Laser-Accelerated Ions from a Shock-Compressed Gas Foil.

We present results of energetic laser-ion acceleration from a tailored, near solid density gas target. Colliding hydrodynamic shocks compress a pure hydrogen gas jet into a 70 µm thick target prior to the arrival of the ultraintense laser pulse. A density scan reveals the transition from a regime characterized by a wide angle, low-energy beam (target normal sheath acceleration) to one of a more focused beam with a high-energy halo (magnetic vortex acceleration). In the latter case, three-dimensional simulations show the formation of a Z pinch driven by the axial current resulting from laser wakefield accelerated electrons. Ions at the rear of the target are then accelerated by a combination of space charge fields from accelerated electrons and Coulombic repulsion as the pinch dissipates.