O. Renner

Simulation of X-ray line transfer in a cylindrically expanding plasma

Abstract A method for calculating the X-ray resonance line emission from a cylindrically expanding laser-produced plasma is presented. A multi-frequency line transfer algorithm, incorporated as a post-processor to a time-dependent 1-D hydrodynamics and non-LTE atomic physics calculation, is used to compute spatially-resolved spectral profiles of the line emission. Coupling of photon re-absorption to ionic state populations is achieved by use of the escape probability method. We use the code to investigate the effects of high expansion velocities, and the influence of neighbouring lines, on the opacity and detailed shape of optically thick lines. As an example we consider the profile of the Lyman-α (1s-2p) resonance line of hydrogenic aluminium.

Laser-driven ablation through fast electrons in PALS-experiment at the laser radiation intensity of 1–50 PW/cm 2

The paper is directed to the study of high-temperature plasma and ablation plasma formation as well as efficiency of the laser energy transfer to solid targets irradiated by laser pulses with intensities of 1–50 PW/cm and duration of 200–300 ps, i.e., at conditions corresponding to the characteristics of the laser spike designed to generate the igniting shock wave in the shock ignition concept. The experiments have been performed at Prague Asterix Laser System. The iodine laser delivered 250 ps (full width at half maximum) pulses with the energy in the range of 100–600 J at the first (λ1= 1.315 μm) and third (λ3= 0.438 μm) harmonic frequencies. The focal spot radius of the laser beam on the surface of Al or Cu targets made was gradually decreased from 160 to 40 μm. The diagnostic data collected using three-frame interferometry, X-ray spectroscopy, and crater replica technique were interpreted by two-dimensional numerical and analytical modeling which included generation and transport of fast electrons. The coupling parameter Iλ was varied in the range of 1 × 10−8 × 10 Wμm/cm covering the regimes of weak to intense fast electron generation. The dominant contribution of fast electron energy transfer into the ablation process and shock wave generation was found when using the first harmonic laser radiation, the focal spot radius of 40–100 μm, and the laser energy of 300–600 J.

High-resolution measurements of X-ray emission from dense quasi-1D plasma: Line merging and profile modification

The precise measurements of the spectral line profiles near the short-wavelength limit of the Lyman series of Al are reported. The spatially resolved narrow-band spectra were attributed to macroscopic plasma parameters by using the results of 1D and 2D hydrodynamic plasma modeling. The gradual merging and broadening of the spectral lines Al Ly e–θ was compared to the synthetic spectra produced by atomic physics codes. The discrepancies between the experimental and theoretical data indicate the limits for applicability of standard theoretical models.

Experimental and simulated profiles of the Al XIII Ly-α resonance line from a cylindrically expanding plasma

Abstract Spectral profiles of optically thick line radiation emitted by an expanding cylindrical laser-produced plasma have been recorded in high resolution using a vertical dispersion variant of the double-crystal spectrometer. By uniformly irradiating a narrow wire with a 1 ns pulse a quasi-steady plasma expansion is generated with a velocity gradient in excess of 10 9 s −1 . Such velocity fields strongly influence the transfer of radiation through a plasma, resulting in a complex distortion of the lineshapes of optically thick lines. We have computed theoretical profiles of the Al XIII Ly-α resonance line. Simulation of the laser-plasma interaction is performed using the 1D Lagrangian hydrocode MED103 which incorporates a non-LTE time-dependent excitation and ionisation package based on the average-atom model. Line emission profiles are calculated by the method of multi-frequency line transfer in the co-moving frame. We investigate the sensitivity of the lineshape on plasma parameters such as the velocity profile, comparing our theoretical predictions with experiment.

New methods of X-ray spectroscopy of laser-produced plasmas with one-dimensional spatial resolution

Two novel high-dispersion, high-resolution spectroscopic methods are described. The properties of the vertical dispersion variants of the double-crystal spectrometer and the Johann spectrometer with a cylindrically bent crystal are discussed and compared with those of standard spectroscopic schemes. Preliminary experimental data demonstrate the good luminosity and extreme spectral and 1-D spatial resolution of these methods, which should prove useful in high-precision X-ray spectroscopic measurements of laser-produced plasmas.

An effect of the target position relative to the laser focus on X-ray emission from the laser plasma

The first harmonics beam generated by an iodine laser system was focused by an f/2 optics on an Al foil target. The X-ray output from the laser plasma both in the line and broad-band spectra was registered over an interval around the “ideal” focus. It was found that the maximum X-ray power is not obtained in the focus itself but for a somewhat larger focal spot outside the focus. To explain this phenomena, temperature and density measurements were in addition made. The plasma temperature evaluated from both the line (He-like Al XII resonant line and j, k, l satellites) and broad-band spectra (two foil method) was also measured and found to be largely constant in the vicinity of the focus. The line and broad-band temperatures differ, the broad-band temperature being about 25% higher. The electron density was equally determined using an intercombination line.

Laser-produced aluminum plasma expansion inside a plastic plasma envelope

Previous experimental results demonstrated that the plasma pressure decreases with the growing atomic number of the target material. In this context, a question arose if the Al plasma outflow could be collimated using the plastic plasma as a compressor. To solve this problem, an experiment using a plastic target with an Al cylindrical insert was performed. The focal spot diameter substantially larger than that of the insert ensured simultaneous heating both target materials. This experiment proved that a production of Al plasma jets collimated by an action of outer plastic plasma is feasible [Kasperczuk et al., Laser Part. Beams 30, 1 (2012)]. The results of investigations presented here provide additional information on distributions of electron temperature in the outflowing plasma and time and space characteristics of ion emission, both registered at bare and constrained-flow Al targets. The experiment was carried out at the Prague asterix laser system iodine laser facility. The laser provided a 250 ps ...

Aluminum Lyman α group formation at high-intensity, high-energy laser-matter interaction

Abstract The satellite structure accompanying the Al Lyα emission from plasma regions close to the surface of the laser-irradiated targets is studied. In spectra produced at constant laser energy but variable pulse duration we identify the situations where the resonance line is suppressed and only the emission from long-wavelength satellites is observed. The experimental data are compared with results of calculations based on two-dimensional hydrodynamics and multilevel collisional-radiative codes. Simulations indicate that these spectra originate from cold dense plasma created inside the crater several tens of micrometers under the surface of the solid target.

Experimental evidence of thermal smoothing in a double-pulse produced plasma

The smoothing mechanism, that is due to the thermal conductivity in the laser plasma generated by a double pulse, is examined. Plasma preformed by a second harmonics prepulse serves as a low density gradient plasma for impacting the main pulse, the frequency of which was tripled for improving the laser-target coupling. The effect of the preformed plasma on the thermal smoothing of the heating pulse, which was split to create two foci on the target surface, was explored by varying the time delay between the prepulse and the main pulse. The smoothing effect was monitored by a pair of pinhole cameras: one viewing side-on and the other one placed at the rear side of the target. Spatially resolved X-ray emission spectra were recorded to determine the density and temperature distributions in the plasma. The maximum smoothing effect was observed for the time delay t = 0.4-0.7 ns.

Plasma-wall interaction studies with optimized laser-produced jets

The production of the laser-produced plasma jets at burnt-through low-Z foils was optimized by using three-frame interferometry. When striking secondary targets, these jets of energetic particles represent an efficient tool for the investigation of transient phenomena at surfaces of the plasma-exposed solids. Two sets of precisely measured x-ray spectroscopic data demonstrate diagnostic potential of the collimated jets in the plasma-wall interaction studies: Blue Doppler shifts of the Al jet self-emission visualize ion deceleration in the near-wall region. Local depressions found in Al Lyγ profiles emitted from Al/Si(PMMA) targets indicate charge exchange between the Al XIII and fully stripped C ions.