Abstract
Accurate modelling of ponderomotive laser self-focusing may represent a key for the success of inertial confinement fusion, especially within the shock ignition approach. From a numerical ...point of view, implementation of a paraxial complex geometrical optics (PCGO) method has improved the performance of the hydrodynamics code CHIC, but (1) overestimating ponderomotive speckle self-focusing in reduced two-dimensional geometry, and (2) not accounting for speckle intensity statistics. The first issue was addressed in our previous work (Ruocco
et al
2019
Plasma Phys. Control. Fusion
61
115009). Based on those results, here we propose a novel PCGO scheme for modelling spatially modulated laser beams by (1) creating Gaussian speckles, and (2) emulating the realistic speckle intensity statistics. Self-focusing of spatially modulated beams in a homogeneous stationary plasma with this method is studied. This investigation evidences that plasma smoothing does not reduce the speckle intensity enhancement at long time scales when the average beam intensity is twice above the speckle critical intensity. Comparison against electromagnetic simulations confirms that this approach improves the description of self-focusing of high-intensity speckles within the PCGO model.
A model based on optical Bloch equations is developed to describe the interaction of femtosecond laser pulses with dielectric solids, accounting for optical-cycle-resolved electron dynamics. It ...includes the main physical processes at play: photoionization, impact ionization, direct and collisional laser heating, and recombination. By using an electron band structure, this approach also accounts for material optical properties as nonlinear polarization response. Various studies are performed, shedding light on the contribution of various processes to the full electron dynamics depending on laser intensity and wavelength. In particular, the standard influence of the impact ionization process is retrieved.
Today, glass and other similar dielectric materials are widely used in modern manufacturing. However, glass is a brittle and a heat sensitive material. Laser technology is used to process glass but ...quality and throughput are still a key issue. In the present paper, we investigate dual-wavelength double ultrashort laser pulses in order to control free electrons dynamics and subsequent ablation for fused silica processing, and further improve the understanding of this laser-material interaction. We used a high average power Yb-doped femtosecond laser source (100 W) with two optical lines exhibiting different pulse durations and wavelengths (500 fs at 515 nm; and 1 or 10 ps at 1030 nm) with various fluences and delays. The best configuration in terms of ablation efficiency is expected to take place when the green pulse first induces free electrons, followed by their heating by the red pulse. The obtained results are discussed in terms of optical transmission as well as ablated volume, and are compared with single pulse ablation. Our experimental results are supported by absorbed energy density calculations based on a model considering the two-color laser induced electron dynamics, including photoionization, laser heating of free electrons, and their recombination. We demonstrate that there is an optimal cooperating effect between the two sub-pulses for a 1-ps delay, nevertheless there is no beneficial effect in splitting the beam for optimizing fused silica ablation compared with the single-pulse green configuration.
Femtosecond Bessel–Gauss beams are attractive tools to a large area of laser processes including high aspect ratio volume nanostructuration in dielectric materials. Understanding the dielectric ...material response to femtosecond Bessel–Gauss beam irradiation is key in controlling its modifications and designing new structures. In this work, we show how the material ionization affects the propagation of the femtosecond Bessel–Gauss laser beam and can limit the laser energy deposition. By performing 2D/3D numerical simulations, we evaluate the absorbed laser energy and subsequent material modifications. First, we model the electron dynamics in the material coupled to the 3D laser propagation effects. Then, we consider 2D thermo-elasto-plastic simulations to characterize the medium modifications. Results show that the laser ionized matter induces a screening of the incident gaussian beams which form the Bessel-Gauss beam. This effect leads to a limitation of the maximum laser energy deposition even if the incident laser energy increases. It can be reduced if a tigthly focused femtosecond Bessel–Gauss beam is used as the angular aperture of the cone along which the incident gaussian beams are distributed is larger.
The purpose of this study was to assess the changes in duodenal composition in three nutritional states: fasted, fed, and fat-enriched fed state. Two isocaloric meals were administered to healthy ...subjects on nonconsecutive days. Subsequently, duodenal samples were collected every 30min after which they were characterized with respect to pH, lipolytic products, bile salts, phospholipids, osmolality, and surface tension. The resulting time profiles displayed fluctuating patterns, which reflect high inter- and intrasubject variability. Duodenal composition was not altered by the higher fat percentage of the fat-enriched liquid meal. Monoglycerides, amounting from 5% to 88% of total lipids, were the dominant lipolytic species, followed by free fatty acids. Within 30min after meal administration, individual intraduodenal concentrations of lipid products were 0.0–5.5, 1.0–14.9, and 3.1–22.4mg/mL in fasted, fed, and fat-enriched fed state, respectively. The corresponding values for bile salts were 2.0–9.0, 6.9–9.3, and 4.4–30.3mM and for phospholipids 0.06–2.4, 2.6–5.7, and 1.4–9.3mM, respectively. Specific trends though, were not detected. This study illustrates the variable intraluminal conditions that can result after food intake. As intraduodenal events (e.g., intraduodenal dissolution) affect absorption of poorly water soluble and/or highly lipophilic drugs, this variability may possibly contribute to the highly variable drug plasma-time profiles often observed.
Experiments have been performed evidencing significant stimulated Raman sidescattering (SRS) at large angles from the density gradient. This was achieved in long scale-length high-temperature plasmas ...in which two beams couple to the same scattered electromagnetic wave further demonstrating for the first time this multiple-beam collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. These findings obtained in plasmas conditions relevant of inertial confinement fusion experiments similarly apply to the more complex geometry of these experiments where anomalously large levels of SRS were measured.
The electron dynamics in dielectric materials induced by intense femtosecond laser pulses is theoretically addressed. The laser driven temporal evolution of the energy distribution of electrons in ...the conduction band is described by a kinetic Boltzmann equation. In addition to the collisional processes for energy transfer such as electron-phonon-photon and electron-electron interactions, a non-collisional process for photon absorption in the conduction band is included. It relies on direct transitions between sub-bands of the conduction band through multiphoton absorption. This mechanism is shown to significantly contribute to the laser heating of conduction electrons for large enough laser intensities. It also increases the time required for the electron distribution to reach the equilibrium state as described by the Fermi-Dirac statistics. Quantitative results are provided for quartz irradiated by a femtosecond laser pulse with a wavelength of 800 nm and for intensities in the range of tens of TW cm−2, lower than the ablation threshold. The change in the energy deposition induced by this non-collisional heating process is expected to have a significant influence on the laser processing of dielectric materials.
Objective: To test the dose-response effect on low-density lipoprotein cholesterol (LDL-c) of plant sterols (PS) from different sources in a low-fat spread. Methods: Dose responses of soybean oil ...(BO), tall oil (TO) and a mix of tall oil and rapeseed oil (TO/RP) as fatty acid esters were tested in a parallel design in free-living subjects recruited from the general community who had elevated cholesterol concentrations. Subjects received either control for 6 weeks or 1.6 g PS per day for 3 weeks, then 3.0 g/day for 3 weeks.Results: LDL-c was lowered significantly by consumption of 1.6 g/day of PS (-10.4%, range -7.3 to -11.4%). Increasing the dose to 3.0 g/day modestly reduced LDL-c concentrations further to -14.7%. TO, containing 78% sitosterol, produced an increase in serum sitosterol of 6.5 nmol/ml, while BO, containing only 27% campesterol, produced an increase in serum campesterol of 9.5 nmol/ml in 6 weeks. After PS withdrawal, serum sterols declined by 50% within 2 weeks. Conclusion: Different PS sources were equally effective in lowering serum LDL-c concentrations. The decrease in absolute concentrations of LDL-c was dependent on the baseline concentrations.
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