A Hydrodynamic Study of Laser-Induced Vaporization of Aluminum in Vacuum
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A HYPROPINAMIIC STUinY nF LASEP-IHDnCFn VAPOP.I7ATInti
OF ALIIP1111?4 IM VAMUIRIp. n.A.
RROST*, C.L. RnHn**, Mq.L.
CRAIIFfnln**,
Ann R.w. 1t1ILLIIS**
*Frank J. Seller Research Laboratory, IISAF Academy, Colorado Sprinos, Cn 80040-652R
"**Depart of
Academy, Colorado .prings, Cn POP40-57ni Physics, I.USAF
ARSTPACT
The three-dimensional hydrodynamics of laser-Induced, steady state vaporization of aluminum (2024) in vacuim was investigated with the aid of both a hydrodynamic computer code and a Mach-7ender interferoneter. Provided the laser beam is sufficiently intense, a vapor plasma forms, resultinq in a sustained region of high pressure near the target. The pressure is distributed asymmetrically over the footprint in the case of oblique incidence. ItiPOnlhCTIltl
The interaction of an intense laser bean with a solid in vacuum has been a topic of considerable interest in recent years. Fxperimentalists have focused on measuring the salient effects inherent in such an Interaction, most of which concern plasma ignition, mechanical coupling, or thermal coupling. In reviewing this topic, Peilly has provided an extensive compilation of the experimental work.r[l Theoreticians have attempted to understand these effects using either one-dimensional or spherical vaporization models. Exanples of such work are the papers of Rasnv, et. al., and Hlelsen.j2,3] Notwithstandin RaBsov's hydrodynamic experiments,Q2] a conseauence of this overall approach is a dearth of information on the three-dimensional hydrodynamics of vacuum interactions,
especially those which develop when the laser beam strikes the target surface obl iquely. In this paper, we describe aspects of the three-dinensional hydrodynamics of laser-metal interactions in vacuum. A hydrodynamic computer code was used to study vacuum interactions in which a steady-state vapor plasma forms at the target surface, colocated with the bean footprint. The hydrodynamics predicted hy the code nualitatively agreed with those observed experimentally. We shalI show here that (1) the plasna forms a region of high pressure which hlows vapor outward away from the laser spot and which blows molten material out of the residual crater, and that (2) hydrodynamic asymmetries develop when the laser bean strikes the target surface ohlinuely, thereby creating a phenomenology more complicated than that of normal incidence. THEORETICAL Itf'VFTIGATIO!I Ve enployed
a
two-dinensinnal,
rectanqular,
Eulerlan
hydrodynamic
computer code to solve the eouations for conservation of mass, momentnm, and energy. This code has beer described in detail in earlier papers on the theory of laser-solid interactions in air.r',`51] The "vacuum" consisted of a rarified atmosphere with a pressure of 10-6 atmospheres and a density of 1n-6 of STP air. Accordingly, the temperature of the vacuum was "room temperature", a condition which would characterize a typical laboratory vacuum. The laser bean was modeled as a spatially and temporally uniform slab of O.. rm half-thickness, 30 t.IW/cmn intensity, Mat. Res
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