Laser-Fusion experiments and experiments of high energy astrophysics interest

Séminaire IPAG de Laurent Gremillet (CEA DAM), jeudi 3 décembre 2015 à 11h00, IPAG seminar room

In a few words :
Powerful lasers have been built in the world and especially in France in Bordeaux, in order to develop experiments of compression of targets constituted of a mixture of deuterium-tritium in the purpose of getting thermonuclear fusion and an energy gain. These experiments have also been motivated by projects of fundamental physics, in particular by astrophysical projects. High energy astrophysics is rich of topics of investigations particularly well adapted to these experiments.

Additional details :
Collisionless turbulent shocks between energetic outflows and the ambient medium are believed to happen in powerful astrophysical objects (e.g., gamma-ray bursts, supernova remnants, pulsar winds, etc.), where they are held responsible for generating nonthermal particles and radiations through, respectively, Fermi diffusion and synchrotron emission [1]. Both these processes require that part of the flow kinetic energy be converted into magnetic fluctuations moving at different velocities upstream and downstream of the shock front. Recent theoretical advances have clarified the mechanisms of the turbulence generation and shock formation [2]. The emerging picture is that of a self-regulated structure, whereby the particles Fermi-accelerated in the turbulence and injected back into the upstream medium also drive the Weibel-type instabilities [3] [4] at the source of this turbulence. Much of this progress has been made possible by the use of ab initio particle-in-cell (PIC) simulations. The accuracy of these simulations, however, comes at the cost of heavy computational load, and they are usually conducted under simplified physical conditions and geometries. The experimental study of collisionless shocks would be of great value for validating the theoretical predictions. Laser-matter interaction is likely to play a crucial role in this respect, because of the unique ability of powerful lasers to drive relatively dense plasmas at high velocities [5]. A major difference concerns the scales at which the astrophysical and experimental phenomena take place because of the considerable disparity of the plasma densities at play (from ne 10−3 − 103 cm−3 in astrophysical settings, to ne 1018 − 1023 cm−3 in laser experiments). The challenging goal will then be to create systems of similar normalized scales with respect to the plasma skin depth. After a short review of the ongoing experimental efforts, we will first present an analytical model of the nonlinear development of the ion-Weibel instability induced by the collision of two non-relativistic ion beams [6]. From comparison with PIC simulations, this model will be shown to correctly capture the evolution of the ion beams up to a stage close to shock formation, and to provide predictions consistent with recent experimental results. In a second part, we will investigate the ability of intense (0 > 1020 W.cm−2) short-pulse ( ps) lasers to induce turbulent electron-ion shocks in solid density plasmas [7]. By means of PIC simulations and kinetic theory, we will demonstrate that, in contrast to the standard astrophysical scenario, the early- time magnetic fluctuations generated by the laser-heated electrons are strong enough to isotropize the target ions and, therefore, cause shock formation [8]]. Finally, we will explore the possibility of generating pair shocks from the interaction of two counterstreaming pair jets driven from solid foils irradiated at extreme laser intensities (0 1024 W.cm−2). We will show that the dissipation caused by the ultra-strong (> 106 T) magnetic modulations resulting from the Weibel instability is amplified by intense synchrotron emission, which enhances the magnetic confinement and compression of the colliding jets [9].