Temperature Measurements at the Focus of a Converging Spherical Shock Wave

Abstract

Converging shock waves generate high energy concentrations at the implosion focus and have been studied since Guderley’s pioneer analytical treatment [1]. The similarity solution for converging cylindrical and spherical shocks describes a continuously accelerating shock with ever-increasing temperature and pressure behind the shock front as it approaches the focal singularity. In reality, however, the acceleration and achievable energy concentrations seem to be limited by the symmetry and stability of the shock wave. While an expanding shock is stable, the converging is not. This means that even an asymmetric explosion in a confined space will eventually result in a smooth expanding, and at the same time, weakening shock. Nevertheless, extreme conditions are created as the wave converges at the focus and reflects. The resulting temperatures are so high that the compressed gas becomes radiating. The light emission enables temperature determination from spectrometric measurements. Experimentally, cylindrical converging shock waves were first produced by Perry and Kantrowitz [2] in an annular shock tube with a tear-formed inner body. Further experiments in tubes following the basic principles of their design have been made [3–5]. A special attention was given to the problem of shock wave stability during the implosion. It was shown that the form of the shock is of pivotal importance for its stability. By forming dynamically stable initially polygonal shock fronts, it was possible to produce experimentally extreme and repeatable conditions in gas at the focal region of cylindrical converging shocks [4, 5].