L. M. Goldman

Convective stimulated Raman scattering instability in UV laser plasmas

Time‐resolved stimulated Raman scattering (SRS) spectra from UV laser‐produced plasmas are reported. In analyzing these spectra the theory for SRS in inhomogeneous plasma is extended. This theory is applied to obtain temporally resolved electron temperatures in the plasma corona. Typical coronal temperatures for plastic targets irradiated at 1015 W/cm2 range up to 1.8 keV. A comparison is made with the predictions of hydrodynamic simulations.

Diagnostic value of odd-integer half-harmonic emission from laser-produced plasmas

The diagostic value of odd‐integer half‐harmonic emission from laser‐produced plasmas is evaluated in the light of recent spectral measurements on UV irradiation experiments. It has been found that under certain conditions a sharp, slightly red‐shifted feature is observed in the ω/2 spectra. This feature has been identified with a particular mode of the 2ωp decay instability for which one of the plasmon wave vectors vanishes. This feature is eminently well suited for coronal electron temperature measurements. Another half‐harmonic (blue‐shifted) feature is more easily observed and may serve as a secondary—though less accurate—temperature diagnostic. In contrast, the spectral splitting of the (3)/(2) harmonic emission proved to be ill suited for temperature diagnostics because of its sensitivity to irradiation and observation geometry. Either ω/2 or 3ω/2 emission is, however, a good qualitative indicator for the presence of the 2ωp decay instability although quantitative inferences on the level of the 2ωp ...

Raman scattering in inhomogeneous laser‐produced plasma

A model of Raman scattering in inhomogeneous laser‐produced plasma is described, which invokes enhanced Thomson scattering. The enhancement is caused by pulses of hot electrons arising from laser–plasma interactions at the critical or quarter‐critical surfaces. A simple model predicts the locations of two enhanced frequency bands, one between ω0 and ω0/2, and the other between 2ω0 and ω0. The model is shown to fit the results of eight experiments, including a new experiment using the six‐beam Omega laser facility [Phys. Rev. Lett. 48, 1179 (1982); Phys. Fluids 27, 2181 (1984)], converted to 351 nm. In this latter experiment, simultaneous measurement of the upscattered and downscattered bands is carried out, and good agreement is found with the enhanced Thomson model.

Thermal transport measurements in 1.05 μm laser irradiation of spherical targets

Transport and implosion experiments have been conducted on the OMEGA 24‐beam, uniform‐irradiation facility. Thermal transport in spherical irradiation was found to be different than in comparable, single‐beam target irradiation and could not be described in terms of a flux‐inhibited model. Deep energy deposition in spherical irradiation (by electrons on the tail of the thermal velocity distribution) was found to lead to a temperature profile which is not as steep as predicted by a flux‐inhibited model. This apparently leads to more explosive implosion (i.e., higher core temperature) than predicted by using such a model.

Evidence of parametric instabilities in second harmonic spectra from 1054 nm laser‐produced plasmas

Second harmonic spectra emitted from 1054 nm laser‐produced plasmas have been observed in side‐ and backscattering and are shown to have angularly dependent, complex structures. Sidescattered spectra show regularly spaced (∼20 A) Stokes components at intensities above 5×1013 W/cm2. The Stokes components are polarized and stronger in the plane of the polarization of the incident laser. These Stokes components are interpreted to be caused by a combination of electron plasma waves generated by the parametric decay instability and the subsequent electron decay instability. Backscattered spectra show additional finely spaced red‐shifted satellites above 4×1014 W/cm2, indicative of the electron decay instability of plasmons generated from resonance absorption.

Brillouin scattering, two‐plasmon decay, and self‐focusing in underdense ultraviolet laser‐produced plasmas

Underdense foam targets were irradiated with a single UV laser beam at intensities up to 1015 W/cm2. The incident laser propagated into the foam a distance of 500–1000 μm, depending on the average target density. The backscattered Brillouin and 3ω0/2 radiation were observed to have the same dependence on the incident laser intensity, indicating a strong interrelation of these processes. A possible coupling mechanism is proposed.

Observations of high‐energy electron distributions in laser plasmas

A very energetic (20–60 keV) electron distribution is observed in laser plasmas produced with both 1054 and 351 nm drivers. This component, believed to be generated by the 2ωp instability, is present at intensities above approximately 2×1014 W/cm2 and contains less than 0.1% of the incident energy. For 1054 nm irradiation, this component appears in addition to the 2 to 7 keV distribution attributed to resonance absorption.

Spectroscopic study of scattered light at around the fundamental wavelength in UV laser‐produced plasmas

Spectroscopic studies of scattered light from UV laser plasmas near the fundamental wavelength are reported. Three distinct spectral components are observed in these experiments. Doppler‐broadened spectra are observed in transmitted light from thin targets and in specularly reflected light from tilted targets. At intensities above 1014 W/cm2, stimulated Brillouin scattered spectra are measured in backscatter for both normal and oblique incidence of the laser. An additional new component is observed in backscattering from targets at normal incidence. This component is broad and blue‐shifted, and is consistent with analytical model calculation based on filamentation.

〈ρR〉 measurements in laser‐produced implosions using elastically scattered ions

The measurement of the number of elastically scattered deuterium and tritium ions is shown experimentally to be an effective means of determining the 〈ρR〉 of the fuel in a laser fusion implosion. A method is described for clearly separating the unwanted proton background signal from the desired d and t knockon signals. The details of the analysis are presented for an experiment resulting in a measured 〈ρR〉 of 1.2×10−3 g/cm2.

Thermal transport measurements in six‐beam, ultraviolet irradiation of spherical targets

Thermal transport, mass ablation rates, and preheat have been studied in spherical irradiation at λ=351 nm, using six of the 24 beams of the OMEGA spherical irradiation laser system at the Laboratory for Laser Energetics. Mass ablation rates are higher at 351‐nm than at 1054‐nm irradiation, even when compared at the same absorbed irradiance. Similar to the case of 1054‐nm irradiation, very deep burnthrough was found at 351 nm. However, the shallow‐gradient temperature profile at the heat front, characteristic of the experiments at 1054 nm, was not observed here, nor was the large difference between uniform and tight focus irradiation of spherical targets. Ablation pressures derived from charge‐collector data rise from 10 to 100 Mbar for absorbed irradiance in the range of 4×1013 to 9×1014 W/cm2.