Temporally shaped current pulses on a two-cavity linear transformer driver system

An important application for low impedance pulsed power drivers is creating high pressures for shock compression of solids. These experiments are useful for studying material properties under kilobar to megabar pressures. The Z driver at Sandia National Laboratories has been used for such studies on a variety of materials, including heavy water, diamond, and tantalum, to name a few. In such experiments, it is important to prevent shock formation in the material samples. Shocks can form as the sound speed increases with loading; at some depth in the sample a pressure significantly higher than the surface pressure can result. The optimum pressure pulse shape to prevent such shocks depends on the test material and the sample thickness, and is generally not a simple sinusoidal-shaped current as a function of time. A system that can create a variety of pulse shapes would be desirable for testing various materials and sample thicknesses. A large number of relatively fast pulses, combined, could create the widest variety of pulse shapes. Linear transformer driver systems, whose cavities consist of many parallel capacitor-switch circuits, could have considerable agility in pulse shape. We will show results from initial experiments in pulse shaping on a system with two inductively isolated cavities in series. Each cavity contains forty pairs of high voltage capacitors and forty gas-insulated spark gap switches. The capacitors are arranged in a bipolar configuration; the spark gap switches must withstand twice the capacitor voltage. A pulse applied to the switch trigger electrodes initiate closure of each switch. We have arranged triggering in groups of ten switches in each cavity, for a total of eight separate trigger points in the system. The fundamental rise time of each capacitor circuit is roughly 70 nanoseconds; this defines the fastest possible output pulse transition time. The pulse rise time can be made longer, and given features on the rise, by delaying triggers to- some of the switches. We will show initial experimental results from tests of the two-cavity system.