X-ray imaging to characterize MeV electronics propagation through plastic targets Page: 3 of 7
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X-ray imaging to characterize MeV electrons propagation through plastic targets
Fusion Group, General Atomics
P.O. Box 85608
San Diego CA 92186-5608
M. Key, J. Koch, and D. Pennington
Lawrence Livermore National Laboratory
Livermore, CA 94550
A high intensity laser pulse incident on an overdense plasma generates high energy
electrons at the critical surface which propagate into the plasma. The details of this
propagation is critical to the Fast Ignition process. The energetic electrons emerge as a
jet on the far side, but the spread and propagation direction of the jet within the plasma is
not well known. By embedding several thin high Z layers in a CH film one can directly
image the progress of the electron beam. It loses enough energy to heat the medium
through which it travels to hundreds of eV. At that temperature a film, even buried under
CH, emits sufficiently hard thermal x-rays to allow imaging the heated area with an x-ray
pinhole camera. The film can be thin enough to also see the emissions from another layer
near the front of the film. If these two images are 1p4Isible simultaneously, one can
measure the beam spread and propagation direction within the plastic.
The fast ignition (FI) inertial fusion concept  depends critically on the converting the
photon ignition pulse to a stream of -1 MeV electrons, and propagating them through a
dense plasma to heat a compact spot in the target. Conversion of the energy carriers from
photons to electrons is required because the plasma becomes opaque at a density of -1
g/cc, considerably before it reaches the >200 g/cc ignition volume. For these densities,
the plasma frequency, below which the plasma is strongly absorbing, is in the x-ray
range . To date, the direction of energy transport within such plasmas has been largely
assessed by looking at the jets emerging from the back of a foil up to a few hundred
microns thick . These jets are seen to propagate normal to the back surface; some data
shows that this is independent of the direction of the incident photon beam or the
orientation of the front surface . Some modeling suggests that the diameter of these
jets is strongly decreased by the presence of the back surface . So we need a more
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Key, M H; Stephens, R B; Koch, J & Pennington, D. X-ray imaging to characterize MeV electronics propagation through plastic targets, article, September 3, 1999; California. (digital.library.unt.edu/ark:/67531/metadc623302/m1/3/: accessed February 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.