Laser-driven ablation through fast electrons in PALS- experiment at the laser radiation intensity of 1–50 PW/cm 2 S. YU. GUS’KOV, 1 N.N. DEMCHENKO, 1 A. KASPERCZUK, 2 T. PISARCZYK, 2 Z. KALINOWSKA, 2 T. CHODUKOWSKI, 2 O. RENNER, 3 M. SMID, 3 E. KROUSKY, 4 M. PFEIFER, 4 J. SKALA, 4 J. ULLSCHMIED, 4 AND P. PISARCZYK 5 1 P.N. Lebedev Physical Institute of RAS, Moscow, Russia 2 Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland 3 Institute of Physics ASCR, v.v.i., Prague, Czech Republic 4 Institute of Plasma Physics ASCR, Prague, Czech Republic 5 Warsaw University of Technology, Warsaw, Poland (RECEIVED 2 September 2013; ACCEPTED 13 November 2013) 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/cm 2 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 (λ 1 = 1.315 μm) and third (λ 3 = 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 Iλ 2 was varied in the range of 1 × 10 14 -8 × 10 16 Wμm 2 /cm 2 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. Keywords: Crater Volume; Fast Electrons; Laser Energy Transfer; Laser Interferometry; Plasma Temperature 1. INTRODUCTION This paper focuses on the research of physical processes partly related to the shock ignition (SI) (Scherbakov, 1983) that is one of the most perspective methods for targets ignition in inertial confinement fusion (ICF). This approach requires fulfillment of strict requirements for parameters of igniting shock wave and conditions of its generation. Namely, the pressure of igniting shock wave excited by laser spike action on ICF target preliminary compressed to the density of 10 g/cm 3 should not be smaller than 300 Mbar, and at the same time, the laser spike intensity and duration should vary in quite narrow ranges of 1–10 PW/cm 2 and 200–500 ps, respectively (Scherbakov, 1983; Betti et al., 2007; Ribeyre et al., 2008). Interaction of laser pulses at the above mentioned relatively high inten- sities with the plasma is strongly affected by non-collisional absorption mechanisms leading to a generation of fast super- thermal electrons. Therefore, the effects of the fast electron energy transport on the ablation pressure forming, shock wave generation and on the target compression belong to the most important problems in SI concept. In addition, the fast electron energy transfer may lead to a preheating of the target, which results in a decreased target compression. To eliminate the target preheating, the ICF investigations are usually carried out under conditions, which avoid the generation of fast electrons. In the presence of the resonance absorption of the laser radiation, such conditions correspond to relatively small values of the parameter I L λ 2 (I L and λ — intensity and wavelength of laser radiation), which should not exceed the value of 10 14 Wμm 2 /cm 2 . In the middle of 177 Address correspondence and reprint requests to: T. Pisarczyk, Institute of Plasma Physics and Laser Microfusion, 23 Hery St., 00-908 Warsaw, Poland. E-mail: tadeusz.pisarczyk@ifpilm.pl Laser and Particle Beams (2014), 32, 177–195. © Cambridge University Press, 2014 0263-0346/14 $20.00 doi:10.1017/S0263034613000992 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0263034613000992 Downloaded from https://www.cambridge.org/core. IP address: 107.173.130.238, on 24 Apr 2020 at 13:42:19, subject to the Cambridge Core terms of use, available at