Subnanosecond interferometry of plasma filaments in a laser-produced spark

Subnanosecond interferograms with high spatial resolution have been made of the sparks produced in several gases by the output of a single-mode ruby laser. This has resulted in the observation of a plasma filament having transverse dimensions an order of magnitude less than the diameter of the focal region, propagating toward the laser, prior to the main plasma formation. The development of this filament is observed, and its production by a self-focusing mechanism is discussed.