Laser Generation of 100-kbar Shock Waves in Solids

Originally published in Shock Compression of Condensed Matter 1991, S.C. Schmidt, R.D. Dick, J.W. Forbes, D.G. Tasker (editors) copyright 1992 Elsevier Science Publishers B.V.
Authored by Craig T. Walters

Neodymium-glass laser pulses (1.06-mm wavelength, 25-ns pulse width) have been used to generate shock waves with peak pressures in the 5- to 120-kbar range at the front surface of solids. Relatively uniform irradiance levels were employed with circular beam areas in the 0.4- to 1.5-cm2 range and single pulse energy up to 800 J (fluences ranged from 200-2000 J/cm2). At 1000 J/cm2, the resulting peak shock pressure is about 35 kbar. By confining the plasma with a transparent glass overlay, this peak pressure was raised to 120 kbar. The nature of the plasma initiation process has been revealed through careful simultaneous temporal resolution of the beam-power, temperature, and stress-wave details.

INTRODUCTION
High-power laser pulses have been used to produce stress waves in materials for more than two decades. Most of the measurements of stress wave amplitude have been performed with pulse fluences less than 100 J/cm2. Laser exposures with fluences many orders of magnitude greater than this have been conducted, but pressures have not been directly measured in these cases. We report here measurements of stress-wave amplitudes in laser interactions with fluences in the range 200-2000 J/cm2 with and without plasma confinement (transparent overlays). A map of pressures that may be achieved with single laser pulse interactions is presented in Figure 1 in terms of peak power density. The dashed line at high intensity follows the set of ablation pressures estimated by Cottet et al.1,2 in the correlation of thin aluminum foil spallation data. These estimates follow a 0.7 power dependence on intensity. Similar estimates made by Eliezer et al.3 and Gilath et al.4 are shown in Figure 1 by the dotted line for aluminum and the chain dashed line for carbon/epoxy at lower intensity. The open circles are peak pressures measured recently with 20- to 30-ns pulses at Battelle 5,6 at the front surface of stress gage packages coated with either graphite or carbon/polymer (black paint). These data agree with the estimate of Reference 3 at low intensity, but rise nearly linearly up to 5 x 1010 W/cm2 in contrast to the model. The solid circles show the effect of confining the plasma at these intermediate intensities with a transparent overlay. These pressures were in the 90 to 120 kbar range and are believed to be the highest directly recorded pressures generated in laser interactions with solids. The solid squares and triangles present the measurements of confined interactions by Fairand and Clauer7 and by Ballard et al 8 for 30-ns pulses. These data are highly consistent up to about 3 x l09 W/cm2 (90 J/cm2). For fluences in the 100- to 1000-J/cm2 range, the detailed nature of the transparent overlay probably takes on increased importance as breakdown processes interfere with energy delivery to the absorbing interface. Our data indicate that some benefit in increased pressure from a correctly designed overlay may be possible at even higher fluences than those investigated, contrary to the plateau seen in the Ballard data. Careful examination of the pressure histories with different coatings and with and without overlays has also revealed some detail of the plasma initiation process as discussed below.
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