title: Proton acceleration by irradiation of isolated spheres with an intense laser pulse.
abstract: We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3×10^{20}Wcm^{-2}. With a laser focal spot size of 10 μm full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3 μm. Maximum proton energies of ∼25 MeV are achieved for targets matching the focal spot size of 10 μm in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.|Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany.
authors.forename: T M
authors.initials: TM
authors.lastname: Ostermayr
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Haffa
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Hilz
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: V
authors.initials: V
authors.lastname: Pauw
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Allinger
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: K-U
authors.initials: KU
authors.lastname: Bamberg
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Böhl
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Bömer
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: P R
authors.initials: PR
authors.lastname: Bolton
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Deutschmann
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: T
authors.initials: T
authors.lastname: Ditmire
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: M E
authors.initials: ME
authors.lastname: Donovan
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: G
authors.initials: G
authors.lastname: Dyer
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: E
authors.initials: E
authors.lastname: Gaul
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: J
authors.initials: J
authors.lastname: Gordon
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: B M
authors.initials: BM
authors.lastname: Hegelich
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Kiefer
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Klier
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Kreuzer
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: M
authors.initials: M
authors.lastname: Martinez
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: E
authors.initials: E
authors.lastname: McCary
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: A R
authors.initials: AR
authors.lastname: Meadows
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Moschüring
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Rösch
authors.affiliation: Ludwig-Maximilians-Universität München, Theresienstr. 37, 80333 München, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Ruhl
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: M
authors.initials: M
authors.lastname: Spinks
authors.affiliation: Center for High Energy Density Science, C1510, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: C
authors.initials: C
authors.lastname: Wagner
authors.affiliation: Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany.|Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Schreiber