Halo-shape structure formation of laser-accelerated intense proton bunch in dense plasma

Résumé fourni par la source

Abstract The transport of a laser-accelerated intense proton beam through a well-characterized dense plasma ( L ∼ 1 mm , T ∼ 17 eV , n e ∼ 4 × 10 20 cm 3 ) was experimentally studied. The plasma was produced by irradiating a foam target ( ρ = 2 mg cm − 3 ) with soft x-ray radiation from a laser-driven hohlraum. A symmetric setup allowed part of the proton bunch to propagate through the plasma, while the other part passed through an adjacent empty channel without target material. This enabled us to do in a single laser shot a comparison of the beam properties with and without plasma interaction while also mitigating shot-to-shot fluctuations. Measurements revealed a transformation of the initially flat beam profile into a halo structure after plasma traversal. Particle-in-cell simulations attribute this effect to magnetic fields generated at the plasma boundary, where the beam exhibits a sharp edge due to collimation by the target holder. These results constitute direct experimental evidence for self-modulation instability of an intense proton bunch in dense plasma. This work has implications for high-energy-density physics and fast ignition scenarios, where controlled proton beam transport in dense plasmas is critical. The presented method also provides a platform for systematic investigations of beam–plasma interactions across wider parameter ranges.

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