Characterization of hot electrons generated by laser-plasma interaction at shock ignition intensities

In an experiment carried out at the Prague Asterix Laser System at laser intensities relevant to shock ignition conditions (I > 1016 W/cm2), the heating and transport of hot electrons were studied by using several complementary diagnostics, i.e., Kα time-resolved imaging, hard x-ray filtering (a bremsstrahlung cannon), and electron spectroscopy. Ablators with differing composition from low Z (parylene N) to high Z (nickel) were used in multilayer planar targets to produce plasmas with different coronal temperature and collisionality and modify the conditions of hot-electron generation. The variety of available diagnostics allowed full characterization of the population of hot electrons, retrieving their conversion efficiency, time generation and duration, temperature, and angular divergence. The obtained results are shown to be consistent with those from detailed simulations and similar inertial confinement fusion experiments. Based on the measured data, the advantages, reliability, and complementarity of the experimental diagnostics are discussed

Simulations of laser imprint reduction using underdense foams and its consequences on the hydrodynamic instability growth

The mechanisms of laser imprint reduction on a surface of a planar foil performed using an underdense foam are presented. The consequences on the Rayleigh–Taylor instability growth at the ablation front when the foil is accelerated are studied. The analysis is based on numerical simulations using a chain of codes: the electromagnetic paraxial code Parax provides the modifications of the intensity perturbation spectrum while the laser beam is crossing the foam. Two-dimensional axially symmetric simulations with the radiation hydrodynamic code CHIC describe the foam expansion and the foil dynamics. Finally, the perturbed flow calculations and the instability growth are investigated with the two-dimensional CHIC version in the planar geometry by using the initial and smoothed perturbation spectra. The dominant role of temporal laser smoothing during the time of foam crossing by the laser beam is demonstrated. Applications to the direct drive targets for inertial confinement fusion are discussed

Fast electron energy deposition in aluminium foils: Resistive

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 1019 Wcm-2 laser pulses interacting with Al foils of thicknesses 10-200 μm. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ≈407 nm (the second harmonic of the laser light) and ≈500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (Thtail ≈ 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (Thbulk=500 ± 100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2ω0 emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 μm. Beyond that depth, because of the beam divergence, the incident current goes under 1012 Acm-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 μm depth

Laser-driven collisionless shock acceleration of protons from gas jets tailored by one or two nanosecond beams

International audienceIt was proposed recently that laser-ion acceleration in gas jets may be significantly improved if each side of a gas jet target is tailored by an auxiliary nanosecond laser pulse [Marquès et al., Phys. Plasmas 28, 023103 (2021)]. In the present study, the proton acceleration by electrostatic shock in these one- or two-side tailored plasmas is investigated using particle-in-cell simulations. It is demonstrated that the formation of a thin plasma layer with a steep density profile and a maximum density of the order of the critical density strongly improves the proton acceleration in the forward direction with a maximum ion energy of tens of MeV with mildly relativistic laser pulses. Proton acceleration up to tens of MeV is predicted using realistic plasma density profiles obtained from tailored gas jet targets compared to a few MeV reported in other publications

Energetic α -particle sources produced through proton-boron reactions by high-energy high-intensity laser beams

In an experiment performed with a high-intensity and high-energy laser system, α-particle production in proton-boron reaction by using a laser-driven proton beam was measured. α particles were observed from the front and also from the rear side, even after a 2-mm-thick boron target. The data obtained in this experiment have been analyzed using a sequence of numerical simulations. The simulations clarify the mechanisms of α-particle production and transport through the boron targets. α-particle energies observed in the experiment and in the simulation reach 10–20 MeV through energy transfer from 20–30 MeV energy incident protons. Despite the lower cross sections for protons with energy above the sub-MeV resonances in the proton-boron reactions, 108–109α particles per steradian have been detected

Gas-induced smoothing of laser beams studied by interaction with thin foils

International audienceWe studied the process of laser beam homogenization in a gas medium placed in front of a thin metallic foil. Experiments were performed using the Prague PALS iodine laser working at 0.44 µm wavelength and irradiances of about 10^15 W cm^−2. Homogenization was detected both by directly analysing the transmitted laser beam and by studying the shock breakout on the foil rear side. Results show that the gas ionization by the laser pulse induces a strong refraction and produces an effective smoothing of large-scale intensity non-uniformities

Direct evidence of gas-induced laser beam smoothing in the interaction with thin foils

The process of laser beam homogenization in a gas medium placed in front of a thin metallic foil has been studied. Experiments were performed using the Prague Asterix Laser System iodine laser [ Jungwirth et al., Phys. Plasmas 8, 2495 (2001) ] working at 0.44 μm wavelength and irradiance of about 10^15 W/cm^2. Homogenization was detected both by directly analyzing the transmitted laser beam and by studying the shock breakout on the foil rear side. Results show that the gas ionization by the laser pulse induces a strong refraction and produces an effective smoothing of large-scale intensity nonuniformities