Theory and measurements of convective Raman side scatter in inertial confinement fusion experiments.

Raman side scatter, whereby scattered light is resonant while propagating perpendicularly to a density gradient in a plasma, was identified experimentally in planar-target experiments at the National Ignition Facility at intensities orders of magnitudes below the threshold for absolute instability. We have derived a new theoretical description of convective Raman side scatter below the absolute threshold, validated by numerical simulations. We show that inertial confinement fusion experiments at full ignition scale, i.e., with mm-scale spot sizes and density scale lengths, are prone to increased coupling losses from Raman side scatter as the instability can extend from the absolute regime near the quarter-critical density to the convective regime at lower electron densities.

Suppressing Two-Plasmon Decay with Laser Frequency Detuning.

Three-dimensional laser-plasma interaction simulations show that laser frequency detuning by an amount achievable with current laser technology can be used to suppress the two-plasmon decay (TPD) instability and the corresponding hot-electron generation. For the plasma conditions and laser configuration in a direct-drive inertial confinement fusion implosion on the OMEGA laser, the simulations show that ∼0.7% laser frequency detuning is sufficient to eliminate TPD-driven hot-electron generation in current experiments. This allows for higher ablation pressures in future implosion designs by using higher laser intensities.

Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments.

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length (∼500 to 700 µm), electron temperature (∼3 to 5 keV), and laser intensity (6 to 16×10^{14} W/cm^{2}) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is ∼0.7% to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from ∼4×10^{14} to ∼6×10^{14} W/cm^{2}. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

Multiple beam two-plasmon decay: linear threshold to nonlinear saturation in three dimensions.

The linear stability of multiple coherent laser beams with respect to two-plasmon-decay instability in an inhomogeneous plasma in three dimensions has been determined. Cooperation between beams leads to absolute instability of long-wavelength decays, while shorter-wavelength shared waves are shown to saturate convectively. The multibeam, in its absolutely unstable form, has the lowest threshold for most cases considered. Nonlinear calculations using a three-dimensional extended Zakharov model show that Langmuir turbulence created by the absolute instability modifies the convective saturation of the shorter-wavelength modes, which are seen to dominate at late times.

Nonuniformly driven two-plasmon-decay instability in direct-drive implosions.

Half-harmonic emission spectra and images taken during directly driven implosions show that the two-plasmon decay (TPD) instability is driven nonuniformly over the target surface and that multibeam effects dominate this instability. The images show a spatially limited extent of the TPD instability. A prominent spectral feature is used to determine the electron temperature in the corona. Near threshold the temperatures agree with one-dimensional hydrodynamic predictions but exceed them by ∼10% above the TPD threshold. Two-dimensional hydrodynamic simulations indicate that a significant part (∼20%) of the laser intensity must be locally absorbed by the TPD instability (i.e., by collisional damping of the electron plasma waves) to maintain these temperature islands.

Experimental validation of the two-plasmon-decay common-wave process.

The energy in hot electrons produced by the two plasmon decay instability, in planar targets, is measured to be the same when driven by one or two laser beams and significantly reduced with four for a constant overlapped intensity on the OMEGA EP. This is caused by multiple beams sharing the same common electron-plasma wave. A model, consistent with the experimental results, predicts that multiple laser beams can only drive a resonant common two plasmon decay 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.

Saturation of the two-plasmon decay instability in long-scale-length plasmas relevant to direct-drive inertial confinement fusion.

Measurements of the hot-electron generation by the two-plasmon-decay instability are made in plasmas relevant to direct-drive inertial confinement fusion. Density-scale lengths of 400 µm at n(cr)/4 in planar CH targets allowed the two-plasmon-decay instability to be driven to saturation for vacuum intensities above ~3.5×10(14) W cm(-2). In the saturated regime, ~1% of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to 90 keV as the laser beam intensity is increased from 2 to 7×10(14) W cm(-2). This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.

Optimizing electron-positron pair production on kilojoule-class high-intensity lasers for the purpose of pair-plasma creation.

Expressions for the yield of electron-positron pairs, their energy spectra, and production rates have been obtained in the interaction of multi-kJ pulses of high-intensity laser light interacting with solid targets. The Bethe-Heitler conversion of hard x-ray bremsstrahlung [D. A. Gryaznykh, Y. Z. Kandiev, and V. A. Lykov, JETP Lett. 67, 257 (1998); K. Nakashima and H. Takabe, Phys. Plasmas 9, 1505 (2002)] is shown to dominate over direct production (trident process) [E. P. Liang, S. C. Wilks, and M. Tabak, Phys. Rev. Lett. 81, 4887 (1998)]. The yields and production rates have been optimized as a function of incident laser intensity by the choice of target material and dimensions, indicating that up to 5 x 10 (11) pairs can be produced on the OMEGA EP laser system [L. J. Waxer, Opt. Photonics News 16, 30 (2005)]. The corresponding production rates are high enough to make possible the creation of a pair plasma.

High-areal-density fuel assembly in direct-drive cryogenic implosions.

The first observation of ignition-relevant areal-density deuterium from implosions of capsules with cryogenic fuel layers at ignition-relevant adiabats is reported. The experiments were performed on the 60-beam, 30-kJUV OMEGA Laser System [T. R. Boehly, Opt. Commun. 133, 495 (1997)10.1016/S0030-4018(96)00325-2]. Neutron-averaged areal densities of 202+/-7 mg/cm2 and 182+/-7 mg/cm2 (corresponding to estimated peak fuel densities in excess of 100 g/cm3) were inferred using an 18-kJ direct-drive pulse designed to put the converging fuel on an adiabat of 2.5. These areal densities are in good agreement with the predictions of hydrodynamic simulations indicating that the fuel adiabat can be accurately controlled under ignition-relevant conditions.

Effects of moisturization on epidermal homeostasis and differentiation.

Moisturizers are commonly used for routine skin care. This study assessed the effects of a moisturizer on barrier function, epidermal architecture, keratinocyte proliferation, and physiological regulation of the epidermis in photoaged but otherwise normal skin. Fifteen women with moderately photoaged forearms were treated twice a day for 4 weeks with a moisturizer containing dimethicone and glycerine. Baseline and post-treatment transepidermal water loss (TEWL) and ipsilateral forearm biopsies were obtained. Epidermal thickness, melanin levels, keratinocyte proliferation, and expression of keratins were evaluated. Induction of keratins 6 and 16, commonly associated with keratinocyte proliferation and wound healing, was observed. Epidermal thickness increased by 0.019 mm (P = 0.005), barrier function improved (TEWL decreased by 13%) and melanin intensity decreased (P = 0.004). Even nonxerotic, photoaged skin may appear younger, benefiting structurally and functionally from routine use of moisturizers containing dimethicone and glycerine.

Time-dependent electron thermal flux inhibition in direct-drive laser implosions.

We simulate direct-drive CH target implosions with square laser pulses by a one-dimensional Fokker-Planck solver combined with a hydrodynamic code, and compare the results with those simulated by the flux-limited Spitzer-Härm model. We find that the electron thermal flux inhibition is time dependent, resulting in longer density scale length, larger laser absorption, and smaller growth of Rayleigh-Taylor instability. The time of peak neutron production calculated from Fokker-Planck simulations agrees with experiments for both 1-ns and 400-ps pulses.

Multibeam effects on fast-electron generation from two-plasmon-decay instability.

Experiments with multiple laser beams have been carried out in both spherical and planar geometry to study two-plasmon-decay instability, the predominant source of suprathermal electrons in direct-drive inertial confinement fusion experiments. These electrons are observed using the hard x rays generated through electron-target interactions. The experiments show for the first time that the total overlapped intensity governs the scaling of the suprathermal-electron generation regardless of the number of overlapped beams, in contrast to conventional theories that are based on the single-beam approximation.

Multibeam stimulated brillouin scattering from hot, solid-target plasmas.

We report on multibeam, laser-plasma interaction experiments in plasmas relevant to future direct-drive-ignition experiments. Six interaction beams are incident on preformed plasmas containing critical density. Stimulated Brillouin scattering (SBS) shows strong evidence of electromagnetic wave seeding of side- and backscattering. The data are also consistent with shared ion waves driven by the six symmetrically arranged interaction beams. Early SBS quenching is observed and attributed to the hydrodynamics of the plasma.