Drive Target Designs,” which describes low-adiabat, cryogenic-deuterium–tritium, and warm, plastic-shell polar-drive (PD)–implosion designs for the OMEGA laser. The designs are at two different on-target laser intensities, each at a different in-flight aspect ratio (IFAR). The first design permits studies of implosion energetics and target performance closer to ignition-relevant intensities (7 × 10¹⁴ W/cm² at the quarter-critical surface), where nonlocal heat conduction and laser–plasma interactions can play an important role, but at lower values of IFAR (~22). The second design permits studies of implosion energetics and target performance at lower intensity (3.0 × 10¹⁴ W/cmsup>2) but at higher IFAR (~32), where the shell instability can play an important role. The higher IFAR designs are accessible on the existing OMEGA Laser System only at lower intensities. Implosions at ignition-relevant intensities can only be obtained by reducing target radius, although only at smaller values of IFAR. PD geometry requires repointing the laser beams to improve shell symmetry. The higher-intensity designs optimize target performance by repointing beams to a lesser extent and compensate for the reduced equatorial drive by increasing beam energies for the repointed beams and using custom beam profiles that improve equatorial illumination at the expense of irradiation at higher latitudes. These designs will be studied when new phase plates for the OMEGA Laser System, corresponding to the smaller target radii and the custom beam profiles, are obtained. Implosion results from the combined set of high-intensity and high-IFAR implosions should yield valuable data to validate models of laser-energy deposition, heat conduction, nonuniformity growth, and fuel assembly in PD geometry.

Additional highlights of research presented in this issue include the following:

• A comprehensive review of the cryogenic-deuterium and deuterium–tritium implosions that have been performed on the Omega Laser System over the last decade is presented. The success of ignition target designs in inertial confinement fusion (ICF) experiments critically depends on the ability to maintain the main the fuel entropy at a low level while accelerating the shell to ignition-relevant velocities of V_imp > 3 × 10⁷ cm/s. The fuel entropy is inferred from the experiments by measuring fuel areal density near peak compression. Measured areal densities up to ⟨ρR⟩_n = 300 mg/cm²(larger than 85% of predicted values) have been 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 these OMEGA designs are predicted to achieve ⟨ρR⟩_n = 1.2 g/cm², sufficient for ignition demonstration in direct-drive ICF experiments.
• The effect of medium-Z doping of plastic ablators on laser imprinting and Rayleigh–Taylor instability growth using direct-drive implosions on the OMEGA Laser System is studied. The targets were spherical plastic (CH) shells volume doped with a varied concentration of Si (4.3% and 7.4%) and Ge (3.9%). The targets were imploded by 48 beams with a low-adiabat, triple-picket laser shape pulse with a peak intensity of 4 × 10¹⁴ W/cm²and x-ray radiographed through a 400-µm 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. Simulations using the two-dimensional, radiation–hydrodynamics code DRACO show good agreement with the measurements.
• 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 to the ablation surface. This enables one to measure the radius of the imploding shell with an accuracy better than 1 µm and determine a 200-ps average velocity to better than 2%.
• Shock-ignition experiments on OMEGA that use a novel beam configuration that has separate low-intensity compression beams and high-intensity spike beams are discussed. Significant improvements in the performance of plastic-shell, D₂ implosions were observed with repointed beams. The analysis of the coupling of the high-intensity spike beam energy into the imploding capsule indicates that absorbed hot-electron energy contributes to the coupling. The backscattering of the laser energy was measured to reach 36% at single-beam intensities of ~8 × 10¹⁵ W/cm². Hard x-ray measurements revealed a relatively low hot-electron temperature of ~30 keV independent of intensity and timing. At the highest intensity, stimulated Brillouin scattering occurs near and above the quarter-critical density and the two-plasmon-decay instability is suppressed.
• A single-shot, electro-optic data-acquisition for system with a 600:1 dynamic range for the NIF Dante instrument is demonstrated. The prototype system uses multiple optical wavelengths to allow for the multiplexing of up to eight signals onto one photodetector and provides optical isolation and a bandwidth of 6 GHz.