Using a Voitenko accelerator [1-3], a series of experiments were performed with the goal of attaining shock velocities in gases approaching 90 km/s. Typically, the basic apparatus comprises a hemispherical bowl filled with a gas at atmospheric pressure; a metal piston across its diameter; and a small bore evacuated shock tube at its apex, Fig. 1. The evacuated shock tube is separated from the gas bowl by a thin diaphragm. A combination of a plane wave explosive lens and a high explosive pad accelerates the piston to a velocity of the order of 4 km/s and subsequently compresses the gas in the bowl. The thin diaphragm at the other end of the bowl then ruptures and the high pressure (shock compressed) gas escapes into the shock tube.