Characterization of near-LTE, high-temperature and high-density aluminum plasmas produced by ultra-high intensity lasers
Introduction
Ultra-high-intensity (UHI) laser pulses are efficient tools to generate hot plasmas at near-solid density. These novel states of matter are promising platforms to address problems associated with stellar opacities [1] and atomic physics in general [2], [3]. The mechanisms whereby the laser-accelerated electrons propagate and deposit their energy through dense materials have been extensively investigated over the past years, mainly in the framework of the fast ignition approach to inertial confinement fusion [4], [5], [6], [7] and, more recently, to address atomic physics phenomena arising in dense plasmas [8], [9], [10], [11]. The non-thermal high energy electron density is usually high enough so that, in addition to the direct collisions with the target particles, the dominant heating process is the ohmic dissipation of the inductive return current formed by collisional background electrons [12], [13], [14]. As early as 1996, it has been demonstrated that dense targets can be heated in excess of 100 eV before any significant hydrodynamic motion takes place [15], [16], [17]. To achieve this goal high-intensity-contrast pulses are required to prevent the sample from expanding before the peak of the laser pulse, which may be achieved using frequency doubling or plasma mirrors [9], [18]. In these experiments, the plasma conditions are commonly inferred from spectroscopic measurements, the interpretation of which implies accurate modeling not only of the radiative properties of the heated sample but also of the early-time heating dynamics. In this paper, we present a detailed analysis of K-shell spectra obtained from laser-driven plastic buried Al samples at the ELFIE facility. In order to produce synthetic spectra to compare to the experiment, we have employed a suite of simulation tools describing the major physical processes arising during the fast electron generation and relaxation phases, as well as the subsequent radiative-hydrodynamic evolution of the target. Our results highlight the great potential of high-contrast UHI lasers to create and explore high energy density (HED) states in the laboratory.
Section snippets
Experimental setup and results
Fig. 1 shows a schematic set-up of the experiment performed with the ELFIE laser at LULI. Frequency-doubled pulses (λ = 0.527 μm) were used to increase the laser intensity contrast to be greater than 107. This served to improve the laser-solid coupling efficiency by minimizing the plasma formation before the arrival of the main pulse [9]. Despite frequency-doubling, however, a residual prepulse of intensity 1012 − 1013 Wcm−2 was observed 60 ps before the laser peak. Consequently, a preplasma of
Step 1- kinetic simulation of the laser-solid interaction
To simulate the UHI laser-solid interaction and the heating processes during the first picosecond, we make use of the two-dimensional (2-D) Cartesian particle-in-cell (PIC) code calder [25]. A three-layer target composed of 1 μm C1+/0.5 μm Al3+/5 μm CH1+ is considered in the simulation. The initial electron and ion temperatures are set to Te = Ti = 10 eV. An exponential density profile of 0.4 μm scale-length is added on the front carbon layer to mimic the effect of the laser prepulse. The other
Near-LTE plasma effects
Fig. 7 presents the time evolution, over a 10 ps window, of the mean Al ion charge, Z∗, obtained after post-processing by spect3d using the LTE, CRSS and CRTD options. Due to the high temperatures reached in the Al layer, the maximum of ionization (Z∗ ∼ 12) is attained 500 fs after the peak of the laser irradiation. The slight overestimate of Z∗ between LTE and nLTE calculations shows that the electron density is not large enough to yield complete LTE conditions at such extreme temperatures
Conclusions
Hot, solid-density Al plasmas were generated using a 4 × 1018 Wcm−2, 350 fs laser pulse. Time-integrated K-shell spectra and time-resolved Heβ-line emission were used to infer the plasma parameters following the laser irradiation. A suite of simulation tools was employed to describe the laser-solid interaction and the subsequent radiative-hydrodynamic processes. The space-time integrated spectra alone are reasonably well reproduced using an LTE atomic physics model for a mean temperature of
Acknowledgments
Heartfelt thanks are due to the ELFIE technical team for their day-to-day support. We also acknowledge useful discussions with Igor Golovkin and Iain Hall from Prism Computational Sciences about the code spect3d.
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