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

TL;DR

This study probes electron-density and electron-temperature evolution in nanosecond laser-produced plasmas at atmospheric pressure in Ar and Ar–3%H$_2$O using a trio of diagnostics: broadband imaging, laser Thomson scattering (LTS), and optical emission spectroscopy (OES). The 23 ns, 1064 nm drive pulse creates plasmas that reach peak densities near $n_e \approx 2\times 10^{17}\ \mathrm{cm^{-3}}$ with $T_e$ around $7$ eV, and exhibit a two-stage decay governed by ambipolar expansion and two- then three-body recombination, with a long-tail modeled by $n_e(t) \approx 6.65\times 10^{17}/(1+10t)^{0.74}$. A transition from collective to non-collective Thomson scattering occurs as the plasma evolves, with early-time $n_e$ values corroborated by Stark-broadened H$_\alpha$ and H$_\beta$ lines measured by OES; both Ar and Ar–H$_2$O show remarkably similar decay dynamics, though water vapor enhances early breakdown and shot-to-shot reproducibility. The results provide benchmark data for modeling nanosecond laser discharges and validate the reliability of combining LTS with Stark broadening in atmospheric laser sparks, with implications for optimizing EUV and related plasma processes.

Abstract

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.

Figures (13)

• Figure 1: Schematic of the laser Thomson scattering experimental setup. Features of the drive and probe laser beams are also provided. OES measurements were performed using the same collection path as LTS. Imaging is performed on the opposite side to LTS measurements.
• Figure 2: Schematic of the vacuum chamber hosting the laser sparks in argon. Laser scattering signals are collected through the viewport at the center while plasma imaging is conducted through the opposing viewport.
• Figure 3: Time-resolved plasma images in pure Ar. The camera gate width is 5 ns. Single-shot and 5-shot accumulated plasma images are shown. Time zero is taken as the arrival time of the 1064 nm drive beam at the focal point inside the test vessel.
• Figure 4: Time-resolved plasma images in Ar-H$_{2}$O. The camera gate width is 5 ns. Single-shot and 5-shot accumulated plasma images are shown. Time zero is taken as the arrival time of the 1064 nm drive beam at the focal point inside the test vessel.
• Figure 5: Sample pure rotational Raman spectrum in N$_{2}$ at 760 Torr and room temperature. The camera gate width is 20 ns. Spectrometer entrance slit width = 50 $\mu$m. The spectral resolution is about 190 pm. 30,000 laser shots were accumulated, with 30 seconds of exposure time repeated 20 times. The shaded region corresponds to the part of the spectrum rejected by the notch filter.