Optimal laser-pulse energy partitioning for air ionization

We investigate the pulse partitioning of a 6.3-mJ, 450-fs pulse at 1030 nm to produce plasma channels. At such moderate energies, splitting the energy into several subpulses reduces the ionization efficiency and thus does not extend the plasma lifetime. We numerically show that when sufficient energy to produce multifilamentation is available, splitting the pulse temporally in a pulse train increases the gas temperature compared to a filament bundle of the same energy. This could improve the mean free path of the free electrons, therefore enhancing the efficiency of discharge triggering.

Figure 1

Experimental setup. Blue WP, half-wave plate; green PBS, polarizing beam splitter.

Figure 2

Effect of energy partitioning on the ionization yield. The delay between the pulses is 4 ns and the electrode gap is 3 cm.

Figure 3

Free electron lifetime when excited by a double pulse (0.5-ns delay), as a function of the relative intensity between the two pulses, in a 3-cm gap under 15 kV.

Figure 4

Free electron lifetime as a function of their initial density. The red dots are the experimental data points and the blue squares the simulation.

Figure 5

(a) Free electron lineic density and (b) heavy species temperature reached with respect to time for one laser pulse of 6.3 mJ or two laser pulses of, respectively, 3.15 mJ and 6.3 mJ each.

Figure 6

(a) Free electron lineic density on the beam propagation axis and (b) heavy species temperature with respect to time for a bundle of 10 filaments and a train of 10 subpulses of 63-mJ total energy.