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Laser-driven collisionless electrostatic shocks (E. Boella - IST, GoLP/IPFN, Portugal)

Shocks in collisionless plasmas are known to be an efficient mechanism for accelerating particles in space physics and astrophysics. Modern laser technology allows the generation of such nonlinear waves in laboratory and therefore shock acceleration has been suggested as a possible route towards a table-top ion accelerator. Ab initio two and three dimensional kinetic simulations of the generation and propagation of electrostatic shocks in near-critical density plasmas are presented. It is shown that lasers of average intensities can be used to excite moderate Mach number shocks that are able to accelerate high-quality proton beams. A detailed parameter scan has been performed in order to investigate the role of the plasma density profile, demonstrating that an optimal density profile is necessary to achieve a beam with low energy spread and high charge. Shocks in under-critical plasmas will be also addressed. In this case, ions are accelerated by the volumetric variant of TNSA down to a long density gradient. The presence of a decreasing electrostatic field causes the ions to overtake each other, leading to the formation of a shock wave. The wave is then responsible to further accelerate the ions located in the upstream, ahead of the shock front. Finally, it will be shown how electrostatic shocks, which are relatively easy to obtain in laboratory, can provide an insight on how collisionless shock waves interact. In fact, the collision of shocks is a pervasive event in astrophysics: for instance, the phenomenon is often invoked as the cause of particle acceleration and magnetic field generation in Gamma-Ray Bursts. A novel set-up to investigate the binary collision of shock waves exploiting two counter-propagating laser beams incident on a multilayered target is proposed and results of explorative particle-in-cell simulations will be discussed.

Salle de conférence - LULI - 16h