W. Grigsby

Laser-induced spallation of aluminum and Al alloys at strain rates above 2×106s−1

Material microstructure is a significant determinant of the tensile stress at which materials fail. Using a high-energy laser to drive shocks in thin slabs, we have explored the role material microstructure plays on the spall strength of high-purity and alloyed aluminum at strain rates of (2–7.5)×106s−1. Slabs of pure recrystallized Al and recrystallized or cold worked Al+3wt% Mg were shock driven using the Z-Beamlet Laser at Sandia National Laboratories. Velocity interferometer measurements determined the spall strength of the materials, and postshot target analysis explored the microscopic fracture morphology. We observed the greatest spall strength for large-grained, recrystallized high-purity aluminum, with the dominant failure mode being ductile and transgranular. We observe for the first time at these strain rates fracture features for a fine-grained Al+3wt% Mg that were a combination of brittle intergranular and ductile transgranular fracture types. Postshot analysis of target cross sections and hy...

Picosecond time scale dynamics of short pulse laser-driven shocks in tin

The dynamics of high strain rate shock waves driven by a subnanosecond laser pulse in thin tin slabs have been investigated. These shocks, with pressure up to 1 Mbar, have been diagnosed with an 800 nm wavelength ultrafast laser pulse in a pump-probe configuration, which measured reflectivity and two-dimensional interferometry of the expanding rear surface. Time-resolved rear surface expansion data suggest that we reached pressures necessary to shock melt tin upon compression. Reflectivity measurements, however, show an anomalously high drop in the tin reflectivity for free standing foils, which can be attributed to microparticle formation at the back surface when the laser-driven shock releases.

LASER‐INDUCED SPALL OF ALUMINUM AND ALUMINUM ALLOYS AT HIGH STRAIN RATES

We conducted laser‐induced spall experiments aimed at studying how a materials microstructure affects the tensile fracture characteristics at high strain rates (>106 s−1). We used the Z‐Beamlet Laser at Sandia National Laboratory to drive shocks and to measure the spall strength of aluminum targets with various microstructures. The targets were recrystallized, high‐purity aluminum (Al‐HP RX), recrystallized aluminum + 3 wt.% magnesium (Al‐3Mg RX), and cold‐worked aluminum +3 wt.% magnesium (Al‐3Mg CW). The Al‐3Mg RX and Al‐3Mg CW are used to explore the roles that solid‐solution alloying and cold‐work strengthening play in the spall process. Using a line‐VISAR (Velocity Interferometer System for Any Reflector) and analysis of recovered samples, we were able to measure spall strength and determine failure morphology in these targets. We find that the spall strength is highest for Al‐HP RX. Analysis reveals that material grain size plays a vital role in the fracture morphology and spall strength results.

Diagnosis of MBAR Laser Produced Shocks in Tin Using Short Pulse Probes

We are studying shock induced melting using laser produced shock waves in tin foils. The diagnostics used for these studies include pump‐probe reflectivity and interferometry using 40fs pulses. Rear surface expansion data from the interferometer suggests that we have reached pressures necessary to shock melt tin upon compression. We have seen no unambiguous change in the reflectivity data to date.

High Repetition Rate kJ-class Nanosecond to Femtosecond Lasers

Using novel liquid cooled slab laser amplifier technology we have developed laser systems capable of amplifying nanosecond laser pulses to energy of ~1 kJ at repetition rate up to 0.1 Hz. The design and performance of these liquid cooled amplifiers at 18 cm aperture will be described along with plans to scale this technology to larger aperture and higher repetition rate.

Development of third harmonic generation as a short pulse probe of shock heated material

We are studying high‐pressure laser produced shock waves in silicon (100). To examine the material dynamics, we are performing pump‐probe style experiments utilizing 600 ps and 40 fs laser pulses from a Ti:sapphire laser. Two‐dimensional interferometry reveals information about the shock breakout, while third harmonic light generated at the rear surface is used to infer the crystalline state of the material as a function of time. Sustained third harmonic generation (THG) during a ∼100 kbar shock breakout indicate that the rear surface remains crystalline for at least 3 ns. However, a decrease in THG during a ∼300 kbar shock breakout suggests a different behavior, which could include a change in crystalline structure.

Time-resolved third harmonic generation from laser- melted semiconductors

To develop shock melting diagnostics, we are studying laser melted semiconductors using nonlinear optical probes. We find a rapid response in THG from Si and GaAs, with both linearly and circularly polarized incident radiation.

Dynamic Acceleration Effects in Explosions of Laser Irradiated Heteronuclear Clusters

Based on the model for heteronuclear cluster explosions developed by Last and Jortner, we present experimental confirmation of the theoretically predicted ion energy enhancement of the light ion species in methane clusters