V. C. Rupert

The interaction of 1.06 μm laser radiation with high Z disk targets

Gold disks have been irradiated with 1.06 μm laser light at intensities between 7 × 1013 and 3 × 1015 W/cm2, and pulse lengths between 200 and 1000 psec. Due to the high Z and long pulse, inverse bremsstrahlung becomes an important absorption mechanism and competes strongly with resonance absorption and stimulated scattering. In addition to measured absorptions, data on the temporal, spatial, angular, and spectral characteristics of the x‐ray emission are presented. Temporally and spectrally resolved back‐reflected light, and polarization‐dependent sidescattered light are detected, providing estimates for the amount of stimulated scattering and of the coronal electron temperature. Inhibited electron thermal conduction and nonlocal thermodynamic equilibrium ionization physics play key roles in bringing numerical simulations of these experiments into agreement with all of the above‐mentioned data.

Laser irradiation of disk targets at 0.53 μm wavelength

Results and analyses are presented for laser irradiation of Be‐, CH‐, Ti‐, and Au‐disk targets with 0.53 μm light in 3–200 J, 600–700 psec pulses, at nominal incident intensities from 3×1013 to 5×1015 W/cm2. The measured absorptions are higher than observed in similar 1.06 μm irradiations, and are largely consistent with modeling which shows the importance of inverse‐bremsstrahlung and Brillouin scattering. Observed red‐shifted back‐reflected light shows that Brillouin scattering occurs at low to moderate levels. Backscattering fractions up to 30% were observed in the f/2 focusing lens. The measured fluxes of multi‐keV x rays indicate hot‐electron fractions of 1% or less, with temperatures of 6 to 20 keV which are consistent with resonance absorption or perhaps 2ωpe. Measurements show 30%–50% efficient conversion of absorbed light into sub‐keV x rays, with time‐, angular‐, and spatial‐emission distributions which are generally consistent with non‐local‐thermodynamic‐equilibrium modeling using inhibited th...

Irradiation of parylene disks with a 1.06 μm laser

Parylene (C8H8) disks have been irradiated with Nd:YAG‐glass laser pulses focused to flux levels in the 1015 to 1017 W/cm2 range. The flux level was varied by changing the pulse length (50–150 psec), the laser energy (5–15 J), and the axial position of the target with respect to the f/1.1 focusing lens. An extensive array of diagnostics was used to measure the temporal and energy distribution of the focused laser light at the target, the temporal and angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron energy distributions. The experimental results, together with two‐dimensional numerical simulations imply absorption via collective processes, laser generation of suprathermal electrons, and transport inhibition consistent with the presence of mega‐Gauss level thermoelectric magnetic fields.

Interaction of 1.06 μm laser radiation with variable Z̃ targets

Parylene (C8H8) and tungsten‐glass (W2O/P2O5) disks have been irradiated with 150–400 psec Nd:YAG‐glass laser pulses focused to diameters of 250–300 μm with flux levels in the 1013–1015 W/cm2 range. An extensive array of diagnostics was used to measure the temporal and energy distributions of the focused laser light at the target, the angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron distributions. Analysis of the experimental results indicates that the laser‐plasma interaction was characterized by a variety of collective phenomena which appeared stronger in the tungsten‐glass experiments.

Evidence for profile steepening in laser‐irradiated plasmas

Experimental evidence of a steepened electron density profile near critical density has been obtained by studying the light scattered by targets (10‐μm thick disks and 100‐μm diam glass spherical microshells filled with deuterium and tritium gas) illuminated by linearly polarized, 1.06‐μm light. Scale lengths on the order of 1 μm have been inferred both from the polarization of the reflected light and from the azimuthal asymmetry (asymmetry about the beam axis) of the time‐integrated scattered light with respect to the laser electric field. Azimuthally asymmetric heating of the microshells targets is indicated both by x‐ray micrographs and by the spatial distribution of the plasma blowoff.

Calorimeters for measurement of ions, x rays, and scattered radiation in laser‐fusion experiments

Several types of calorimeters have been developed for measuring ions, x rays, and scattered radiation produced by fusion experiment targets imploded by laser pulses. One version for ions and x rays uses metal absorbers in a differential arrangement to compensate for the small fraction of scattered laser radiation that is also absorbed; the other version uses a glass absorber which transmits most of the scattered laser radiation. The scattered‐radiation calorimeters use a transparent glass shield to remove ions and x rays and colored glass to absorb the radiation.

Use of thin wall imaging in the diagnosis of laser heated hohlraums

High-Z, laser heated hohlraums can be made thick enough to contain thermal radiation, yet thin enough to let out x rays >∼6 keV produced by hot, relatively dense blow-off plasma. We use such “thin wall hohlraums” to observe the physical location of hot, dense, laser produced hohlraum plasmas. This technique has allowed us to come to some understanding of laser transport/deposition, plasma stagnation, and bulk plasma filling.

Studies of K‐ and L‐shell x‐ray emission from laser plasmas for use as a high‐energy backlighter (abstract)

We are developing a backlighter in the 4–10 keV energy range using K‐shell spectra from elements with Z≤30 and the L‐shell spectra from elements with Z≥58. Using one beam of the Nova laser yielded about 3.0 kJ of 0.53 μm wavelengths light in a 1.0 ns square pulse which was focused to a spot of less than 100 μm in diameter on 12.5 and 25.0‐μm‐thick foil targets. Our initial results include time‐resolved spectra from slab (2 mm×2 mm) and disk (100 μm diameter) targets of Ce (Z=58), Zn (Z=30), and Ti (Z=22). The Zn and Ti exhibit strong emission from the cold Kα line and satellite states, while the Ce spectra show prominent L‐band structure. By exploring the differences between disks matched to the beam focus and extended slabs we hope to obtain information on plasma cooling mechanisms.