Hot Electrons Transverse Refluxing in Ultraintense Laser-Solid Interactions

LULI, Ecole Polytechnique, CNRS, CEA, UPMC, route de Saclay, 91128 Palaiseau, France.
Physical Review Letters (Impact Factor: 7.51). 07/2010; 105(1):015005. DOI: 10.1103/PHYSREVLETT.105.015005
Source: PubMed

ABSTRACT We have analyzed the coupling of ultraintense lasers (at ∼2×10{19}  W/cm{2}) with solid foils of limited transverse extent (∼10  s of μm) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

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Available from: Thomas E Cowan, Sep 28, 2015
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    • "Normally, this mechanism just represents a loss of absorbed laser energy, which is converted to hot electrons but not contributing to the quasi static sheath built up at the target rear side. In a recent study [15], however, using very small diameter targets, the refluxing of transversely spreading electrons were found to enhance and smooth the sheath field for TNSA from the rear surface. "
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    ABSTRACT: A coated hollow core microsphere is introduced as a novel target in ultra-intense laser-matter interaction experiments. In particular, it facilitates staged laser-driven proton acceleration by combining conventional target normal sheath acceleration (TNSA), power recycling of hot laterally spreading electrons and staging in a very simple and cheap target geometry. During TNSA of protons from one area of the sphere surface, laterally spreading hot electrons form a charge wave. Due to the spherical geometry, this wave refocuses on the opposite side of the sphere, where an opening has been laser micromachined. This leads to a strong transient charge separation field being set up there, which can post-accelerate those TNSA protons passing through the hole at the right time. Experimentally, the feasibility of using such targets is demonstrated. A redistribution is encountered in the experimental proton energy spectra, as predicted by particle-in-cell simulations and attributed to transient fields set up by oscillating currents on the sphere surface.
    New Journal of Physics 02/2011; 13(1). DOI:10.1088/1367-2630/13/1/013030 · 3.56 Impact Factor
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    ABSTRACT: The employment of microstructured targets for efficient ion acceleration by short intense laser pulses is discussed. We further examine recently proposed targets, such as foil with hole or foil with slice joint to its front surface, by two dimensional particle-in-cell (PIC) simulations. These microstructured targets enable to accelerate protons to higher maximum energies compared with simple thin foils due to additional acceleration of hot electrons during laser-target interaction. On the other hand, we found that the energy fluencies of protons accelerated in those specially designed targets are reduced compared with thin foils irradiated by obliquely incident laser pulses.
    Laser Optics 2010; 07/2010
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    ABSTRACT: A technique developed to measure in time and space the dynamics of the electron populations resulting from the irradiation of thin solids by ultraintense lasers is presented. It is a phase reflectometry technique that uses an optical probe beam reflecting off the target rear surface. The phase of the probe beam is sensitive to both laser-produced fast electrons of low-density streaming into vacuum and warm solid density electrons that are heated by the fast electrons. A time and space resolved interferometer allows to recover the phase of the probe beam sampling the target. The entire diagnostic is computationally modeled by calculating the probe beam phase when propagating through plasma density profiles originating from numerical calculations of plasma expansion. Matching the modeling to the experimental measurements allows retrieving the initial electron density and temperature of both populations locally at the target surface with very high temporal and spatial resolution (~4 ps, 6 μm). Limitations and approximations of the diagnostic are discussed and analyzed.
    The Review of scientific instruments 11/2010; 81(11):113302. DOI:10.1063/1.3499250 · 1.61 Impact Factor
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