Plasma Decay of Nanosecond Pulsed Laser-Produced Ar and Ar-H2O Sparks at Atmospheric Pressure

Time-resolved diagnostics were applied to investigate free-electron properties in nanosecond laser-produced discharges sustained at atmospheric pressure in Ar and in Ar with 3% H2O. The discharges were generated using 23 ns, 1064 nm laser pulses. Broadband plasma imaging and laser Thomson scattering were combined with optical emission spectroscopy, with particular emphasis on Stark broadening of the Halpha and Hbeta lines. The plasma exhibited a bright emission that persisted for up to 30–40 us after breakdown, followed by a very weak glow lasting up to 19 ms. Peak electron number density of about 2 x 10^17 cm-3 and electron temperature of about 7 eV were measured. Excellent agreement between both techniques was obtained for absolute electron number densities. The inferred temporal decay of free electrons is consistent with processes dominated by ambipolar expansion and two- and three-body electron-ion recombination. These results provide benchmark data for modeling nanosecond laser discharges and demonstrate the reliability of combining Thomson scattering with Stark broadening in atmospheric laser sparks.

💡 Research Summary

The authors investigated the temporal evolution of free‑electron properties in nanosecond laser‑produced plasmas (LPPs) generated at atmospheric pressure in pure argon and in argon containing 3 % water vapor. A 23 ns, 1064 nm Nd:YAG pulse (≈64 mJ, ~1.6 × 10¹⁰ W cm⁻²) was used as the drive laser to create the spark, while a second Nd:YAG laser frequency‑doubled to 532 nm (≈24 mJ, ~1.8 × 10⁹ W cm⁻²) served as the probe for laser Thomson scattering (LTS). The probe beam was collected at 90°, passed through two volume Bragg‑grating notch filters to suppress the elastic 532 nm line, and directed into a high‑resolution spectrograph equipped with an intensified sCMOS detector. The same collection optics were employed for optical emission spectroscopy (OES); Stark broadening of the Hα (656 nm) and Hβ (486 nm) lines was used to infer electron density.

Time‑resolved broadband imaging (5 ns gate) revealed that the plasma initially appears as 2–3 bright kernels aligned with the laser propagation direction (x‑axis) within the first 10–30 ns. These kernels expand rapidly in the transverse (y) direction, reaching ~1 mm by 0.3–1 µs, while the axial length grows to ~5 mm by ~10 µs. Between 2.5 and 15 µs the discharge develops “branches” in y, maintaining an elongated shape in x. After 30–40 µs the emission becomes predominantly transverse, and by 100 µs (pure Ar) or 40 µs (Ar‑3 % H₂O) the plasma contracts back to an axial filament. A weak after‑glow persists up to 19 ms, indicating long‑lived heating of the surrounding gas rather than sustained plasma.

LTS measurements showed electron densities rising from ~5 × 10⁸ cm⁻³ at the earliest detectable time to a peak of ~2 × 10¹⁷ cm⁻³ around 0.5 µs, with an electron temperature of ~7 eV (≈8 × 10⁴ K). The density then decays rapidly; the decay curve matches a model that includes ambipolar expansion followed by two‑body (e⁻ + Ar⁺) and three‑body (e⁻ + Ar⁺ + Ar) recombination. Stark‑broadened Hα/Hβ analysis yielded electron densities within 10 % of the LTS values, confirming the reliability of both diagnostics even at atmospheric pressure where collisional broadening dominates.

The presence of water vapor lowers the effective ionization threshold (12.6 eV vs. 15.8 eV for Ar) and increases laser energy absorption, leading to more reproducible discharges and faster plasma quenching. In Ar‑3 % H₂O the transition back to an axial filament occurs at ~40 µs, compared with ~100 µs in pure Ar, reflecting enhanced three‑body recombination rates in the humid mixture.

The authors discuss the origin of the observed two‑lobe structure. They suggest that early‑time refractive‑index gradients can lens the laser (plasma lensing), while later hydrodynamic shock interactions, driven by inverse Bremsstrahlung heating, can also generate forward and backward shock fronts that maintain the lobes. The flat‑top spatial profile of the drive beam, rather than temporal mode beating, is identified as a likely source of the observed micro‑structures.

Overall, the work provides a comprehensive benchmark dataset for atmospheric‑pressure nanosecond LPPs, demonstrates that LTS and Stark broadening can be combined to yield consistent absolute electron densities, and clarifies that plasma decay is governed by ambipolar expansion followed by electron‑ion recombination. These insights are directly applicable to modeling laser‑induced ignition, plasma‑based catalysis, high‑speed flow control, and other technologies that rely on accurate description of laser sparks in gases.