T. Caillaud

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.

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.

Influence of ultrashort laser pulse duration on the X-ray emission spectrum of plasma produced in cluster target

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the n1P1−11S0 (n=5–9) transitions of the helium-like Ar XVII ion for a pulse duration varying from 45 fs to 1.1 ps and a constant fluence of ∼105 J/cm2. The independent modeling of the relative intensities of the transitions from the n=5,..., 10 levels, as well as of the 21P1− 12S0 and 23P1−12S0 lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.

Study of intense femtosecond laser propagation into a dense Ar gas and cluster jet

The propagation of an intense (up to 1017W∕cm2) and short laser pulse (down to 40fs) is studied through a well characterized high density Ar cluster jet obtained at the output of a supersonic nozzle. The x-ray emission from the irradiated clusters is measured as a function of the focusing depth inside the jet, with a spatial resolution of the emitting plasma. A strong refraction of the laser pulse is observed, limiting the interaction at the entrance of the jet and decreasing the effective laser intensity on clusters. Calculations indicate that it is due to the ionization of the residual gaseous phase present in the cluster jet. As the focal volume is modified, this effect should be considered for any quantitative analysis of the laser-cluster interaction.

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.