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An improved high luminosity, easily spectrally tunable backlighting scheme based on a spherically bent crystal is considered in this paper. Contrary to the traditional backlighting scheme, we used crystal far from normal incidence, and the backlighter source was inside the Rowland circle. With the presented configuration, we obtained a spatial resolution up to 8 µm in the desired direction with an X-ray backlighting energy close to 5 keV. Detailed discussions and ray-tracing calculations show that with this convenient scheme resolution down to 5 µm can be achieved. A dedicated application to high energy density physics is presented: the radiography of shock compressed matter.
In this article, we present a laboratory astrophysics experiment
on radiative shocks and its interpretation using simple modelization.
The experiment is performed with a 100-J laser (pulse duration of about
0.5 ns) which irradiates a 1-mm3 xenon gas-filled cell.
Descriptions of both the experiment and the associated diagnostics
are given. The apparition of a radiation precursor in the unshocked
material is evidenced from interferometry diagrams. A model
including self-similar solutions and numerical ones is derived
and fairly good agreements are obtained between the theoretical
and the experimental results.
We have studied the interaction of soft X-ray thermal radiation
with foam-layered metal targets. The X-radiation was produced
by focusing a high energy laser inside a small size hohlraum.
An increment in shock pressure was observed with the foam layer
as compared to bare metal targets.
The paper discusses recent advances in the use of foams in
laser–plasma experiments, concerning in particular: (1)
the use of foam in order to get an efficient smoothing of laser
energy deposition, (2) the problem of hydrodynamics of layered
foam-payload targets, (3) the use of foam for shock pressure
amplification in equation-of-state experiments, (4) the study
of the equation of state of foams in the Megabar regime, and
(5) the use of foams for astrophysics relevant experiments,
here in particular shock acceleration experiments.
We present some preliminary results on the equation of
state (EOS) of water in a pressure regime of astrophysical
interest. In the experiments, structured targets made of
an aluminum step followed by a water layer are irradiated
by the laser at an intensity up to 4·1014
W·cm−2 to generate a shock wave.
Velocities are measured in the two materials using a VISAR
interferometric diagnostic for water, and a streak camera
to measure target self-emission for Al. EOS points for water
are obtained with the impedance mismatch method using Al EOS
as a reference. Water reflectivity was also measured.
We studied the influence of foams on laser produced
shocks. Experiments were performed at LULI using a Nd laser
converted to second harmonic, and at MPQ (Max Planck Institut
für Quantenoptik) using the iodine Asterix laser converted
to third harmonic. In both cases, sub-ns lasers with pulse
energies of several tens of joules were focused on large
focal spots (hundreds of microns) to reduce 2D effects.
The laser beams were optically smoothed with phase zone
plates (PZP) and directly focused on layered targets made
of a foam layer on the laser side and a stepped Al layer
on the other side. A visible streak camera was used to
detect shock breakthrough at the base and at the step of
the Al target, allowing shock velocity to be determined.
Using the well known SESAME Al equation of state, we determined
shock pressure. A stronger pressure increase was measured
when foam was present, compared to what was obtained by
focusing the laser beam directly on the Al target. This
was due to the impedance mismatch effect at the Al-foam
The first experimental study of the propagation of electrons
created by an intense laser in shock-compressed matter has
been performed with the VULCAN laser facility at the
Rutherford Appleton Laboratory, to investigate one of
the fundamental phases of the fast ignitor concept for
inertial confinement fusion. Plastic plane targets were
irradiated on one side with two pulsed laser beams, each
with I ≈ 1014 W/cm2,
t ≈ 2 ns, E ≈ 80 J per pulse, to
generate a planar shock wave; on the opposite side of the
target, a chirped pulse amplification (CPA) laser beam
(I ≈ 1016 W/cm2,
t ≈ 3 ps, E ≈ 10 J) was focused
to generate the fast electrons. The results show an increase
of hot electron penetration in compressed matter with respect
to an ordinary one. Experimental results have been analyzed
with computer simulations.
Experimental results of heating measurements of
matter carried out in a study of laser-driven shock waves
in aluminum (Batani et al. 1997) are discussed.
The measured temporal evolution of the “color”
temperature of the rear surface of the target is compared
with that computed by a numerical code. It has been established
that the target preheating can substantially affect optical
signal features recorded from the rear side of the target,
and consequently influence upon the accuracy of temperature
and pressure measurements of the material behind the shock
Time- and space-integrated emission spectra measurements have been performed in plasma produced by 308 nm wavelength XeCl laser radiation (IL = (4–10)·1012 W/cm2, τ = 10 ns) and by 248 nm wavelength KrF laser pulse train radiation (IL = 5·1015 W/cm2, τ = 7 ps, 16 pulses in train) on CF2 plane target. Theoretical modelling of Lyman series and He-like ion resonance series of fluorine and its fit of experimental data show considerable differences in the absorption of laser radiation in the two plasmas.
We have analyzed the shock wave propagation experiments performed at LULI and presented at ECLIM'94. The targets were aluminium foils with thickness from 5 to 25 μm. Simulations were performed with the SARA-1D multigroup radiation code. We have shown a small level of preheating caused by the absorption of X-rays with energies close to the K-edge of aluminum. Several sets of opacities were used in order to study this effect, including experimental values for cold aluminum. Simulations show a small level of visible emission induced by X-ray preheating before the arrival of the shock.
Experimental results on the determination of the color temperature in shock waves produced with lasers are presented. The method is based on imaging the target rear side in two different spectral windows and on using phased zone plates to produce high-quality shocks. The shock velocity is also measured, allowing, with the use of the equation of state, the real shock temperature to be deduced and compared with the measured color temperature.
Experimental results are presented on shock-wave generation in solid samples, irradiated directly by optically smoothed laser beams. Random phase plates and phased zone plates have been successfully used. In particular, the last technique allowed the production of uniform shock fronts that have been used for equation of state experiments at pressures above 10 Mbar. Pressures higher than 35 Mbar were achieved in gold, by using laser pulses with energy E ≈ 100 J, and structured, two-step, two-material targets.
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