F. Dorchies

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.

Modeling Cluster Jets as Targets for High-Power Ultrashort Laser Pulses

A hydrodynamic model is formulated that describes the formation of clusters in atomic gas jets expanding into vacuum, which are used as laser plasma targets. Detailed model calculations performed for an argon gas jet describe spatial distributions of the density of gas and cluster phases formed in the Laval nozzle at room temperature in a broad range of entrance gas pressures. The cluster density distribution is significantly inhomogeneous. The cluster distribution features revealed by the model calculations were qualitatively confirmed by the X-ray spectroscopic measurements of the spatial distribution of emission from the plasma created in the jet tar-gets by high-power ultrashort laser pulses.

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.

Characterization of argon cluster jets for laser interaction studies

Abstract Atomic clusters can be produced from rare gas, by spontaneous condensation during its expansion and cooling in a supersonic nozzle. When irradiated by intense laser pulses, these near-solid density clusters were observed to absorb a large fraction of the laser energy, leading to the production of highly charged ions. Based on this interaction regime, several international research groups have already demonstrated the ability to produce intense and highly repetitive X-ray and neutron sources without debris, with a relatively small experimental setup. In order to understand the different mechanisms involved in laser – cluster interaction, a complete characterization is needed for the target composed by clusters and surrounded by gas. We present a method based on two different optical diagnostics (Mach–Zehnder interferometry and Rayleigh scattering) and supported by a numerical simulation describing the gas expansion and cluster growth in the nozzle. The study was performed considering argon expanding in two types of nozzles: a Laval and a conical one. The experimental spatial profiles were observed to be in good agreement with the simulations, leading to spatial resolution of gas density, cluster size and cluster density.

X-ray spectroscopy of a thin foil plasma produced by a short-pulse high-intensity laser

Abstract High density and temperature plasmas have been generated by irradiating thin foils of various elements with a high-energy subpicosecond laser pulse. The X-ray emission duration was studied by time-resolved X-ray spectroscopy. Frequency domain interferometry provided a measurement of the hydrodynamic expansion of the back of the foil as a function of time. The effect of longitudinal temperature gradients, i.e., gradients perpendicular to the surface, were decreased using very thin foil targets. Additionally, radial gradients effects, i.e., gradients parallel to the surface, were limited by using a 50 μm pinhole on target. The Al, Se and Sm spectra, recorded in the range 7.7– 8 A using a conical crystal spectrometer coupled to an 800 fs resolution streak camera, lasted a few picoseconds. Sm spectra showed no spectral features in this wavelength range, providing a spectrally homogeneous backlighter for future experiments. The main features of the experimental time-resolved spectra have been well reproduced with one-dimensional hydrodynamic simulations of the free expansion of a plasma heated at a given initial temperature obtained from the expansion velocity of the rear critical surface of the plasma.

Time-dependent study of K-shell satellite line structure from L-shell ions in ultra-short laser argon cluster experiments

Abstract The interaction of high-power, ultra-short, femtosecond laser pulses with matter, particularly solids and atomic gas clusters has been an area of extensive research in recent years. In particular, gas cluster targets, with their high densities and resistance to heat conduction, combine the advantages of both gas and solid targets. High temperature and high-density plasmas with satellite lines from multiply charged ions have been observed: 2p-1s emissions in Li-like through F-like ions appear in the experimental spectrum. The experimental satellite spectra show sensitivity to pulse duration, laser contrast, and cluster size. Since the laser energy is deposited in such a short timescale, a time-dependent model is being developed to study the plasma formation in such systems. In the present work, preliminary time-integrated spectra simulated from this model are presented assuming different initial conditions for the pre-plasma. The computational model also includes a provision for non-thermal electrons at energies substantially above the mean plasma temperature.

Observation of dielectronic satellites in the K-spectrum of argon ions in plasma produced by femtosecond laser pulses

The satellite structure of 1s2p1,3P1-1s21S0 lines of the He-like argon ion in plasma produced by a 45-fs laser pulse in a gas-jet cluster target is measured with a high spectral resolution. Radiation transitions 2p → 1s from autoionizing states (AISs) are detected for ions ranging from Li-like to F-like. The spectrum observed is theoretically simulated with the use of the spectroscopic data for the AISs of multicharged ions obtained within the multiconfiguration relativistic Hartree-Fock method. Good agreement with experimental data is obtained when the main population channels of these states are taken into account for typical values of cluster-target plasma parameters.

X‐ray spectromicroscopy of clusters heated by fs laser radiation

The review of systematic investigations of X‐ray radiation properties of different clusters heated by short‐pulse (35–1100 fs) high‐intensive (1016– 1018 W/cm2) Ti:Sa laser radiation is presented. The cluster targets were formed by the adiabatic expansion in vacuum of an Kr or Ar gas jets produced by a pulsed valve with Laval or conical nozzles. The gas pressure is varied from 15 up to 100 bar. High spectrally (λ/δλ=4000–5000) and spatially (40–80 μm) resolved X‐Ray spectra near resonance lines (4‐2 transitions) of Ne‐like ions of Kr, H‐ and He‐like ions of Ar have been obtained and detailed spectroscopic analysis was consistent with a theoretical two‐temperature collisional‐radiative model of irradiated atomic clusters incorporating with an effects of highly energetic electrons. The role of laser prepulse for X‐ray intensity emission investigated in details. X‐ray spectra radiation from plasma with electron density more than 2×1022 cm−3 was observed. Big effect of fast electrons influence on the X‐ray em...

Spectral Studies of Short Pulse Laser Irradiated Argon Clusters

The systematic experimental studies of plasma produced upon the interaction of ultra‐short laser pulses with cluster targets are carried out using the methods of x‐ray spectroscopy. The dependence of the plasma parameters on the initial properties of a cluster target such as the design of a supersonic nozzle, the average size of clusters, and their spatial distribution as well as the laser‐pulse properties of duration and contrast are studied. The plasma diagnostics is performed using the model of spectra formation that provides a good agreement with the experimental data.