Time Lag between High-Speed Pellets and the Ionization in Their Trails

Abstract

An investigation of the spatial relation between high-speed projectiles fired at velocities of from 1 to 2.5 km per sec shows that the ionization and light in their trail does not occur at a point along the projectile path at the same instant the projectile passes. The time lag has been measured by letting the projectile and ionization in its trail obstruct the transmission of microwaves through a short length of wave guide. The time lag between the pellet and the ionization was observed for magnesium, aluminum, and titanium projectiles, but attempts to obtain the same data for copper, steel, and zinc were unsuccessful because these materials did not seem to produce ionization in their flight at the velocities being investigated. Time lags of up to 1000 μsec were measured at the lower velocities for aluminum projectiles. At velocities of 2.5 km per sec no time lag was observed. That is, the ionization occurred immediately as the pellet passed. Measurements of the luminosity associated with these pellets by means of photocells indicated that the ionization and the luminous region occurred simultaneously. The variations of this time lag as a function of velocity is explained by some thermodynamic properties of the metal in the pellet.