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Colliding collisionless shocks appear in a great variety of astrophysicalphenomena and are thought to be possible sources of particle acceleration inthe Universe. To investigate the detailed dynamics of this phenomenon, we haveperformed a dedicated laboratory experiment. We have generated twocounter-streaming subcritical collisionless magnetized shocks by irradiatingtwo teflon (CF$_2$) targets with 100 J, 1 ns laser beams on the LULI2000 laserfacility. The interaction region between the plasma flows was pre-filled with alow density background hydrogen plasma and initialized with an externallyapplied homogeneous magnetic field perpendicular to the shocks. We report hereon measurements of the plasma density and temperature during the formation ofthe supercritical shocks, their transition to subcritical, and finalinterpenetration. We have also modeled the macroscopic evolution of the systemvia hydrodynamic simulations and the microphysics at play during theinteraction via Particle-In-Cell simulations. The main goal was to understandwhat was the effect of the second shock on particle energization. We found thatin the presence of two shocks the ambient ions reach energies around 1.5 timesof the ones obtained with single shocks. Both the presence of the downstreamzone of the second shock and of the downstream zone common for the two shocksplay a role in the different energization: the characteristics of theperpendicular electric fields in the two areas allow, indeed, certain particlesto keep being accelerated or to avoid being decelerated.

Particle energization in colliding subcritical collisionless shocks investigated in the laboratory