T. Pisarczyk, S.Yu. Gus'kova, O. Renner, R. Dudzak, J. Dostal, N.N. Demchenko, M. Smid, T. Chodukowski, Z. Kalinowska, M. Rosinski, P. Parys, J. Badziak, D. Batani, S. Borodziuk, L. A. Gizz, E. Krousky, Y. Maheu6, G. Cristoforettia7, L. Antonellia6, P. Koestera7, F. Baffigia7, J. Ullschmieda4a5, J. Hrebiceka4a5, T. Medrik, M. Pfeifera4a5, J. Skala and P. Pisarczyk
Laser and Particle Beams 34, 94-108 (2016)
The paper is a continuation of research carried out at Prague Asterix Laser System (PALS) related to the shock ignition (SI) approach in inertial fusion, which was carried out with use of 1 omega main laser beam as the main beam generating a shock wave. Two-layer targets were used, consisting of Cu massive planar target coated with a thin polyethylene layer, which, in the case of two-beam irradiation geometry, simulate conditions related to the SI scenario. The investigations presented in this paper are related to the use of 3 omega to create ablation pressure for high-power shock wave generation. The interferometric studies of the ablative plasma expansion, complemented by measurements of crater volumes and K-alpha emission, clearly demonstrate the effect of changing the incident laser intensity due to changing the focal radius on efficiency of laser energy transfer to a shock wave and fast electron emission. The efficiency of the energy transfer increases with the radius of the focused laser beam. The pre-plasma does not significantly change the character of this effect. However, it unambiguously results in the increasing temperature of fast electrons, the total energy of which remains very small (<0.1% of the laser energy). This study shows that the optimal radius from the point of view of 3 omega radiation energy transfer to the shock wave is the maximal one used in these experiments and equal to 200 mu m that corresponds to the minimal effect of two-dimensional (2D)-expansion. Such a result is typical for the ablation process determined by electron conductivity energy transfer under the conditions of one-dimensional or 2D matter expansion without any appreciable effect due to energy transfer by fast electrons. The 2D simulations based on application of the ALANT-HE code and an analytical model that includes generation and transport of hot electrons has been used to support of experimental data.
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