P. Monot

Double plasma mirror for ultrahigh temporal contrast ultraintense laser pulses

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.

Prepulse effect on intense femtosecond laser pulse propagation in gas

The propagation of an ultrashort laser pulse can be affected by the light reaching the medium before the pulse. This can cause a serious drawback to possible applications. The propagation in He of an intense 60-fs pulse delivered by a Ti:sapphire laser in the chirped pulse amplification (CPA) mode has been investigated in conditions of interest for laser-plasma acceleration of electrons. The effects of both nanosecond amplified spontaneous emission and picosecond pedestals have been clearly identified. There is evidence that such effects are basically of refractive nature and that they are not detrimental for the propagation of a CPA pulse focused to moderately relativistic intensity. The observations are fully consistent with numerical simulations and can contribute to the search of a stable regime for laser acceleration.

Multiply charged ions produced in noble gases by a 1 ps laser pulse at lambda =1053 nm

Multiply charged ion production in He, Ne, Ar and Xe has been investigated using a 1 ps Nd-glass laser pulse at lambda =1053 nm with intensities in the 1013-1018 W cm-2 range. All the outer-shell electrons are removed except in Ne gas in which ion charge states up to Ne7+ are observed. The influence of the polarization of the laser radiation is also investigated. Experimental ion production rates are compared with those predicted by tunnelling and barrier-suppression ionization models.

X-ray spectroscopy diagnostic of a plasma produced by femtosecond laser pulses irradiating a cluster target

The parameters of a plasma produced upon the interaction of ultrashort laser pulses with cluster targets are measured by the methods of X-ray spectroscopy. The dependence of the plasma parameters on the initial properties of a cluster target (the design of a supersonic nozzle, the average size of clusters, the spatial inhomogeneity) and the laser pulse properties (its duration and contrast) is studied. The plasma diagnostics is performed using the model of formation of emission spectra, which was proposed earlier and includes a number of fitting parameters, which provide good agreement with experimental spectra. The systematic experimental studies performed by us showed that our model of cluster heating by ultrashort pulses is indeed a physical model, and the fitting parameters represent the average values of plasma parameters in the corresponding space-time regions.

On the Interaction of Femtosecond Laser Pulses with Cluster Targets

The heating of clusters by femtosecond laser pulses is studied theoretically and experimentally. Both the formation of a cluster target and the results of experimental studies of the cluster plasma by the methods of X-ray emission spectroscopy are considered. A numerical model of cluster formation in a supersonic gas jet is proposed. It is shown that detailed studies of two-phase gas-dynamic processes in a nozzle forming the jet give the spatial distributions of all parameters required for the correct calculation of the cluster heating by short laser pulses. Calculations of nozzles of different configurations show that in a number of cases an almost homogeneous cluster target can be formed, whereas in other cases the distributions of parameters prove to be not only inhomogeneous but also even nonmonotonic. A simple physical model of the plasma production by a femtosecond laser pulse and a picosecond prepulse is proposed. It is shown that a comparison of X-ray spectra with detailed calculations of the ion kinetics makes it possible to determine the main parameters of the plasma being produced.

The generation of fast particles in plasmas created by laser pulses with different wavelengths

By means of spatially resolved high-resolution X-ray spectroscopy, we have investigated the generation of fast ions at various laser installations with different flux densities and laser wavelengths. It is demonstrated that the fast ion generation in laser-produced plasma can be achieved for a very low level of the averaged laser intensity on the target. The time-of-flight mass spectrometry ion diagnostics and X-ray spectrographs give very close results for the energy distribution of the thermal ion component. For higher energies, however, we found significant differences: the spatially resolved high-resolution spectrographs expose the presence of suprathermal ions, while the time-of-flight method does not. Suprathermal ion energies Eion plotted as a function of the qλ2 parameter show a large scatter far above the experimental errors. The cause of these large scatters is attributed to a strong nonuniformity of the laser intensity distribution in the focal spot. The analysis by means of hydrodynamics and spectral simulations show that the X-ray emission spectrum is a complex convolution from different parts of the plasma with strongly different electron density and temperature. It is shown that the highly resolved Li-like satellite spectrum near Heαcontains significant distortions even for very low hot electron fractions. Non-Maxwellian spectroscopy allows determination of both the hot electron fraction and the bulk electron temperature.

X-ray Spectroscopic Observations of a Superdense Plasma in Nanoparticles Irradiated by Superintense Femtosecond Laser Radiation

The interaction of femtosecond laser pulses with SiO2 aerogel targets has been analyzed by x-ray spectroscopic methods. The use of an aerogel target with transparent grains makes it possible to considerably reduce the requirements on laser-pulse contrasts for which heating occurs without the formation of a preplasma. A nanoplasma with a density sevenfold higher than the solid-state density has been detected.

Space- and time-resolved observation of extreme laser frequency upshifting during ultrafast-ionization

A 65-fs, 800-nm, 2-TW laser pulse propagating through a nitrogen gas jet has been experimentally studied by 90° Thomson scattering. Time-integrated spectra of scattered light show unprecedented broadening towards the blue which exceeds 300 nm. Images of the scattering region provide for the first time a space- and time-resolved description of the process leading quite regularly to such a large upshift. The mean shifting rate was as high as δλ/δt ≈ 3 A/fs, never observed before. Interferometry shows that it occurs after partial laser defocusing. Numerical simulations prove that such an upshift is consistent with a laser-gas late interaction, when laser intensity has decreased well below relativistic values (a0 ≪ 1) and ionization process involves most of the laser pulse. This kind of interaction makes spectral tuning of ultrashort intense laser pulses possible in a large spectral range.

High-order harmonic generation using plasma mirrors

We discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, which can be clearly identified in numerical simulations: the coherent wake emission (CWE) and the relativistic oscillating mirror (ROM) processes. Using high-contrast multi-terawatt ultrashort laser pulses, harmonics originating from these two processes can also be distinguished experimentally.

High-sensitivity, portable, tunable imaging X-ray spectrometer based on a spherical crystal and MCP

A portable (200 � 100 � 100 mm 3 ), high-luminosity, spherically bent crystal spectrometer was designed to measure very low emissivity X-ray spectra of different elements with spatial resolution in a wide spectral range (1.2� 19.6 ( A). A large (50 � 15 mm 2 ) open aperture mica spherically bent crystal with R ¼ 150 mm was used as dispersive and focusing element. This spectrometer was associated with a large sensitive area (f ¼ 40 mm) micro-channel plates assembly. This apparatus provides simultaneously high spectral (l=dlB1800) and spatial (100–200mm) resolutions. Its large tunability allowed, without any adjustment of the spectrometer set-up, to record spectra in the 1.38–17.5 ( A wavelength range. We used the X-ray emission of femtosecond laser-produced plasmas from different materials ((CF2)n, CaF2, Cu, Al) to test the spectrometer. Thanks to the high sensitivity (high collection efficiency) of the system, high quality space-resolved X-ray spectra of Fluorine and Aluminum plasmas were obtained on a single laser shot. r 2001 Elsevier Science B.V. All rights reserved.