Interferometric, Shadow and Schlieren Studies of Low-Inductance Laser-Triggered Air Sparks in the Nanosecond Regime

In this study we used a transversely excited atmospheric (TEA) nitrogen laser in an oscillator-amplifier configuration to study a low-inductance, atmospheric, air spark for the first 40 nanoseconds of its evolution. The spark was triggered by the same laser used to photograph the events after a time-of-flight delay. We measured the pulse duration of the UV nitrogen laser using streak photography to be about one nanosecond. This configuration gave us a powerful tool to study the spark channel development in the nanosecond regime. The radial expansion of the shock front of an electrical spark in atmospheric air was studied in a narrow gap (0.5 mm) by shadow and schlieren photography using the TEA nitrogen laser ((lamda) = 337.1 nm). Within the first 25 ns of expansion the diameter of the channel grew linearly with time and the index of refraction is less than that in the surrounding gas. The shock speed was measured to be about 7 km/s, which is about 20 times the speed of sound in air at room temperature. Formative time lag and jitter were investigated for two types of laser -triggered spark gaps. The applied voltage was 3 kV. Formative time lags of down to about 3 ns for copper electrodes and 6 ns for stainless steel electrodes were measured. For a constant gap spacing we found that jitter was small only when the spark gap was operated close to its static breakdown voltage, the triggered-laser light was focused on the cathode of the gap, and the nitrogen laser energy exceeded some minimum. We used an interferometric method with the UV nitrogen laser radiation for electron density measurement in the spark channel as a function of radius and time. Interferograms of our spark channel reveal a large negative fringe shift in the channel. The negative fringe shift is due to electrons. The electron density and the index of refraction profiles of the spark channel as a function of radius and time were recovered from the observed axial fringe shift with the use of an Abel inversion. The peak electron density in the center of the channel reached a value of about 6.5 x 10('19) cm('-3) at about 8 nanoseconds after initiation. (Abstract shortened with permission of author.).