Decay of femtosecond laser-induced plasma filaments in air, nitrogen, and argon for atmospheric and subatmospheric pressures

The temporal evolution of a plasma channel at the trail of a self-guided femtosecond laser pulse was studied experimentally and theoretically in air, nitrogen (with an admixture of ∼3% ${\mathrm{O}}_2$), and argon in a wide range of gas pressures (from 2 to 760 Torr). Measurements by means of transverse optical interferometry and pulsed terahertz scattering techniques showed that plasma density in air and nitrogen at atmospheric pressure reduces by an order of magnitude within 3–4 ns and that the decay rate decreases with decreasing pressure. The argon plasma did not decay within several nanoseconds for pressures of 50–760 Torr. We extended our theoretical model previously applied for atmospheric pressure air plasma to explain the plasma decay in the gases under study and to show that allowance for plasma channel expansion affects plasma decay at low pressures.

Figure 1

Experimental setup. The inset shows the images of the filament at different pressures in air.

Figure 2

Temporal evolution of electron density (a) in Ar and [(b)–(d)] in ${\mathrm{N}}_2$ for various gas pressures. Symbols in (a) and closed circles in (b)–(d) correspond to interferometric measurements; the other symbols correspond to terahertz measurements and curves correspond to calculations. Calculations and terahertz measurements were made with considering [solid curves and open circles in (b)–d)] and neglecting [dashed curves and crosses in (c) and (d)] channel expansion. Vertical black straight lines in (d) are error bars for terahertz measurement data. The error of interferometric measurements may be estimated to be about 20% of experimental data statistics in (a).

Figure 3

Temporal evolution of electron density in air for (a) 50 and (b) 2 Torr. Closed circles correspond to interferometric measurements; the other symbols correspond to terahertz measurements and curves correspond to calculations. Calculations and terahertz measurements were made with considering (solid curves and open circles) and neglecting (dashed curves and crosses) channel expansion.

Figure 4

Plasma density at $t=0$ (crosses), 2 (circles), and 7 ns (diamonds) as a function of pressure for (a) air and (b) nitrogen ($t=0$ corresponds to time of filament creation). Symbols correspond to experimental measurements. Dashed and solid curves correspond to calculations for 2 and 7 ns, respectively. The data are obtained with allowance for plasma expansion.

Figure 5

Theoretically calculated temporal evolution of electron temperature in ${\mathrm{N}}_2$ (solid lines) and air (dashed lines) for various gas pressures.

Figure 6

Theoretically calculated temporal evolution of gas temperature in ${\mathrm{N}}_2$ (solid lines) and air (dashed lines) for various gas pressures.

Figure 7

Theoretically calculated temporal evolution of electron loss frequencies for different channels of electron loss in ${\mathrm{N}}_2$ for (a) 2, (b) 50, and (c) 760 Torr: dissociative recombination with ${{\mathrm{N}}_2}^{+}$ (1), dissociative recombination with ${{\mathrm{O}}_2}^{+}$ (2), three-body recombination (3), dissociative recombination with ${{\mathrm{N}}_4}^{+}$ (4), dissociative recombination with ${\mathrm{N}}_2{{\mathrm{O}}_2}^{+}$ (5), and dissociative recombination with ${{\mathrm{O}}_4}^{+}$ (6).

Figure 8

Theoretically calculated temporal evolution of electron loss frequencies for different channels of electron loss in air for (a) 2 and (b) 50 Torr: dissociative recombination with ${{\mathrm{N}}_2}^{+}$ (1), dissociative recombination with ${{\mathrm{O}}_2}^{+}$ (2), three-body recombination (3), dissociative recombination with ${\mathrm{N}}_2{{\mathrm{O}}_2}^{+}$ (4), and dissociative recombination with ${\mathrm{H}}_2\mathrm{O}·{{\mathrm{O}}_2}^{+}$ (5).