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Content archived on 2022-12-23

Non-linear and quantum optics of super-strong laser fields in plasma

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The research was focused on the generation of super-intense laser intensities(up to 1020 W/cm2 ) and the study of the interaction with matter of the associated super-strong high electric fields. The three teams involved in the project have worked together on an investigation of absorption and scattering mechanisms of subpicosecond laser radiation in solid targets at high laser intensities. Laser-produced X-ray sources on solid targets have also been investigated. The co-ordinator team has performed experiments using the laser facility \available at the Laboratoire d'Optique Appliquée. The hot electron pulse emitted by an ultra short (100 fs) and bright (>10 16W/cm2) laser pulse on multi-layered solid targets has been characterised. The resulting Kα yield is 2.5x108photons/steradian/pulse for aluminium at 1.5 keV, The measured hot electron temperature agrees with the one obtained from the theory of resonance absorption in short density scale length plasmas and with particle in cells imulations of vacuum heating provided by the German team. These measurements have been compared with the brilliance of various modem X-ray sources. At a given brilliance, in the energy range above the keV, short pulse X-ray sources from laser plasmas have a duration very much shorter than synchrotron radiation. This advantage could be critical in molecular reactivity studies where a visible, ferntosecond photon, excites (pump) a molecular complex and anX-ray photon (probe) visualises structural and conformation changes of the molecule at different (short) times after the perturbation. The Russian team has developed a short pulse laser using original techniques. For the generation of 100 fs laser pulses in the master oscillator, the following directions of work have been considered: i) cutting a slice in the 1ps pulse of the master oscillator of the phosphate Nd: glass with the help of optical breakdown or with an electro-optical deflector, and ii) formation of100 Is pulses in the master oscillator with a Ti:Al2O3crystal, pumped by the second harmonic of the available phosphateNd: glass laser with passive mode-locking and negative feedback. To increase the frequency band of the Nd: glass systeni amplification, special spectral filters and also simultaneous application of phosphate and silicate Nd:glasses have been studied. On the basis of the available equipment, results of 200 fs laser pulses with a 1 J level of energy have been obtained. Experiments and calculations oil X-ray production from Al and Mg targets with 10 17W/cm2 laser intensity were carried out. Experiments and calculations of second harmonic generation from picosecond laser plasma were reflected radiation on the level of some % from picosecond laser plasma was measured. quasi-linear theory of high intensity supershort laser pulse interaction with strongly inhomogeneous plasma at oblique incidence has been implemented. Non-linear theory of relativistic intensity short laser pulse interaction with overdense plasma at normal incidence was developed. The codes SKINM and KINETM for simulations of laser pulse dense plasma interaction in hydrodynamical and kinetic approximations have been written. Comparisons between the code SKINM and the French code FILM and also KINETM and the Jena PIC code gave a good agreement. Non-local and non-linear theories of ultra-high intensity short laser pulses interaction in vacuum were also developed. The fluid code FILM developed at LULI has been used to compare absorption and soft X-ray yield with earlier experiments by the Jena team at fairly modest intensities (<10 17 Wcm 2 ) using a KrF laser. The Jena PIC code has been used and improved extensively in this research programme. Particular developments include: i) improved accounting of fast electrons and ions, giving a more accurate hot electron spectrum, ii) more flexibility,, in boundary conditions including foil targets and regions of underdense pre-plasma, iii) inclusion of sophisticated Fourier analysis of reflected light to predict harmonic and sub-harmonic spectra.

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