Study of the energy release region of a heavy-ion flux in nanomaterials by X-ray spectroscopy of multicharged ions

X-ray spectroscopic data are reported on the state of a silicon aerogel medium (SiO₂) irradiated by a Ni ion flux with an energy of 4.7 MeV/nucleon. Radiation from the electron transitions to the K shell in multicharged Si ions is detected with spatial resolution along the direction of the beam propagation in the medium. Calculations performed for the ionization state and population of the levels excited in the silicon atoms of the target provide the estimate T_e = 70–120 eV for the medium electron temperature in the ion tack region. Two-dimensional hydrodynamic calculations provide the estimate T_e = 60–116 eV for the same conditions and densities of 0.5–2.5 g/cm³ of the excited target medium.     

Population kinetics in turbulent plasmas: The role of non-Markovian fluctuations

A novel approach to the modeling of atomic populations in turbulent plasmas is applied to ionization-recombination balance calculations. Fluctuations of the fluid parameters are retained using a time-dependent statistical approach, suitable for cases where the turbulence characteristic times are of the same order as or smaller than the typical atomic relaxation times. We show that the populations are sensitive to the shape of the autocorrelation function of the fluctuations. An illustration is proposed through an ideal two-level system forced by non-Markovian temperature fluctuations.     

Turbulence and line shape formation in plasmas

We investigate the role of turbulent fluctuations on line shape formation in optically thin plasmas. A general expression, valid if the turbulent scales are much larger than the atomic scales, is obtained for the profile measured using a passive spectroscopy set up. Well-known limiting cases for Doppler and Stark broadening are identified. Then, the role of temperature fluctuations on Doppler line shapes, and the effect of density fluctuations on Stark broadened lines in the quasi-static regime are investigated.     

Equivalence of the method of the kinetic equation and the fluctuating-frequency method in the theory of the broadening of spectral lines

A new expression for the Stark profiles of spectral lines in plasma has been obtained by the method of the kinetic equation taking into account the dynamics of the plasma microfield. The result represents a dynamic line profile in the form of simple functionals of a static profile. The relation of the new solution with the known fluctuating-frequency method has been analyzed. It has been shown that this method is a discrete analog of the method of the kinetic equation and passes to the latter method in the limit of the continuous fluctuations. Simple formulas (4), (5), and (21) for dynamic line profiles provide ultrafast calculations of the profiles of spectral lines taking into account the dynamics of the plasma microfield.     

Decay of cystalline order and equilibration during the solid-to-plasma transition induced by 20-fs microfocused 92-eV free-electron-laser pulses

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16) W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.     

Intense ion beams as a tool for opacity measurements in warm dense matter

Opacity measurements in warm dense matter (WDM) provide a valuable benchmark for the diverging theoretical models in this regime. Heating of thin foils with intense heavy-ion beams allows one to create isolated samples of warm dense matter suitable for experimental determination of frequency-dependent opacities. A prerequisite for the measurements is the isothermal expansion of the heated foil. Hydrodynamic simulations predict that this condition is fulfilled. The analysis shows that existing ion-beam accelerators are capable to contribute to this field of research.     

Aluminium Lyman-beta satellite emission in non-Maxwellian dense laser produced plasmas

High-energy decay channels of the Al Lyman-β satellite have been observed in X-ray emission from highly ionized plasma jets created by intense laser irradiation of aluminium foil targets. Atomic structure calculations show that the Lyman β satellite emission consists from six emission groups close to the He-like Al 1s²-1s4p (He_γ) and 1s²-1s5p (He_δ) resonance lines. This provides new possibilities for space resolved analysis of high density plasmas. Non-Maxwellian simulations of the plasma emission carried out with the MARIA code demonstrate that the intensity ratios of the Lyman-β satellites and the He_γ and He_δ resonance lines are very sensitive to the bulk electron temperature. In contrast to standard diagnostic methods, parameter studies show that this bulk electron diagnostics is practically unaffected by suprathermal electrons having less than 10% of the bulk electron density.     

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 E _ion 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.     

The impact of dense plasma environments on the 1s3l fine structure levels of He-like ions

Finite-temperature dense plasma relativistic atomic structure calculations are employed to study the energy eigenvalues and transition probabilities related to the 1s3l fine structure levels of He-like ions. We discovered a particular response of the level structure that is distinctly different from simple nuclear charge screening effects that exists also for atoms and ions in vacuum as Z decreases. It is demonstrated that characteristic level crossings are driven by the free electron screening due to a particular dependence on the quantum numbers of each level of the 1s3l-configuration. Levels with large l shift stronger to the continuum than levels with small l whereas spin dependent dense plasma effects are relatively small. As a result, the known energy level fine structure in vacuum is subject to characteristic changes (level crossings, level order). Numerical calculations performed for a wide range of density, temperature and different chemical elements indicate that induced level crossings and change in level order are general characteristics of dense plasma effects.     

Nonadiabatic heating of a plasma produced by the ionization of a gas by a short intense laser pulse

A hydrodynamic model of the interaction of a short, intense laser pulse with moderate-density gases is constructed. The model systematically takes into account both tunneling-ionization processes and the relativistic motion of the electrons that are produced by the ionization of the gas in the relativistically strong field of the pulse. X-ray spectroscopy data agree well with the proposed theory. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 566–571 (10 October 1998)