Initial experimental evidence of self-collimation of target-normal-sheath-accelerated proton beam in a stack of conducting foils

• Published in: Physics of plasmas 2013-08, Vol.20 (8)
• Main Authors: Ni, P. A., Lund, S. M., McGuffey, C., Alexander, N., Aurand, B., Barnard, J. J., Beg, F. N., Bellei, C., Bieniosek, F. M., Brabetz, C., Cohen, R. H., Kim, J., Neumayer, P., Roth, M., Logan, B. G.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ALUMINIUM; Focusing; Foils (structural shapes); HEAVY IONS; Lasers; Lenses; LINEAR ACCELERATORS; PARTICLE ACCELERATORS; PLASMA FOCUS; PLASMA GUNS; PLASMA PRODUCTION; PLASMA SHEATH; Plasmas; PROTON BEAMS; PROTONS; Reduction; Stacks; SYNCHROTRONS
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.4818147

Phenomena consistent with self-collimation (or weak self-focusing) of laser target-normal-sheath-accelerated protons was experimentally observed for the first time, in a specially engineered structure (“lens”) consisting of a stack of 300 thin aluminum foils separated by 50 μm vacuum gaps. The experiments were carried out in a “passive environment,” i.e., no external fields applied, neutralization plasma or injection of secondary charged particles was imposed. Experiments were performed at the petawatt “PHELIX” laser user facility (E = 100 J, Δt = 400 fs, λ = 1062 nm) at the “Helmholtzzentrum für Schwerionenforschung–GSI” in Darmstadt, Germany. The observed rms beam spot reduction depends inversely on energy, with a focusing degree decreasing monotonically from 2 at 5.4 MeV to 1.5 at 18.7 MeV. The physics inside the lens is complex, resulting in a number of different mechanisms that can potentially affect the particle dynamics within the structure. We present a plausible simple interpretation of the experiment in which the combination of magnetic self-pinch forces generated by the beam current together with the simultaneous reduction of the repulsive electrostatic forces due to the foils are the dominant mechanisms responsible for the observed focusing/collimation. This focusing technique could be applied to a wide variety of space-charge dominated proton and heavy ion beams and impact fields and applications, such as HEDP science, inertial confinement fusion in both fast ignition and heavy ion fusion approaches, compact laser-driven injectors for a Linear Accelerator (LINAC) or synchrotron, medical therapy, materials processing, etc.