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We analyze the results of a recent experiment performed at the PALS laboratory and concerning ablation pressure at 0.44 µm laser wavelength measured at irradiance up to 2 × 1014 W/cm2. Using the code “ATLANT,” we have performed two-dimensional (2D) hydrodynamics simulations. Results show that 2D effects did not affect the experiment and also give evidence of the phenomenon of delocalized absorption of laser light.
Hot electrons may significantly influence interaction of ultrashort
laser pulses with solids. Accurate consideration of resonant absorption
of laser energy and hot electron generation at a critical surface was
achieved through the developed physical and mathematical models. A
two-dimensional (2D) ray-tracing algorithm has been developed to
simulate laser beam refraction and Bremsstrahlung absorption with
allowance for nonlinear influence of a strong electromagnetic field.
Hot electron transport was considered as a straight-line flow weakening
by a friction force calculated in the approximation of the average
state of ionization. Developed models were coupled with the 2D
Lagrangian gas dynamic code “ATLANT” that takes into
account nonlinear heat transport. The developed program has been
applied to simulate irradiation of Al foils by picosecond laser double
pulses. Hot electron transport and heating resulted in thin foil
explosions. The transition from the exploding foil regime to the
ablative one with foil thickening has been simulated and analyzed at
various values of laser light intensity. In second series of
calculations we have modeled the interaction of a nanosecond iodine
laser with a two-layered target.
The electron-beam-pumped KrF laser installation GARPUN with a 100-J
output energy and long 100-ns pulse duration has been used to
investigate laser–target interactions in a broad range of laser
intensities for small (150 μm) and large (∼1 cm) irradiated
spots. For higher intensities (up to 5 × 1012
W/cm2), a conical shock wave was generated in condensed
matter by megabar pressure at the ablation front. It propagated with a
supersonic velocity in a quasisteady manner together with a conical
shock wave inside a target. Evaporated target material moving with a
velocity of ∼50 km/s formed an extended plasma corona of ∼5
mm length with an electron temperature of ∼100 eV. Emission spectra
of plasma have been investigated in the extreme UV range 120–250
Å. For lower intensities (108–109
W/cm2), planar shock waves in normal density air were
produced with initial velocities up to 4 km/s in the forward
direction and 7 km/s in the opposite direction toward incident
radiation. In rarefied air, the forward shock wave kept velocities
constant whereas the backward ones were accelerated up to 30 km/s.
Planar compression waves in transparent condensed matter were also
demonstrated propagating with sonic velocity.
Formation of transverse inhomogeneities in the corona of a
solid target irradiated by an inhomogeneous main laser pulse
and a uniform background pulse was observed experimentally via
side-on shadowgraphy. The experimental results were successfully
interpreted using a two-dimensional hydrodynamics code. Our
simulations identified the onset of sharp contact boundaries
between plasma streams of different expansion velocities. The
formation and the decay of the contact boundaries is investigated
in detail. When the background pulse is used as a laser prepulse,
a layer of coronal plasma is formed that enhances main pulse
collisional absorption in underdense plasma and creates conditions
for an efficient thermal smoothing of the transverse inhomogeneities.
Laser prepulse effect on thermal smoothing of non-uniformities
of target illumination is studied by means of 2D Lagrangian
hydrodynamics simulation, based on parameters of real experiment.
A substantial smoothing effect is demonstrated in case
of an optimum delay between the prepulse and main heating
laser pulse. The cause of enhancement of thermal smoothing
effect by laser prepulse is the formation of long hot layer
between the region of laser absorption and ablation surface.
Comparison with experimental results is presented.
A possibility of input of high-power laser pulse into a
cavity through a hole was studied by means of 2D numerical
calculations. Such tasks appear in view of investigation
of the effective targets with internal input of energy
(Bessarab et al. 1992; Basov et al. 1998),
“cannon-ball” (Hogan 1989), “Greenhouse”
targets (Gus'kov et al. 1995).
We have used two Euler codes “NUTCY” and “FAKEL”
to model the problems of laser beam input into a cavity
through the holes.
Two series of experiments on laser irradiation
of the different thickness Al-foils were made using laser
facilities “GARPUN” and “PICO”
(Lebedev Physical Institute, Moscow). “GARPUN”
is the KrF-laser with pulse energy Elas
≈ 100 J and pulse duration τ ≈ 100 ns. “PICO”
is a Nd-laser facility. The laser energy is
Elas1 ≈ 20 J and τ ≈
3–4 ns in a single beam. The burn through time
(tb) of different thickness
foils was studied. We have varied the foil thickness:
d = 20–500 μm for “GARPUN”
facility experiments, and d = 3–12 μm
for the case of “PICO” experiments. It was
discovered that the rates of the foil burn through are
much higher than those obtained in (Dahmani et al.,
1991a,b). The experimental data were analyzed with the
help of 2D numerical simulations, using the 2D Euler code
“NUTCY.” Good agreement was obtained between
numerical and experimental results. In the first case the
rate of foil “enlightment” is defined by transversal
displacement of matter (“drilling effect”).
With allowance for the effect of “hot spots formations”
it was possible to explain the burn through of thick foils
and low laser energy at the rear side of films in “PICO”
facility experiments (“microdrilling effect”).
The methods of the diminishing of the influence of microdrilling
effect (or “imprint” effect) on the nonuniformity
of ablation pressure are discussed.
The results of the experiments at the installation
“PICO,” with thin foils heating by laser radiation
pulses of nanosecond duration are reported. The Al foils
with thickness in the range from 3 μ up to 40 μ
were used as targets. The flux density was varied from
1013 W/cm2 to 1014 W/cm2.
The sharp dependence of the portion of laser energy that
passed through the target on foil thickness was observed.
This phenomena was accompanied by a relatively small decrease
of the passed radiation pulse duration. The anomalously
high speed burning through of thin foil was observed in
these experiments and the conclusion on the possible mechanism
of this phenomena has been done on the base of comparison
of experimental data with theoretical calculations. The
observed phenomena can be interpreted with a conjecture
about the local burning through of a target, in the small
areas of the target surface, with many more values of flux
density than the average one following laser radiation
self-focusing and formation of “hot spots.”
Multistage, e-beam-pumped, 100 J-class KrF laser
installation “GARPUN” is described with the
emphases to high-power laser beam control and target irradiation
experiments. The ablation pressures in the megabar range
were measured and hydrodynamic flow was investigated both
experimentally and by numerical simulations for laser intensities
up to 5×1012 W/cm2, pulse duration
of 100 ns, and focal spot diameter 150 μm. Graphite-diamond
phase transformation under laser loading was observed by
dynamic and Raman scattering methods. Some approaches to
the fast ignition inertial confinement fusion, using the
simultaneous amplification of long and short laser pulses
in KrF drivers, are considered.
The development of hydrodynamic instability (HDI) in laser targets is studied by means of 2D numerical code “ATLANT.” The scaling of HDI development for the condition of “DELFIN-1” laser setup experiments is derived. It is shown that there is the possibility to improve the shell compression condition for the case of long-wave perturbation of laser flux (determined by particular target irradiation geometry) by using the shell with “relief.”
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