Laser heating of solid matter by light pressure-driven shocks Page: 4 of 12
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High-intensity laser matter interaction and energy transport by laser generated MeV
electrons are widely investigated, both for novel basic science and for applications which
include bright sources of multi-10 keV x-rays for probing high energy density matter ,
production of collimated proton plasma jets at energies up to almost 50 MeV [2, 3], and fast
ignition of inertial confined fusion . Understanding absorption of laser radiation at high
intensities, the production of MeV electrons, and energy transport with associated isochoric
heating is crucial to the development of these applications.
The conversion efficiency of the laser energy to MeV electrons has been measured to
reach > 30% at intensities of > 3 x 1019 Wcm-2 [5, 6]. Fast ignition experiments heating
a compressed plasma via a hollow cone indicated laser-to-plasma energy coupling efficiency
of - 20% [7, 8]. The formation of a hot surface layer by sub ps pulsed laser irradiation at
intensities > 1019 Wcm-2 has been reported by Nishimura et al.(2005) and by Theobald
et al.(2006), showing respectively a 0.5 pm thick hot layer at 1019 Wcm-2 and a 1.0 pm
thick hot layer at - 4 x 1020 Wcm-2. At such high intensities, the collisional range of
the hot electrons produced by absorption of the laser radiation is more than two orders of
magnitude greater than the thickness of the heated layer and the physics of the heating
challenges current understanding.
In this letter, we report a more detailed study of hot surface layer formation at intensities
~ 5 x 1020 Wcm-2 using K- shell spectroscopy of layered targets, monochromatic imaging
of K-shell emission and 2D PIC modeling. The hot surface layer is attributed to heating by
a light pressure -driven shock wave.
The experiments used the Vulcan Petawatt laser system at the Rutherford Appleton
Laboratory . The laser pulse had a wavelength of 1.054 pm, 0.8 ps duration, 400J energy
and a peak intensity of 5 x 1020 Wcm-2. An f/3 off-axis parabola focused the beam to
7 pm diameter at half peak intensity with - 30% of the energy in a 15 pm diameter and
the remaining energy distributed over wings. The amplified spontaneous emission (ASE)
contrast ratio was 10-7 in intensity and 10-4 in energy. The targets were 400 pm x 400 pm x
5 pm square foils of Mo with two thin tracer layers: a 0.5 pm thick Ni layer located at the
irradiated (front) surface or under various depths of Mo and a 1 pm thick V layer located
at the rear surface.
A spherically bent Bragg crystal imager [10, 11] normally used to image Cu K, (8.05 keV)
recorded nominally Ni Ly (8.07 keV) images, taking advantage of the quasi-continuum of
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Akli, K; Hansen, S B; Kemp, A J; Freeman, R R; Beg, F N; Clark, D et al. Laser heating of solid matter by light pressure-driven shocks, article, May 4, 2007; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc896559/m1/4/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.