Laser-driven ablation through fast electrons in PALS-experiment at the laser radiation intensity of 1–50 PW/cm²

Abstract

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/cm2 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 Iλ2 was varied in the range of 1 × 1014−8 × 1016 Wμm2/cm2 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-current stream of energetic α particles from laser-driven proton-boron fusion

“…With kT = 1.5 keV, ρd = 1.3×10 −2 g/cm 2 , S = 5.03 ×10 −5 cm 2 (corresponding to a spot radius of 40 μm), we find E h 70 J. This value is well below the energy absorbed in the interaction between the laser beam and the target, which is estimated [37] at a fraction of 30% of E L in our irradiation conditions, i.e., 200 J. We present this as a mere argument of plausibility for our simple model, well aware of the large uncertainties affecting the experimental α yield, expected proton yield, stopping power models, and estimate of the bulk temperature.…”

“…Thermonuclear yield: Initially, we have tried to explain the α yield in terms of thermonuclear reactions occurring in the plasma plume and the warm, shocked target bulk [37]. In the following, we denote by η = ρ/ρ 0 the compression ratio between the density under shock, ρ, and the initial (ordinary) bulk density, ρ 0 .…”

“…As an estimate of the plasma temperature in our experiment, we have used the analytical model for plume expansion of Gus'kov et al [37] and found a value of the order of 1 keV, which is in good agreement with, e.g., the temperature of C ions (1.8 keV) measured under similar irradiation conditions [40]. At kT = 1 keV, one finds N α,th 3 × 10 −25 n p (cm −3 ) for E L = 600 J and τ ∼ 1 ns.…”

Effect of ‘wandering’ and other features of energy transfer by fast electrons in a direct-drive inertial confinement fusion target

“…In the second case, this is due to the high intensity of a laser pulse, about 10 16 W/cm 2 , which is necessary for generation of an igniting shock wave. The effect of increasing ablation pressure due to energy transfer by fast electrons to dense plasma regions was experimentally observed in experiments [21,22] and in other ones [23,24,25] devoted to study of shock ignition.…”

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

“…In our previous papers (Kalinowska et al, 2012;Gus'kov et al, 2014;Pisarczyk et al, 2014), it has been shown that the interferometric measurements in combination with the crater volume analysis can provide essential information about mechanisms of the laser radiation absorption. The ratio N/V cr (where N is the total electron number in the plasma plume and V cr is the crater volume in cm 3 ) defines a number of thermal electrons participating in the creation of crater volume unit (1 cm 3 ).…”