Search

We report results and modelling of an experiment performed at the Target Area West Vulcan laser facility, aimed at investigating laser–plasma interaction in conditions that are of interest for the shock ignition scheme in inertial confinement fusion (ICF), that is, laser intensity higher than ${10}^{16}$   $\mathrm{W}/{\mathrm{cm}}^2$ impinging on a hot ($T>1$ keV), inhomogeneous and long scalelength pre-formed plasma. Measurements show a significant stimulated Raman scattering (SRS) backscattering ($\sim 4\%{-}20\%$ of laser energy) driven at low plasma densities and no signatures of two-plasmon decay (TPD)/SRS driven at the quarter critical density region. Results are satisfactorily reproduced by an analytical model accounting for the convective SRS growth in independent laser speckles, in conditions where the reflectivity is dominated by the contribution from the most intense speckles, where SRS becomes saturated. Analytical and kinetic simulations well reproduce the onset of SRS at low plasma densities in a regime strongly affected by non-linear Landau damping and by filamentation of the most intense laser speckles. The absence of TPD/SRS at higher densities is explained by pump depletion and plasma smoothing driven by filamentation. The prevalence of laser coupling in the low-density profile justifies the low temperature measured for hot electrons ($7\!{-}\!12$ keV), which is well reproduced by numerical simulations. [ABSTRACT FROM AUTHOR]

The design and the early commissioning of the ELI-Beamlines laser facility's 30 J, 30 fs, 10 Hz HAPLS (High-repetition-rate Advanced Petawatt Laser System) beam transport (BT) system to the P3 target chamber are described in detail. It is the world's first and with 54 m length, the longest distance high average power petawatt (PW) BT system ever built. It connects the HAPLS pulse compressor via the injector periscope with the 4.5 m diameter P3 target chamber of the plasma physics group in hall E3. It is the largest target chamber of the facility and was connected first to the BT system. The major engineering challenges are the required high vibration stability mirror support structures, the high pointing stability optomechanics as well as the required levels for chemical and particle cleanliness of the vacuum vessels to preserve the high laser damage threshold of the dielectrically coated high-power mirrors. A first commissioning experiment at low pulse energy shows the full functionality of the BT system to P3 and the novel experimental infrastructure. [ABSTRACT FROM AUTHOR]

The hot electron role in shock generation and energy deposition to the hot dense core is crucial for the shock ignition scheme, implying the need for their characterization at laser intensities of interest for shock ignition. In this paper, we analyze the experimental results obtained at the PALS laboratory and provide an estimation of hot electron temperature and conversion efficiency using a semi-analytical approach, including Harrach-Kidder's model. [ABSTRACT FROM AUTHOR]

Abstract: High-energy decay channels of the Al Lyman-β satellite have been observed in X-ray emission from highly ionized plasma jets created by intense laser irradiation of aluminium foil targets. Atomic structure calculations show that the Lyman β satellite emission consists from six emission groups close to the He-like Al 1s2-1s4p (Heγ) and 1s2-1s5p (Heδ) resonance lines. This provides new possibilities for space resolved analysis of high density plasmas. Non-Maxwellian simulations of the plasma emission carried out with the MARIA code demonstrate that the intensity ratios of the Lyman-β satellites and the Heγ and Heδ resonance lines are very sensitive to the bulk electron temperature. In contrast to standard diagnostic methods, parameter studies show that this bulk electron diagnostics is practically unaffected by suprathermal electrons having less than 10% of the bulk electron density. [Copyright &y& Elsevier]

Abstract: Theory predicts that the presence of strong single-frequency electric fields results in appearance of satellite or dip structures in X-ray spectral lines emitted from hot dense plasmas. Emission from multicharged ions is measured to determine the effects of laser field. A ps-laser beam was split into two parts: the first created an expanding plasma, while the second, which was temporally synchronized, irradiated the plasma at a varying distances in a direction perpendicular to the target normal. The field introduced by the second beam perturbed the plasma environment in the vicinity of radiators. The spatially resolved X-ray spectra were recorded using the high-resolution toroidally bent crystal spectrometer combined with a CCD detector. Spectrally resolved features are observed in broadened Al Heβ line profiles that are consistent with predicted spectra. The predicted spectra are derived from a combination of hydrodynamic plasma modeling post-processed by theoretical models that include the effect of externally introduced laser fields. The possible mixing of higher-intensity fields is qualitatively explained by a combination of fluid and one-dimensional PIC simulations that indicate redistribution of the fields and density fluctuations due to a presence of parametric instabilities. [Copyright &y& Elsevier]

Detailed measurements of the angular distribution of 3 ω0/2 radiation are presented in short scale length plasmas (0.8–7 μm) generated by laser radiation at intensities reaching the relativistic level (1016–6×1018 W/cm2). The experimental results are in very good agreement with theoretical predictions based on two-plasmon decay and stimulated Raman scattering instabilities. New three-halves harmonic generation mechanisms are an identified characteristic of femtosecond laser induced parametric instabilities. These are the joint interaction of incident and reflected laser beams as well as stimulated Raman scattering. It is shown both experimentally and theoretically that the three-halves harmonic radiation is a useful preplasma diagnostic tool. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Prague Asterix Laser System (PALS) is a new international laboratory where research teams are invited to compete for the beam time. The PALS Center runs an iodine photodissociation high-power laser system delivering up to 1.2 kJ of energy in ∼400 ps pulses at the wavelength of 1.315/ μm. Optional doubling and tripling of the frequency is assured by large-diameter nonlinear crystals. The ASTERIX IV laser [H. Baumhacker et al., Appl. Phys. B 61,325 (1995)], transferred from Garching into a new laser hall in Prague, was updated and put into operation on 8 June 2000. These upgrades include new beam delivery options and a twin interaction chamber, which is designed flexibly for a broad spectrum of applications. Results of the first series of experiments are presented and some planned upgrades are briefly described. These include implementation of adaptive optics, replacement of the iodine master oscillator by a more flexible solid state oscillator based on fiber optics, and a femtosecond extension of the laser output to reach the petawatt pulse power region. [ABSTRACT FROM AUTHOR]

Spatially resolved x-ray spectra of the Al Lyman α group emitted from the surface of the laser- irradiated targets display plasma regions where the resonance line is suppressed and the dielectronic satellites dominate. Calculations based on multilevel collisional-radiative code and two-dimensional hydrodynamic simulations relate this spectral distribution to a presence of the cold overcritical density plasma inside the crater at the target surface. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Strong oscillating electric fields modify the emission spectra of ions and can result in the appearance of additional field-induced satellites, field ionisation, and alter spectral line shapes. In addition, these high fields may alter the atomic kinetics. Experiments observing these strong oscillating electric fields in highly ionised systems are limited. Here we describe experiments using a combination of long and short pulse lasers to look for field-induced satellites, and spectral line shape modification in uniform well-characterised plasmas. A hydrogen-like and helium-like aluminium plasma was preformed using a long-duration (∼1ns), moderate intensity (∼1014 W/cm2) laser pulse. A second short-duration (10ps), high-intensity (∼5×1016 W/cm) laser pulse was propagated transversally through this plasma to locally modify the oscillating electric field. The X-ray spectra were recorded using high-dispersion, high-luminosity spectrometers providing spatial resolution. Current analysis indicates that the predicted strong field effects have not been identified. [Copyright &y& Elsevier]