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Hot electrons generated from laser plasma instabilities degrade performance of direct drive implosions by preheating the deuterium and tritium (DT) fuel resulting in early decompression and lower areal densities at stagnation. A technique to quantify the hot electron preheat of the dense DT fuel and connect it to the degradation in areal density is described in detail. Hot electrons are measured primarily from the hard x-rays they emit as they slow down in the target. The DT preheat is inferred from a comparison of the hard x-ray signals between a DT-layered implosion and its mass equivalent ablator only implosion. The preheat energy spatial distribution within the imploding shell is inferred from experiments using high Z payloads of varying thicknesses. It is found that the electrons deposit their energy uniformly throughout the shell material. For typical direct-drive OMEGA implosions driven with an overlapped intensity of ∼ 9 · 10 14 W / cm 2 , approximately ∼ 0.02 % – 0.03 % of the laser energy is converted into preheat of the stagnated fuel which corresponds to areal density degradations of 10%–20%. The degradations in areal density explain some of the observed discrepancies between the simulated and measured areal densities. [ABSTRACT FROM AUTHOR]

In inertial confinement fusion, an externally applied magnetic field can reduce heat losses in the compressing fuel thereby increasing neutron-averaged ion temperatures and neutron yields. However, magnetization is only beneficial if the magnetic pressure remains negligible compared to the fuel pressure. Experiments and three-dimensional magneto-hydrodynamic simulations of cylindrical implosions on the OMEGA laser show ion temperature and neutron yield enhancements of up to 44% and 67%, respectively. As the applied axial magnetic field is increased to nearly 30 T, both experiments and simulations show yield degradation. For magnetized, cylindrical implosions, there exists an optimal magnetic field that maximizes the increase in yield. Limiting the fuel convergence ratio by preheating the fuel can further increase the benefit of magnetization. The results demonstrate that it is possible to create a plasma with a density of order 1 g / cm 3 and an ion temperature greater than 1 keV with a magnetic pressure comparable to the thermal pressure, a new regime for laser-produced plasmas on OMEGA. [ABSTRACT FROM AUTHOR]

Analyzes enhanced x-ray emission from long-pulse-width laser-produced plasmas. Application of methods in the investigation such as the Nova laser facility; Results of experimental procedures with regard to the pulse-width dependence of thermal x-ray conversion efficiencies in laser-heated gold plasmas; Background on the temporal profiles of x-ray emission intensities.

Recent advances in laser science and technology have opened new possibilities for the study of high energy density plasma physics. The advances include techniques to control the laser spatial and temporal coherence, and the development of laser architectures and optical materials that have led to the demonstration of compact, short pulse (τ≤10-12 sec) high brightness lasers, capable of irradiating plasmas with intensities ≥1018 W/cm2. Experiments with reduced laser coherence have shown a substantial decrease in laser-driven parametric instabilities and have extended the parameter range where inverse bremsstrahlung absorption is the dominant coupling process. Beam smoothing with short wavelength lasers should result in inverse bremsstrahlung dominated coupling in the irradiance parameter regimes of the millimeter scale-length plasmas envisioned for the megajoule class lasers for ignition and gain in inertial fusion. In addition new regimes of laser–plasma coupling will become experimentally accessible when plasmas are irradiated with I≥1018 W/cm2. Relativistic effects, extreme profile modification, and electrons heated to energies exceeding 1 MeV are several of the phenomena that are expected. Numerous applications in basic and applied plasma physics will result from these new capabilities. [ABSTRACT FROM AUTHOR]

A study of the absorption and scattering of laser light by planar, laser-produced plasmas is reported. Up to 4 kJ of 0.53 μm laser light irradiated thick, gold targets in 1 nsec pulses at intensities up to 4×1015 W/cm2. The absorption was determined from measurements of the scattered light by an array of about 100 detectors. Stimulated Brillouin sidescattering, identified by the angular distribution and spectrum of the scattered light, exceeded 10% of the laser energy for intensities above 1015 W/cm2. The observations are compared to those of previously reported experiments. It is suggested that a combination of collisional absorption, enhanced backscattering, and convective sidescattering produces the observed dependence on laser intensity. [ABSTRACT FROM AUTHOR]

We have observed enhanced emission of the O VIII Hα line at 121 eV from a recombining plasma produced by laser heating of a thin Formvar foil with two opposed cylindrically focused (0.3×1.4 cm) 5-TW laser beams. The time-dependent intensity of the Hα line increases by nearly an order of magnitude as the plasma length is increased from 0.27 to 1.27 cm. This increase in intensity is greater than that expected from a medium exhibiting no gain. A comparison of the measured time history of the emission with that of non-local thermodynamic equilibrium time-dependent calculations suggests that a small-signal gain ∼0.5 cm-1 is obtained. [ABSTRACT FROM AUTHOR]

Experiments to obtain time-resolved, soft-x-ray emission from laser-driven plasmas are succinctly described. The spectra (0.19 kev ≤ hv ≤ 1.3 keV) at various times have been deconvolved and energy integrated to obtain time-resolved yields from Au disk targets. The temporal profiles of the total thermal x-ray output power follow the overall laser temporal shape but do not show the high-frequency fluctuation observed in the laser pulse. The behavior of the ratio of the instantaneous x-ray yields over the laser absorption is studied. The studies were done at the LLNL Nova laser facility. Single pulses and pulses in "picket fence" configuration were used to heat the gold targets. [ABSTRACT FROM AUTHOR]