Isochoric Heating of Solid Aluminum by Ultrashort Laser Pulses Focused on a Tamped Target

Abstract

We have studied the K-shell emission of an Al plasma which was generated by focusing a high contrast 150 fs laser pulse at a wavelength of 395 nm and intensity of 5 3 10 17 W͞cm 2 on a flat Al target tamped by a thin surface layer of MgO. The measured resonance lines (Ly a , He a , and He b ) and their Li-like and He-like satellites are extremely broadened and show a red polarization shift. Analysis of the Ly a and He b satellites yields an electron temperature of ഠ300 eV and an electron density of ͑5 10͒ 3 10 23 cm 23 . [S0031-9007(99)09405-3] PACS numbers: 52.50. Jm, 52.25.Nr, 52.70.Kz One fascinating aspect of the interaction of intense, ultrashort-duration laser pulses with matter is the possibility to generate plasmas at solid state density at high temperatures in the range 0.1 to 1 keV. Under these conditions the ion coupling parameter G [1] exceeds one and the plasma is thus in a strongly coupled state [2]. Such plasmas are of particular interest in inertial confinement fusion (ICF) and astrophysics. For example, it is possible to study the x-ray opacity of matter under conditions found in stellar interiors [3]. The importance for ICF originates from the fact that fs-laser generated plasmas approach temperatures and densities similar to the values currently attained in indirect drive experiments [4] and may therefore be of interest to investigate x-ray spectroscopy diagnostics needed for ICF plasmas [5]. In contrast to ICF plasmas, which require huge laser facilities, fs-laser plasmas can be generated by small tabletoplike lasers with a high repetition rate.Here we report an experiment in which we focused a frequency doubled 150-fs Ti-Sapphire laser on tamped targets, which consisted of solid Al covered by a thin surface layer of MgO. We measured the Al K-shell emission by means of time-integrated high resolution crystal spectroscopy. The resonance and satellite lines were considerably broader than previously reported [6][7][8][9]. For the detailed spectral analysis, we used simultaneously the He-like satellites of the Ly a line and the He b line which is strongly merged with its Li-like satellites. To our knowledge these features have not been considered in previous studies of the x-ray emission from fs-laser plasmas. Our analysis indicates that we achieved a higher density compared to previous experiments where the electron density did not exceed a few times 10 23 cm 23 . We attribute this result to the fact that we avoided early expansion by using a high contrast fs-laser pulse and tamped targets. Also the short wavelength of 395 nm may be helpful because it leads to absorption of the laser at a high critical density (n c 7.2 3 10 21 cm 23 ). Altogether, it was thus possible to produce ultrafast heating of solid Al before any significant expansion took place (i.e., isochoric heating).The ATLAS Ti-Sapphire laser at the MPQ-Garching was used to produce pulses with 150 fs (FWHM), and 200 mJ at l 790 nm. To achieve a high contrast ratio, we frequency doubled the pulses and obtained 65-75 mJ at l 395 nm. ...

Nonrelativistic structure calculations of two-electron ions in a strongly coupled plasma environment

“…In table 10, we have displayed the comparison of present results for transition energies of C 4+ , Al 11+ and Ar 16+ with experimental measurements [22][23][24] and existing theoretical calculations [36]. Table-10 clearly depicts that the present results are in good agreement with the experimental measurements as compared to other theoretical results.…”

Atomic photoionization in a changing plasma environment

“…Finally, we would like to point out that the red-shifted narrow absorption resonances with large peak cross section such as the one shown in the bottom plot of Fig. 1 may offer the opportunity for observation with sufficient initial state population and the currently available highly intense continuous light sources for the laser-produced plasmas similar to those already realized [17,18]. …”

“…In addition to the solar plasma discussed earlier, for the laser-produced dense plasma at an electronic temperature of a few hundred  and a density of 10 22 − 10 24  −3 [17,18], the Debye length would be comparable to the size of the atoms. Although the laser-produced plasma is far from a thermodynamically equilibrium system, a detailed theoretical estimate based on the Debye-Hückel model may help to better understand the physics behind the observed spectra, particularly the shift of the spectral lines.…”

Short-Pulse Laser-Produced Plasmas

“…Time duration of these kilovolt emissions have been studied with a sub-ps resolution streak camera [77,78], and conversion efficiencies have been determined for more energetic x-rays [79]. Spectroscopy of ultrafast plasmas has been performed with contrast-clean laser pulses obtained through frequency doubling [80,81] and tamped Al-tracer layer-targets [82]. Isochoric heating of solid aluminum at temperatures around 300-eV has been achieved.…”