Shock formation and the ideal shape of ramp compression waves.

We derive expressions for shock formation based on the local curvature of the flow characteristics during dynamic compression. Given a specific ramp adiabat, calculated for instance from the equation of state for a substance, the ideal nonlinear shape for an applied ramp loading history can be determined. We discuss the region affected by lateral release, which can be presented in compact form for the ideal loading history. Example calculations are given for representative metals and plastic ablators. Continuum dynamics (hydrocode) simulations were in good agreement with the algebraic forms. Example applications are presented for several classes of laser-loading experiment, identifying conditions where shocks are desired but not formed, and where long-duration ramps are desired.

Laser-plasmas in the relativistic-transparency regime: Science and applications.

Laser-plasma interactions in the novel regime of relativistically induced transparency (RIT) have been harnessed to generate intense ion beams efficiently with average energies exceeding 10 MeV/nucleon (>100 MeV for protons) at "table-top" scales in experiments at the LANL Trident Laser. By further optimization of the laser and target, the RIT regime has been extended into a self-organized plasma mode. This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency. This mode involves self-generation of persistent high magnetic fields (∼104 T, according to particle-in-cell simulations of the experiments) at the rear-side of the plasma. These magnetic fields trap the laser-heated multi-MeV electrons, which generate a high localized electrostatic field (∼0.1 T V/m). After the laser exits the plasma, this electric field acts on a highly structured ion-beam distribution in phase space to reduce the energy spread, thus separating acceleration and energy-spread reduction. Thus, ion beams with narrow energy peaks at up to 18 MeV/nucleon are generated reproducibly with high efficiency (≈5%). The experimental demonstration has been done with 0.12 PW, high-contrast, 0.6 ps Gaussian 1.053 µm laser pulses irradiating planar foils up to 250 nm thick at 2-8 × 1020 W/cm2. These ion beams with co-propagating electrons have been used on Trident for uniform volumetric isochoric heating to generate and study warm-dense matter at high densities. These beam plasmas have been directed also at a thick Ta disk to generate a directed, intense point-like Bremsstrahlung source of photons peaked at ∼2 MeV and used it for point projection radiography of thick high density objects. In addition, prior work on the intense neutron beam driven by an intense deuterium beam generated in the RIT regime has been extended. Neutron spectral control by means of a flexible converter-disk design has been demonstrated, and the neutron beam has been used for point-projection imaging of thick objects. The plans and prospects for further improvements and applications are also discussed.

Quasi-isentropic compression by ablative laser loading: response of materials to dynamic loading on nanosecond time scales.

The TRIDENT laser was used to induce quasi-isentropic compression waves to approximately 15 GPa in samples of Si, by ablative loading using a laser pulse whose intensity increased smoothly over 2.5 ns. The intensity history of the pulse and the velocity history at the opposite surface of the sample were recorded. Experiments were performed using samples of two different thicknesses simultaneously, in which the evolution of the compression wave was clearly visible. Isentropic stress states deduced were consistent with the previously investigated response of Si to uniaxial loading. The ablative loading was simulated using radiation hydrodynamics, with different equations of state in the plasma and condensed regions and including elasticity in the solid Si. These calculations reproduced the measured velocity histories quite well, demonstrating that quasi-isentropic compression was induced with no preheat from the laser drive. Normal continuum behavior was demonstrated to hold below nanosecond time scales for isentropic compression waves, with no evidence for nonequilibrium effects in the crystal lattice. Details of the velocity history over about 10 GPa were reproduced less well, suggesting a deficiency in the model used for compressed Si, which may be consistent with recent theoretical predictions of uniaxial compression at high strain rates.

High-temporal contrast using low-gain optical parametric amplification.

We demonstrate the use of low-gain optical parametric amplification (OPA) as a means of improving temporal contrast to a detection-limited level 10(-10). 250 microJ, 500 fs pulses of 1053 nm are frequency doubled and subsequently restored to the original wavelength by OPA with >10% efficiency.