Studies of Laser-Driven Shock Waves in Aluminum

Abstract

We have observed the structure and velocity of laser-driven shock waves in aluminum foils. We have measured shock velocities as high as IS km/s and shock luminosity risetimes less than 50 ps, and we have inferred pressures of 200 GPa and shock-fvont thicknesses < 0.7 jum. These results suggest that such techniques may be used for measuring equation-of-state parameters and studying the detailed structure of shock fronts.The development of high-power pulsed lasers and high-sensitivity, ultrafast streak cameras has opened possibilities for studying hydrodynamic phenomena in a pressure and temperature regime previously obtainable only with nuclear explosives. We have initiated a research pro-gram exploiting these new tools for investigation of temporal and spatial shock structure and measurement of equation-of-state (EOS) parameters. In this Letter we report the results of experiments in which, for the first time, laser-driven shock waves are observed by their own luminosity. We include measurements of shock velocity, shock risetime and luminosity structure, and material motion.Several experimenters have used high-energy lasers to measure EOS parameters. Van Kessel and Sigel^'^ observed shock waves traveling through thick, transparent samples by directing a second laser through the sample onto a camera, and they were able to deduce pressures, densities, and temperatures behind the shock front. Billon et al.^ photographed target blowoff with a second laser and deduced an average shock velocity in the target by neglecting the shock-formation time and assuming that the shock traversed the entire thickness of the foil. For thin, opaque targets, where we cannot use another laser to sidelight the shock front, we chose to measure the arrival of the shock at the target surface by observing the shock luminosity at two depths in a target made with a thin evaporated layer covering half of the shock area. Figure 1 shows a schematic of the target. The laser energy is initially deposited near the surface of the foil (solid circle) where it generates hot electrons which in turn redeposit some of the energy throughout the nearby volume (dashed line). The sudden heating causes a shock wave to propagate through the target. If the temperature behind the shock front is high enough, we can observe the shock emergence first at the back of the substrate and later from the added layer.^ It is important that the substrate be thicker than the range of the fast electrons to minimize preheating, but it must also be thin enough to prevent a rarefaction from overtaking the shock before its emergence. Measurements by Giovanielli^ show that 90% of the hot electrons from laser pulses similar to ours are stopped by 8 iim of aluminum. For our experiments the substrate was 13 jLtm thick and the evaporated layer ranged in thickness from 2 to 5 jum. We also studied some targets without steps to learn about the time history of the light from them.The 1.06-jLtm neodymium-glass laser delivered from 20 to 40 J in 300 ps to the target inside a 1m-diam vacuum ...

Line-imaging velocimeter for shock diagnostics at the OMEGA laser facility

“…At OMEGA and these other facilities we have implemented variations and additions to the basic arrangement that are not included in the description above. Four of the most useful variations include: (1) schemes to lengthen the duration of the probe pulse; (2) schemes for simultaneous dual wavelength operation, for example at the fundamental (1064 nm) and second harmonic (532 nm) of the probe laser; (3) the addition of an optical recording channel using the existing imaging relay to record thermal luminescence data simultaneous with the VISAR data; and, (4) the addition of dove prisms to enable arbitrary image orientation on the detector.…”

“…The earliest laser-driven shock wave experiments studied shock propagation by detecting the thermal luminescence produced when a strong shock breaks through the surface of an opaque stepped target [1,2]. This technique requires the shock to be sufficiently strong that the thermal luminescence is readily detectable after breakout of the shock front; it has been utilized in a number of shock detection studies at high energy laser facilities [3][4][5][6][7].…”

Experimental evidence of the generation of multi-hundred megabar pressures in 0.26 μm wavelength laser experiments

“…Pressures well below 10 Mbar are currently obtained with traditional laboratory experiments; it appeared that this limit could be extended by using laser-driven shock waves (Veeser et al 1978;Trainor et al 1979). Recently, some short-wavelength laser experiments realized at 0-35 /xm (Yaakobi et al 1981) and at 0-26 urn laser wavelengths have shown the feasibility of easily increasing this range to reach pressures which can only be obtained in the vicinity of nuclear explosions (Ragan III 1980 and; unlike the laser driven shock wave method, this last method has the advantage of achieving the required accuracy necessary to compare, meaningfully, the experimental results and theoretical models of the equation of state of the materials.…”

Laser driven shock experiments at PALS

“…These include the measurement of ablation thickness using time resolved X-ray spectroscopy [3], recoil momentum measurements [4], layered target burn trough measurements [2], time resolved X-ray radiography [1], ballistic pendula [9], Faraday cups and plasma energy calorimeters [8], and shock velocity measurements [5,6]. Here we follow the last experimental method, taking advantage of the recent advancement in the generation of high quality shocks and in the measurements of shock velocities with stepped targets [25][26][27].…”