Shock Loading and Impact Facilities

A. Shock Loading and Impact Cratering Apparatus

We have constructed five projectile launching, shock-loading facilities which are being exclusively employed for research on impact effects on rocks and minerals and equations of state of minerals.

a) 40 mm propellant gun (Fig. 1)

b) 90 mm/25 mm light gas gun (Fig. 1)

c) 20 mm/6 mm light gas gun (Fig. 2)

d) 20 mm propellant gun

e) 40 mm/13 m vertical drop tube
Fig. 1. General view of the Shock Wave Laboratory looking east. In the left of the photo the rear of the 40 mm propellant gun (a) is visible. A spare 25 mm light gas gun barrel rests on white supports, to the immediate right of the 40 mm gun. To the right, Dr. N. Evans and Mssrs. M. Long and E. Gelle are shown around the impact chamber of the light gas gun. Overhead is the 10 ton traveling bridge crane which allows gun assembly (e.g. Fig. 3).
The 40 mm apparatus (a) is used for low pressure (~50 GPa) equation of state shock temperature and recovery measurements. For minerals of shock impedance similar to anhydrite, shock pressures of ~30 to 40 GPa are produced by the direct impact of tungsten flyer plates. This gun is also set up for oblique impact, jetting, experiments. This gun launches 90 g projectiles at speeds of 0.01 to 2.5 km/sec with a compressed gas or propellant breech. Projectile velocity is measured to 0.5% using timed laser beam obscuration and flash radiometry methods. Instrumentation shared with the 90 mm/25 mm light gas gun (b) include a Velocity Interferometr System for Any Reflector (VISAR) to measure shock and release wave profiles, a high speed 6-channel radiative pyrmeter for measuring shock temperatures, and a TRW Image Converter Streak Camera and Beckman & Whitley rotating mirror streak camera with xenon light source for measuring shock velocity. Supporting instrumentation include Hewlett Packard 54111D, and Gould 4074 digitizing oscilloscopes. For the purpose of carrying out equation of state experiments on molten iron, silicates, oxides, and sulfides to 400 kbar, we are using a Lepel 10 KW radiofrequency induction heating power supply.
Fig. 2. The 20 mm/ 6 mm light gas gun. Dr. N. Evans is in foreground
The 20 mm/ 6 mm (d) light gas gun launches a ~1 gm projectile to speeds of 6.9 km/sec. This gun uses a magnetic detection system to measure velocity and is primarily used for impact cratering and shock devolatilization experiments

The Shock Wave Laboratory apparatus is being continually upgraded (Fig. 3).
Fig. 3. Photo of old 6" naval gun, previously used for a pump tube being removed from the Laboratory. This was replaced in 1991 by a shiny new barrel seen in Fig. 1. Below the 6" gun hanging from the sling is the 40 mm propellant gun.

B. Shock Temperature Apparatus

Our present pyrometer is a fiber optic image dissecting system conceptually improved device over that first described in Lyzenga and Ahrens (1979). It now records emitted light from 450 to 900 nm via 6, ~50 nm filtered photodiodes whose outputs are recorded with 250 MHz digital oscilloscopes and/or 100 MHz digital transient recorders to obtain absolute radiances with a time resolution of ~5-10 nsec. This apparatus can be mounted onto the impact chambers of either the 40 mm propellant (a) or 90 mm/25 mm two-stage light gas gun (b). The apparatus has been used to obtain shock temperatures on mineral samples as thin as 1.5 mm in the 2300 to 6000 K range and for metal films 500-5x106 backed with initially transparent materials.

C. VISAR (Velocity Interferometer for Any Reflector)

A schematic of the VISAR setup is shown in Fig. 1. Recently, we have upgraded this apparatus to employ fiber optics to make it usable for any of our 4 guns. In this device, a small time delay is introduced in one leg of the velocity interferometer by means of fused silica cylinders while the optical path difference between the two legs is kept close to zero. This allows interference fringes to be formed with a diffuse (spatially incoherent) source. Light from a 3 watt Ar+ laser is focused onto the target surface. The returned light is recollimated before passing through beamshaping and polarization optics. The light is then split, delayed and recombined at the beamsplitter, generating interference fringes which are recorded using photomultipliers and digital oscilloscopes. The VISAR we have constructed is similar to that originally described by Barker and Hollenbach [1972], except that it incorporates the push-pull modification and data reduction scheme of Hemsing [1979].
Fig. 4. Schematic of VISAR setup. The abbreviations are: PBS is polarizing beam splitter, S is polarization scramber, PMT is photomultiplier, P is polarizer, L is lense, F is filter, PZT is piezoelectric translator, M is mirror, and BS is beam splitter.
The VISAR employs a relationship between particle velocity and the number of interference fringes that can be expressed as [Barker and Hollenbach, 1972]:

u(t-t/2) = kF(t)

where u is the surface velocity, t is time, t is the characteristic delay time of the interferometer (1-2 ns), k is the fringe constant in velocity-per-fringe, and F is number of fringes recorded. For the VISAR, k depends on the laser wavelength, the speed of light, the stress-induced change in refractive index of the window material, as well as the thickness, index of refraction, and dispersion relation of the etalon material. The fringe constant, and hence the surface velocity, is constrained to within 1%. The time resolution of the present VISAR is estimated to be 2-3 nsec.
Former graduate students, Wenbo Yang and Kathleen Holland adjust laser detector in Lindhurst Shock Wave Laboratory
In order to compare the properties of minerals at very high pressures and temperatures with the properties of various zones of the Earth's interior, as measured mostly via seismological methods, a program of shock wave research on a wide range of Earth materials is conducted in the Shock Wave Laboratory. Both high-pressure environments and the conditions which exist upon meteorite impact on both silicate and icy planetary surfaces are being studied with four projectile launching (gun) facilities in the Lindhurst Laboratory. These include two one-stage propellant guns and two two-stage light-gas guns. The guns are used to impact minerals with projectiles at speeds varying from 0.1 to over 7 km/sec. Depending on the density of the impactor plate and sample material, dynamic pressures associated with the resulting shock wave range in amplitude from 10 kbar to 4000 kbar. Thus, the behavior of minerals in the pressure range extending from the base of the Earth's crust to very nearly the center of the Earth are being actively studied.

High-speed streak camera recording and flash X-ray photography apparatus are used to measure kinematic properties of the flow associated with the shock wave and hence, the pressure and density. High-speed radiometers are in active use in the laboratory to measure shock temperatures. Facilities for producing impact craters and recovery of intensely shocked materials support active programs of impact cratering, dynamic tensile strength measurements, shock compaction, and impact metamorphism. Samples may be preheated to temperatures of 2000 K, as well as precooled to 20 K, allowing compression studies of both silicate magmas and planetary ices.
Former Planetary Science graduate student, Sarah Stewart-Mukhopadhyay proudly displays her first shock attenuation experiment on 4090 porous ice being conducted with a 120 K sample. The experiment employs an electromagnetic recording of shock waves in ice at stresses less than 1 MPa.