Expansion of nanoplasmas and laser-driven nuclear fusion in single exploding clusters

arXiv:0807.4125v1 [physics.plasm-ph] 25 Jul 2008 Expansion of nanoplasmas and laser-driven nuclear fusion in single exploding clusters F Peano1, JL Martins1, RA Fonseca1,2, F Peinetti3, R Mulas3, G Coppa3, I Last4, J Jortner4, and LO Silva1 1GoLP/Instituto de Plasmas e Fus˜ao Nuclear, Instituto Superior T´ecnico, 1049-001 Lisboa, Portugal 2Departamento de Ciˆencias e Tecnologias da Informa¸c˜ao, Instituto Superior de Ciˆencias do Trabalho e da Empresa, 1649-026 Lisboa, Portugal 3Dipartimento di Energetica, Politecnico di Torino, 10129 Torino, Italy 4School of Chemistry, Tel-Aviv University, Ramat Aviv, 69978 Tel-Aviv, Israel E-mail: fabio.peano@ist.utl.pt, luis.silva@ist.utl.pt Abstract. The expansion of laser-irradiated clusters or nanodroplets depends strongly on the amount of energy delivered to the electrons and can be controlled by using appropriately shaped laser pulses. In this paper, a self-consistent kinetic model is used to analyze the transition from quasineutral, hydrodinamic-like expansion regimes to the Coulomb explosion (CE) regime when increasing the ratio between the thermal energy of the electrons and the electrostatic energy stored in the cluster. It is shown that a suitable double-pump irradiation scheme can produce hybrid expansion regimes, wherein a slow hydrodynamic expansion is followed by a fast CE, leading to ion overtaking and producing multiple ion flows expanding with different velocities. This can be exploited to obtain intracluster fusion reactions in both homonuclear deuterium clusters and heteronuclear deuterium-tritium clusters, as also proved by three-dimensional molecular-dynamics simulations. PACS numbers: 36.40.Gk, 52.38.Kd, 52.65-y Submitted to: Plasma Phys. Control. Fusion Expansion of nanoplasmas and laser-driven nuclear fusion in single exploding clusters2 1. Introduction The interaction of ultraintense lasers with jets of molecular clusters or nanodroplets (with typical size in the range 1 −100 nm and containing 102 −108 particles) is a central research topic [1], with important applications, such as tabletop nuclear fusion for compact neutron sources [2–8], or the laboratory investigations of nucleosynthesis reactions, relevant to astrophysical scenarios [9–11]. Clustered media can be regarded as sparse distributions of tiny solid targets, a peculiar configuration that allows for both a deep penetration of the laser radiation and a strong laser-matter coupling with many individual, overdense targets, thus providing extremely efficient energy absorption [12]. When hit by an ultraintense laser beam, the neutral atoms in a cluster are promptly ionized (cf. Ref. [13] for a detailed analysis of the concurring ionization mechanisms in different laser/cluster configurations) and a dense “nanoplasma” [1, 14] is formed. The free electrons then absorb energy from the laser pulse [15] and start expanding, causing the formation of strong electric fields, which lead to efficient ion acceleration, as first predicted by Dawson [16]. When the energy transferred to the electrons is much smaller than the electrostatic energy stored in the ion core, charge separation is localized to regions much smaller than the cluster [14,17–21], which then remains quasi-neutral and undergoes a hydrodynamic-like expansion [22–26]; in opposite conditions (e.g. with small deuterium clusters exposed to extremely intense laser radiation) the electrons suddenly escape from the cluster and the remaining bare-ion distribution undergoes a pure Coulomb explosion (CE) [27]. In intermediate situations, the expansion dynamics is a mixture of the phenomenology of the two limits, with the expansion process being strongly dependent on the self-consistent dynamics of ions and trapped electrons [21,28]. When increasing the laser energy, or when lowering the cluster size and density, the expansion conditions vary smoothly from quasi-neutral, hydrodynamic-like regimes to pure CE regimes, as confirmed by particle-in-cell (PIC) simulations [29,30] of the self-consistent laser-cluster interaction and by kinetic or fluid modeling of the expansion of finite-size, non-quasi-neutral plasma expansions [21,28,31]. Therefore, the expansion regime can be controlled by regulating the amount of energy transferred to the electrons [32], which can be obtained with appropriately shaped laser beams. An important example is the irradiation of homonuclear deuterium clusters with two sequential laser pulses having different intensities [33]. In this way, one can taylor the expansion dynamics so as to induce overrunning between ions and the consequent formation of expanding shells containing multiple ion flows. Within such structures (here denoted as “shock shells”, following the terminology in [27]), the relative velocities between deuterium ions from different flows can be sufficiently high for energetic collisions and dd fusion reactions to occurr [34]. Since these intracluster reactions occur early in the expansion, and before different exploding clus

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