C. Stenz

Portable, tunable, high-luminosity spherical crystal spectrometer with an x-ray charge coupled device, for high-resolution x-ray spectromicroscopy of clusters heated by femtosecond laser pulses

A portable (200×100×100 mm3), high-luminosity, spherically bent crystal spectrometer was designed for measuring in a wide spectral range of 1.2–19.6 A very low emissivity x-ray spectra of different clusters heated by 35 fs laser radiation. This spectrometer is associated with a custom design x-ray charge coupled device that features a large sensitive area (24.6×24.6 mm2) and a small pixel size (24×24 μm2). This apparatus provides simultaneous high spectral (λ/δλ∼1000–5000) and spatial (40–80 μm) resolution. A large (30×10 mm2) open aperture mica crystal with R=100 mm is used as the dispersive and focusing element. The large tuneability of the spectrometer makes it possible to record high-resolution spectra of H-like ions of oxygen (CO2 clusters) in a spectral range of 15–17 A, Ne-like like ions of Kr in a spectral range of 5–5.7 A, and He-like spectra of Ar in a spectral range of 3.0–3.4 and 3.7–4.4 A without any adjustment of the spectrometer setup. Thanks to the high luminosity (high collection efficien...

X-ray spectra of fast ions generated from clusters by ultrashort laser pulses

The high precision X-ray spectroscopy studies of plasma created from the CO 2 clusters in gas jet targets by the ultrashort laser pulses (35 and 60 fs duration) were performed at the intensities I L ∼ 10 17 –10 18 W cm −2 . The spectral line shape of the H-like and He-like oxygen ions gains an asymmetry with increasing the laser pulse intensity. Theoretical modeling of the line shape shows that the asymmetry can be explained by absorption of the Doppler-shifted line radiation from the essential fraction of ions (over 10 −3 ) with energies above 1 MeV due to photoionization of inner shells of carbon ions. The results obtained demonstrate measurement capabilities of the X-ray spectral measurements of multicharged ions accelerated during the interaction with a laser radiation.

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.

Studies of supersonic, radiative plasma jet interaction with gases at the Prague Asterix Laser System facility

The interaction of laser driven jets with gas puffs at various pressures is investigated experimentally and is analyzed by means of numerical tools. In the experiment, a combination of two complementary diagnostics allowed to characterize the main structures in the interaction zone. By changing the gas composition and its density, the plasma cooling time can be controlled and one can pass from a quasiadiabatic outflow to a strongly radiation cooling jet. This tuning yields hydrodynamic structures very similar to those seen in astrophysical objects; the bow shock propagating through the gas, the shocked materials, the contact discontinuity, and the Mach disk. From a dimensional analysis, a scaling is made between both systems and shows the study relevance for the jet velocity, the Mach number, the jet-gas density ratio, and the dissipative processes. The use of a two-dimensional radiation hydrodynamic code, confirms the previous analysis and provides detailed structure of the interaction zone and energy repartition between jet and surrounding gases.

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.

High resolution X-ray spectroscopy investigations of fs laser irradiated Ar clusters by varying cluster size and laser flux density

Abstract Atomic argon clusters were created using various types of nozzles and gas jet backing pressures. High temperature plasmas were created by irradiating the clusters with high intensity 35 fs laser pulses. The data obtained from these experiments were compared with each other as well as an experiment using a longer pulse ( 60 fs ) higher flux density laser. Detailed spectroscopic analysis of high resolution X-ray data near the Heα,Heβ, and Lyα lines of Ar was consistent with a two-temperature collisional-radiative model incorporating the effects of highly energetic electrons. Each variation has an effect on the rate of cluster expansion; therefore, results of the analysis provide useful insight into the behavior of such plasmas over short time scales. It was demonstrated for the first time, that by using sufficiently large clusters, hydrogen-like ions of Ar with an ionization potential of approximately 4120 eV could be obtained with a 35 fs laser pulse with a relatively low flux density of 1×10 17 W/cm 2 .

Interaction physics for the shock ignition scheme of inertial confinement fusion targets

This paper presents an analysis of laser?plasma interaction risks of the shock ignition (SI) scheme and experimental results under conditions relevant to the corona of a compressed target. Experiments are performed on the LIL facility at the 10?kJ level, on the LULI 2000 facility with two beams at the kJ level and on the LULI 6-beam facility with 100?J in each beam. Different aspects of the interaction of the SI pulse are studied exploiting either the flexibility of the LULI 6-beam facility to produce a very high intensity pulse or the high energy of the LIL to produce long and hot plasmas. A continuity is found allowing us to draw some conclusions regarding the coupling quality and efficiency of the SI spike pulse. It is shown that the propagation of the SI beams in the underdense plasma present in the corona of inertial confinement fusion targets could strongly modify the initial spot size of the beam through filamentation. Detailed experimental studies of the growth and saturation of backscattering instabilities in these plasmas indicate that significant levels of stimulated scattering reflectivities (larger than 40%) may be reached at least for some time during the SI pulse.

Experimental investigation of the stimulated Brillouin scattering growth and saturation at 526 and 351 nm for direct drive and shock ignition

We have designed experiments to study the effect of the laser wavelength (0.527 versus 0.351 μm) on the coupling efficiency in plasma conditions relevant to compression and shock ignition (SI) schemes in different intensity regimes. A difficult issue was to produce interaction conditions that are equivalent for the two wavelengths. This was obtained by using plasma preformed from a solid target with a plasma-preforming beam at the same wavelength as the interaction beam. This produced an almost exponential density profile from vacuum to the critical density of the interaction beam in which all interaction mechanisms are taken into account. The growth and saturation of stimulated Brillouin scattering (SBS) have been measured at the two wavelengths, in backward as well as in near-backward directions. We have found that the SBS intensity threshold is ∼1.5 times higher at 3ω than at 2ω in agreement with the Iλ dependence of the SBS gain. The SBS behaviour is very well reproduced by the linear calculations of ...

Laboratory modeling of supersonic radiative jets propagation in plasmas and their scaling to astrophysical conditions

Supersonic jets propagation over considerable distances and their interactions with surrounding media is one of the important subjects in astrophysics. Laboratory-created jets have completely different scales, however, typical velocities are the same, and the similarity criteria can be applied to scale them to astrophysical conditions. Moreover, by choosing appropriate pairs of colliding plasmas, one can fulfil the scaling conditions for the radiation emission rates. In this paper we present the results of studies of interaction of laser-created jets with gas-puff plasmas at the PALS laser facility. By varying the gas pressure and composition, the nature of the interaction zone changes from a quasi-adiabatic outflow to a strongly radiation cooling jet. The fine scale structures of the interaction zone are studied by means of optical and x-ray diagnostics, and they are interpreted with a semi-analytical model and 2D radiation hydrodynamic simulations. The conclusions from the laboratory experiment are scaled to the astrophysical conditions.