Experimental schematic. The setup consists of a pair of electrodes, the high-voltage pulse generator, a fast-gated ICCD camera for observation, and the femtosecond laser focus closely aligned with and focused between the electrodes.
Schematic of high voltage power supply.
Voltage waveform with parameters of the waveform indicated (left). Typical voltage-current-power characteristics of electrical pulse without laser (right).
Top images: raw image of guided plasma filament (a); optical scheme for three-projection image acquisition (b). Bottom line: discharge path reconstructed from three projection images. Reconstructions without guiding laser (c), and with fs guiding laser with the approximate laser focus shown in red (d).
Schlieren (a) and camera (b) images of the streamer phase of the discharge development. The schlieren photo is taken 500 μs after the shot with an exposure of 0.1 μs. The third image (c) is taken after the laser pulse and shows the strong streamer along the laser-designated path. Both images of the streamer development are captured with the high voltage pulse just below the breakdown threshold.
Voltage waveforms (a) without laser guiding where strong oscillations are the result of streamer branching, and (b) with laser guiding τd = 9.5 μs, where no strong oscillations are evident.
Images of the first stage of the HV discharge propagation at fs laser guiding. Xf = 10 mm; (a) τd = 1 μs; (b) τd = 3 μs, attached filament; (c) τd = 3 μs, filament a bit aside, (d) τd = 5 μs; (e) τd = 80 μs; (f) τd = 150 μs.
Breakdown voltage vs delay time between laser pulse and HV pulse maximum. (a) Air at atmospheric pressure, (b) nitrogen of >0.99 purity. “Nearby electrode” indicates that the axial distance from electrode to top end of the laser filament is Xf ≤ 5 mm; “out of electrode” indicates Xf ≥ 10 mm.
One-dimensional axisymmetric Reynolds-averaged Navier-Stokes simulation of a modeled energy deposition showing (a) Radial density profiles at several fixed times following a modeled energy, and (b) the variation of the centerline density.
(a) Schlieren image HV spark discharge with a laminar He jet (delay 20 μs); jet nozzle is on bottom, left side. (b) Schlieren image of HV spark discharge interaction with heated air jet; delay time 10 μs, air temperature T ≈ 380 K.
Time-evolution of plasma components during the initial decay of a femtosecond laser pulse in mixture at P = 1 atm, T0 = 300 K. The initial density of electrons is Ne 0 = 1.5 × 1017 cm−3. (a) Charged particles, (b) neutral components.
Time-evolution of plasma components on the main stage of plasma decay of a femtosecond laser pulse in N2:O2 = 4:1 mixture at P = 1 atm, T0 = 300 K. The initial density of electrons Ne 0 = 1.5 × 1017 cm−3. Open circles are the experiment data on electron concentration at similar conditions. 18
Temporal dynamics of the polarization time τpol(t) in nitrogen and air at P = 1 atm, T0 = 300 K.
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