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Collisions of high Mach number flows are ubiquitous in astrophysics. Because of the low density of astrophysical plasmas, collisional mean free paths are typically large and therefore observed shock waves must be "collisionless", i.e. due to a localization of ions by self-generated electro-magnetic fields.

Experiments conducted using high power lasers (NIF, OMEGA), producing counter-streaming, high-velocity plasma flows via laser ablation (10¹⁶Wcm^-2) of solid targets, aim to answer questions relevent to collisionless shock physics: the importance of electromagnetic-filamentation (Weibel) instabilities in shock formation, the self-generation of B-fields in shocks, the influence of external B-fields on shock formation, and the signatures of particle acceleration in shocks.

New Thomson scattering measurements provide direct evidence of the development of Weibel filaments. Modulations in the intensity of the ion-feature peaks corresponding to the two interpenetrating ion streams provide a measurement of modulations in the ion current, while modulations in the electron feature indicate the development of a strong transverse modulation in the density of the plasma, consistent with the nonlinear growth phase of the Weibel instability.