A laser-triggered spark gap model

Laser preionization is a method whereby the formative lag time and jitter of spark gaps can be reduced, and breakdown can be initiated at voltages significantly below the self-breakdown value. In the spark gaps of interest (100-ns spark duration), the plasma column expands from an initial laser preionized diameter of approximately 50 μm to an arc of diameter in excess of 1 mm, and conducts greater than 12-kA peak current. Since the time required for the resistive collapse of the plasma is comparable to the spark duration, the spark gap represents a non-negligible resistive loss in the circuit. The objective of this study is to understand the basic mechanisms by which laser-triggered spark gaps develop and to provide a basis to optimize their design and minimize their resistive losses. To study the expansion and conduction phases of laser-triggered spark gaps, a first principles model has been developed. This model includes gas dynamics, electron collision kinetics, radiation transport, and external circuitry in a self-consistent formulation. The formulation of the model is discussed and results are compared to experimental data. We find that growth of the spark column is dominated by gas dynamic expansion of the hot ionized core, augmented by photoionization and thermal ionization at the plasma column boundary. The plasma column is confined within a high-density cylindrical shell of neutral gas that traps ionizing radiation in a region of low E/N (electric field divided by gas density), thereby inhibiting expansion by nonhydrodynamic means (electron avalanche).