TWO STAGE GAS GUN

We use large guns in the Shock Physics Group to generate very high pressures. The guns accelerate a projectile (bullet) to velocities up to 8 km/s, or about 18000 mph. This is about 70% of escape velocity from the Earth. When these projectiles hit a sample in our target chamber, a shock wave is produced in the sample. These shock waves produce very great pressures, up to 700 GPa (7 Mbar = 7 million times atmospheric pressure, or roughly 100 million psi). Not only is the pressure very high, but the shock wave also produces temperatures of up to 15000 K and can increase the density many fold. Under these extraordinary conditions, we can observe remarkable changes in the physical properties and chemistry of liquids and solids. Besides being of fundamental interest in understanding how materials behave under these conditions, the conditions we make in the samples are typical of planetary interiors (the core of the Earth, for example), and powerful explosions.

The variety of materials we study and the changes in them require a wide range of experimental techniques. All of them must be capable of making a measurement in a very short time, a few to several hundred nanoseconds ( 1 nanosecond = 10-9 sec). In our experiments, we can measure shock velocity, electrical and thermal conductivity, and the velocity of the shocked material (mass velocity). We also use a wide range of optical and spectroscopic methods to measure temperature, and to observe molecular vibrations (Raman spectroscopy), Mie scattering, absorption, emission spectroscopy, and optical emissivity. We also employ fast streak and framing photography. In addition, we also can recover intact samples shocked to high pressures. The most fundamental measurement we make is the equation of state, the relationship between pressure, density, and energy or temperature. Unlike sound waves, the velocity of a shock wave increases with the strength of the shock. By measuring the shock velocity and the speed of the projectile, we use simple equations that are derived from the conservation of mass, momentum, and energy to find the pressure, density and energy behind a shock wave. The locus of final states of shocks with varying strength, the Hugoniot, expresses how the pressure, density and energy (the equation of state) change with shock strength. We can also use a variety of methods to determine the properties of materials that are not on the Hugoniot, such as quasi-isentropic compression.

For more information on this project, please contact Neil Holmes.