Recent results from experimental studies on laser–plasma coupling in a shock ignition relevant regime

Abstract

Shock ignition (SI) is an appealing approach in the inertial confinement scenario for the ignition and burn of a pre-compressed fusion pellet. In this scheme, a strong converging shock is launched by laser irradiation at an intensity I λ 2 >10 15 W cm −2 µm 2 at the end of the compression phase. In this intensity regime, laser-plasma interactions are characterized by the onset of a variety of instabilities, including stimulated Raman scattering, Brillouin scattering and the two plasmon decay, accompanied by the generation of a population of fast electrons. The effect of the fast electrons on the efficiency of the shock wave production is investigated in a series of dedicated experiments at the Prague Asterix Laser Facility (PALS).We study the laser-plasma coupling in a SI relevant regime in a planar geometry by creating an extended preformed plasma with a laser beam at ∼7 × 10 13 W cm −2 (250 ps, 1315 nm). A strong shock is launched by irradiation with a second laser beam at intensities in the range 10 15 -10 16 W cm −2 (250 ps, 438 nm) at various delays with respect to the first beam. The pre-plasma is characterized using x-ray spectroscopy, ion diagnostics and interferometry. Spectroscopy and calorimetry of the backscattered radiation is performed in the spectral range 250-850 nm, including (3/2)ω, ω and ω/2 emission. The fast electron production is characterized through spectroscopy and imaging of the Kα emission. Information on the shock pressure is obtained using shock breakout chronometry and measurements of the craters produced by the shock in a massive target.Preliminary results show that the backscattered energy is in the range 3-15%, mainly due to backscattered light at the laser wavelength (438 nm), which increases with increasing the delay between the two laser beams. The values of the peak shock pressures inferred from the shock breakout times are lower than expected from 2D numerical simulations. The same simulations reveal that the 2D effects play a major role in these experiments, with the laser spot size comparable with the distance between critical and ablation layers.

Pre-plasma effect on laser beam energy transfer to a dense target under conditions relevant to shock ignition

“…The role of fast electrons in the laser energy conversion to shock waves was also investigated in our previous research (Gus 'kov et al, 2004;Kalinowska et al, 2012;Koester et al, 2013;Batani et al, 2014a) performed with the PALS iodine laser. At the beginning, massive targets of Al and Cu have been irradiated at various focal spot radii of the fundamental frequency (1ω) and frequency-tripled (3ω) laser beam radiation to identify the mechanisms of the laser absorption and to determine their influence on the absorbed energy transfer to the target Kalinowska et al, 2012).…”

“…Similar to (Koester et al, 2013;Batani et al, 2014a;Pisarczyk et al, 2014), the experiments were performed using Cu massive planar targets covered by 25 μm plastic layer (C 8 H 7 Cl), which were irradiated by the 1ω laser beam (λ = 1.315 μm) at the energy of 250 J. Structure and irradiation geometry of the two-layer target are presented in Figure 1.…”

Benchmark of a parallel fluid code for parametric decay instability in inhomogeneous plasmas

“…During the past four decades, PDIs have already been developed deeply in experiments and theories [1][2][3][4][5][6][7][8][9][10][11]. But there still some deficiencies exist for the large scale space problem.…”

Measurement of reflectivity of spherically bent crystals using Kα signal from hot electrons produced by laser-matter interaction

“…Moreover, by measuring Kα emission at different target depths, we can estimate the average energy of the hot electrons as well as beam divergence. [10][11][12][13][14] Spherically bent crystal reflectivity can be estimated using several models. Nevertheless, measurements are necessary in order to exactly know the reflectivity of a specific crystal.…”