Single-shot spatially resolved characterization of laser-induced shock waves in water

We have developed an optical method for single-shot spatially resolved shock-wave peak-pressure measurements. A schlieren technique and streak photography were used to follow the propagation of the shock wave. The shock position r as a function of time was extracted from the streak images by digital image-processing techniques. The resulting r(t) curves were differentiated with respect to time to yield shock-wave velocities that were converted to shock pressures with the aid of the equation of the state of the medium. Features and limitations of the technique are demonstrated and discussed on the basis of measurements of shock-wave amplitudes generated by laser-induced breakdown in water. For this purpose, laser pulses of 6-ns duration and pulse energies of 1 and 10 mJ were focused into a cuvette containing water. Complete p(t) curves were obtained with a temporal resolution in the subnanosecond range. The total acquisition and processing time for a single event is ~2 min. The shock-peak pressures at the source were found to be 8.4 ? 1.5 and 11.8 ? 1.6 GPa for pulse energies of 1 and 10 mJ, respectively. Within the first two source radii, the shock-wave pressure p(r) was found to decay on average in proportion to r(-1.3?0.2) for both pulse energies. Thereafter the pressure dropped in proportion to r(-2.2?0.1). In water the method can be used to measure shock-wave amplitudes exceeding 0.1 GPa. Because it is a single-shot technique, the method is especially suited for investigating events with large statistical variations.