Laser produced plasma Research Papers (original) (raw)

New types of space resolved X-ray spectra produced in light matter experiments with high intensity lasers have been investigated experimentally and theoretically. This type of spectra is characterised by the disappearance of distinct resonance line emission and the appearance of very broad emission structures due to the dielectronic satellite transitions associated to the resonance lines. Atomic data calculations have shown, that rather exotic states with K-shell vacancies are involved. For quantitative spectra interpretation we developed a model for dielectronic satellite accumulation (DSA-model) in cold dense optically thick plasmas which are tested by rigorous comparison with space resolved spectra from ns-lasers. In experiments with laser intensities up to 1019 W/cm2 focused into nitrogen gas targets, hollow ion configurations are observed by means of soft X-ray spectroscopy. It is shown that transitions in hollow ions can be used for plasma diagnostic. The determination of the electron temperature in the long lasting recombining regime is demonstrated. In Light-matter interaction experiments with extremely high contrast (up to 10^10) short pulse (400 fs) lasers electron densities of ne≈3×10^23cm^−3 at temperatures between kTe=200–300 eV have been determined by means of spectral simulations developed previously for ns-laser produced plasmas. Expansion velocities are determined analysing asymmetric optically thick line emission. Further, the results are checked by observing the spectral windows involving the region about the Heα-line and the region from the Heβ-line to the He-like continuum. Finally, plasmas of solid density are characteristic in experiments with heavy ion beams heating massive targets.
We report the first spectroscopic investigations in plasmas of this type with results on solid neon heated by Ar-ions. A spectroscopic method for the determination of the electron temperature in extreme optically thick plasmas is developed.
1. Introduction
The investigation of dense plasma has received great interest in a widespread community: inertial fusion driven by lasers and heavy ion beams, X-ray lasers, non-coherent X-ray sources, and correlation effects in dense cold plasmas. In these investigations plasma spectroscopy has provided important information for basic research and for the optimisation of desired plasma parameters. X-ray spectroscopy of these dense plasmas, which contain highly charged ions, has turned out to be extremely useful for the determination of the plasma parameters and several models have been successfully developed in the last decades, e.g. see [1] and [2]. The general feature of these traditional spectra are the dominant emission of resonance lines.

Recently, the interaction of radiation with matter by means of powerful lasers with extremely high contrast of up to 1011 produce spectra which differ dramatically from traditional ones, e.g. known from ns-laser experiments. A general feature of these newer spectra is the disappearance of the resonance lines and the appearance of very broad emission structures associated to the resonance lines. Theoretical calculations readily showed that neither Stark-broadening nor opacity effects could account for the experimental observation. Only recently, Rosmej and Faenov [3] proposed a model of accumulated dielectronic satellites (DSA-model) for the interpretation of the experimental findings [4], [5], [6], [7] and [8].

It became immediately clear that spectra from short pulse high-power high-contrast lasers were not appropriate to study the origins of the observed spectra. Transient effects, field ionisation, continuum level depression at high densities and optical thickness made the theoretical interpretation difficult. Therefore, it was appropriate to perform experiments that could illuminate the situation and perform systematic investigations at ns-laser installation. The keypoint in these experiments being the measurement of X-ray spectra with high luminosity at high spectral and spatial resolution. This was realised by means of spherically bent mica crystals providing a spectral resolution of λ/δλ≈104 simultaneously with spatial resolution of m [9], [10] and [11]. Note that spectra emitted from plasmas in traditional ns-laser experiments arising from regions close to the target surface showed emission features similar to those known from high-intensity high-contrast laser pulses.

Not all questions could be addressed in these experiments, in particular the broad emission structures located far from usual resonance line positions [12], [13] and [14] required further study. A major step the atomic data calculations that showed these structures might be due to transitions in hollow ions [7], [8], [14], [15] and [16]. However, the question concerning the excitation mechanisms however remained unresolved.

The similarity of cold, dense plasmas created by short pulse high-contrast lasers and plasmas generated through heating solids with heavy ion beams made the beam–solid interaction experiments attractive to the laser community. Spectroscopic investigations of the first experiments with Ar-ions heating solid neon [17] were thus pursued.

2. Laser produced plasmas
2.1. Observation of unusual X-ray spectra from high-intensity high-contrast laser pulses
Experiments on the interaction of high-intensity high-contrast lasers with solid targets have been performed at the Centre for Ultrafast Optical Science of the University of Michigan [18] and [19]. The pulse had a duration of 400 fs, a wavelength of m, an energy of 1 J, and an intensity contrast of 1010. The diameter of the focal spot was about m and a peak intensity of 0.5×1019 W/cm2 was achieved at the surface of a solid target. In addition, experiments with prepulses were performed with a total energy of 2 J and a time separation from the main pulse of 2 ps. Solid Mg targets were used. X-ray spectra [19] have been recorded with spherically bent mica crystals with a 186 mm radius of curvature and X-ray CCD camera. The geometrical arrangement uses the FSSR-1D scheme [10].