Abstract :
Laser-initiated breakdown of high-voltage spark gaps has been extended to the megavolt range through the use of a novel coaxial triggering geometry. In this configuration, a 100-to 250-megawatt ruby laser of 1.4- to 3.5-joules output was aligned along the interelectrode axis of the spark gap. The laser beam passed through a hemispherical electrode, mounted on a hollow shaft, and was focused by a lens internal to this electrode at the opposite-switch electrode surface. Based on investigations of the effect of polarity on switch performance, i.e., least delay and jitter, it was concluded that 1) irradiation of the charged electrode was preferred and 2) irradiation of positive-rather than negative-charged electrodes gave best performance. Delay times between arrival of the laser pulse and complete gap closure as short as 2 ns with unmeasurable jitter (< 1 ns) were readily attainable under various conditions (high-pressure and high-reduced fields). By recourse to classical arc breakdown theories, i.e., Townsend avalanche and streamer mechanisms, it was concluded that the variation of delay with reduced field follows an avalanche process. However, for an explanation of extremely short delays observed (high-velocity closure rates), a streamer mechanism is necessary. Photographs of laser-triggered breakdown gave consistently single-channel straight arc breakdown with the absence of lossy arrested discharges, while self-breakdown or mechanically triggered breakdown produced longer and irregular breakdown channels with numerous energy-absorbing arrested leaders. This observation is in concert with the extremely short delays observed and leads us to suspect that volume dielectric breakdown or at least arc channel preparation is responsible for the observed results.
