LLE Review 131

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Highlights

This volume of LLE Review, covering April–June 2012, features “Shock-Ignition Experiments with Planar Targets on OMEGA” which describes strong-shock–generation measurements in the presence of pre-plasmas with relevance to shock ignition using the OMEGA Laser System. A planar target was irradiated with a laser pulse consisting of a pre-plasma–generating foot followed by a high-intensity spike, driving a strong shock into the target. The observed shock dynamics inferred from velocity interferometer for any reflector (VISAR) and streaked optical pyrometer (SOP) measurements are reproduced well using two-dimensional DRACO simulations, indicating that plastic-ablator shocks of up to 70-Mbar strength have been generated.

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

• The first x-ray Thomson-scattering (XRTS) measurement of shock-compressed liquid deuterium performed at the Omega Laser Facility is reported. The noncollective, spectrally resolved, inelastic XRTS employs 2.96-keV Cl Ly_α line emission. Microscopic property measurements of shocked deuterium show an inferred spatially averaged electron temperature of 8±5 eV, an electron density of 2.2 × 10²³ cm^-3 and an ionization of 0.8 (-0.25, +0.15). Two-dimensional hydrodynamic simulations using equation-of-state models suited for the extreme parameters also occurring in inertial confinement fusion research and planetary interiors are consistent with the experimental results.
• A low-temperature chemical cleaning approach has been developed to remove manufacturing residues from multilayer dielectric (MLD) pulse-compressor gratings to be used in the OMEGA EP Laser System. The method strips baked-on photoresist, metal contaminants, and debris without damaging the grating’s delicate surface structure. Because targeted cleaning steps remove specific families of contaminants (heavy organics, light organics, metals, and oxides), the process can be adjusted to strip known quantities and types of material. After cleaning, grating samples showed excellent performance in short-pulse (10-ps) laser-damage testing at 1054 nm. Average in-air damage thresholds were 4.06±0.19 J/cm² and 3.32±0.22 J/cm² (beam normal) in the 1-on-1 and N-on-1 regimes, respectively, for a set of nine gratings cleaned at processing temperatures in the range 40°C to 70°C. Post-cleaning diffraction efficiencies were consistently above 96%.
• The measurement of magnetic fields induced by the Rayleigh–Taylor (RT) instability in planar plastic foil with ultrafast proton radiography is presented. Thin plastic foils were irradiated with ~4-kJ, 2.5-ns laser pulses focused to an intensity of ~10¹⁴ W/cm². Target modulations were seeded by laser nonuniformities and amplified during target acceleration by the RT instability. The experimental data show the hydrodynamic evolution of the target and the generated MG-level magnetic fields in the broken foil, which are in good agreement with predictions from 2-D magnetohydrodynamic simulations.
• The high-resolution spectroscopy used to measure ICF neutron spectra to infer the areal density (ρR) of cryogenic DT implosions on OMEGA is described in detail. Neutron time-of-flight (nTOF) techniques are used to measure the spectrum of neutrons that elastically scatter off the dense deuterium (D) and tritium (T) fuel. High signal-to-background data have been recorded on cryogenic DT implosions using a well-collimated 13.4-m line of sight and an nTOF detector with an advanced liquid scintillator compound. An innovative method to analyze the elastically scattered neutron spectra was developed using well-known cross sections of the DT nuclear reactions. The measured areal densities are consistent with alternative ρR measurements and 1-D simulations.
• The two-plasmon–decay common-wave process has been demonstrated at the Omega Laser Facility. The total energy in hot electrons produced in planar target is measured to be the same when using one or two laser beams and significantly reduced with four beams for a constant overlapped intensity. This is caused by multiple beams sharing the same common electron plasma wave in the two-plasmon–decay (TPD) instability. A model, consistent with the experimental results, predicts that multiple laser beams can only drive a resonant common TPD 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.
• A reflective optical transport system has been built for ultraviolet Thomson scattering from electron plasma waves on OMEGA. A Schwarzschild objective that uses two concentric spherical mirrors coupled to a Pfund objective provides diffraction-limited imaging across all reflected wavelengths. This enables the operator to perform Thomson-scattering measurements of ultraviolet (0.263-µm) light scattered from electron plasma waves.