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Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Mar;89(3):033105. Epub 2014 Mar 24.

Investigation of fast-electron-induced Kα x rays in laser-produced blow-off plasma.

Author information

  • 1Center for Energy Research, University of California San Diego, La Jolla, California 92093, USA.
  • 2General Atomics, San Diego, California 92093, USA.
  • 3Center for Energy Research, University of California San Diego, La Jolla, California 92093, USA and Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
  • 4Department of Physics, Ohio State University, Columbus, Ohio 43210, USA.
  • 5Graduate School of Engineering, Osaka University, 565-0871, Japan.
  • 6Photon Pioneers Center, Osaka University, 565-0871, Japan.
  • 7Lawrence Livermore National Laboratory, Livermore, California 94550, USA.


Refluxing of fast electrons generated by high-intensity, short-pulse lasers was investigated by measuring electron-induced Kα x rays from a buried tracer layer. Using planar foils of Au/Cu/CH, the 150-J, 0.7-ps TITAN short-pulse laser was focused on the gold foil to generate fast electrons and the 3-ns, 300-J long pulse beam irradiated on the CH side to create expanding plasma as a conducting medium. By delaying the short-pulse beam timing from the long pulse laser irradiation, the plasma size was varied to change electron refluxing in the target rear. The total yields and two-dimensional images of 8.05-keV Cu-Kα x ray were recorded with an x-ray spectrometer and two monochromatic crystal imagers. The measurements show that the integrated yields decrease by a factor of 10 from refluxing to the nonrefluxing limit. Similar radial profiles of the Kα images in the rear were observed at all delays. Hybrid-particle-in-cell simulations using plasma profiles calculated by a radiation-hydrodynamic code HYDRA agree well with the measured Kα yields. The simulations suggest that conducting plasma with the size of ∼300 μm in the laser direction and ∼600 μm in the lateral direction at the density of 2 × 1020 1/cm3 is sufficiently large to prevent electrons from refluxing in the target. The parameters found in this study can be useful in designing experiments utilizing a Kα x-ray source in refluxing regime or a tracer layer in nonrefluxing regime.

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