Simulation of optical breakdown in nitrogen by focused short laser pulses of 1064 nm wavelength

Abstract

A kinetic model of electron cascade growth in the electromagnetic field of a focused intense laser pulse as used for laser spark generation in gases has been numerically implemented in Visual C code. The effects considered comprise Drude absorption, diffusive kinetic and inelastic losses as well as (three-particle) electron recombination. The objectives were to illustrate the dynamic process of gas ionization, and to clarify the pressure dependence of known breakdown thresholds within a range of about 2 × 104 to 2 × 106 Pa of initial pressure. Two-dimensional (cylindric coordinates) simulations of the optical breakdown in nitrogen were conducted on a commercial PC, using constant values for the collision cross section (2 × 10−19 m2), prevalent electronic excitation states (~4.8 eV), and a laser wavelength of 1064 nm. A certain aerosol concentration on the order of 3 ppb was assumed in order to provide initial electrons for cascade growth. Exemplary results with laser pulse energy of 26 mJ, pulse duration of 14 ns and an 18 µm focal spot size illustrate the dynamic process of ionization within a very short time period of less than 0.5 ns. The kinetic energy of the electrons is found to increase sharply up to more than 100,000 K on breakdown. A series of simulations considered the minimum pulse energy of breakdown (MPE) under variation of initial pressure. Identical laser parameters as in experiments conducted previously were used and the results are in excellent agreement with respect to curve shapes, i.e., MPE ~1/p0.4 in the first experiment and MPE ~1/p0.3 in the second one. The absolute values lie within a factor of two, which is explained by model abstraction and input data uncertainties.

Laser-induced optical breakdown applied for laser spark ignition

“…The two steps leading to optically excited breakdown are, first (Hughes, 1975), the generation of a few free electrons 2 (Tartar et al, 2008) via linear absorption or thermionic processes of impurities like dust or aerosols 3 (multiphoton absorption, as mentioned in detail later, is considered to be of lesser importance). These electrons serve as initial receptors of energy through three-body collisions with photons and neutrals.…”