Pré-Publication, Document De Travail Année : 2025

Ultrafast ps–TALIF and streak camera diagnostics of atomic hydrogen in a helium microplasma jet

Résumé

Picosecond two-photon absorption laser induced fluorescence (ps–TALIF) is combined with a streak camera (∼1 ps highest temporal resolution) to probe the effective lifetime (τ_effH(n=3) ) and absolute density (N_H) of atomic hydrogen in the effluent of a helium atmospheric-pressure microplasma jet (μAPPJ). This approach allows for improved temporal resolution compared to conventional nanosecond diagnostics, enabling measurements of (τ_effH(n=3)) as small as 50 ps, the corresponding N_H reaching down to 10^14 cm^-3. Atomic densities are calibrated by performing identical ps–TALIF measurements in krypton gas contained in a custom-built low-pressure cuvette. Physical mechanisms that may be involved in the TALIF scheme such as photoionisation and/or stimulated emission are also assessed, ensuring reliable studies. The determined τ_effH(n=3) and N_H increase with the helium flow rate (Q_He) in the range Q_He=0.3–1 slm. Both quantities are maximized near the exit nozzle of the μAPPJ, obtaining values below 400 ps and 6×10^14 cm^-3, respectively. As the axial distance increases, τ_effH(n=3) declines with a rate of ≈65 ps/mm for Q_He=1 slm which is about 3 times smaller than for Q_He=0.3 slm. These findings reveal a strong correlation between the experimentally-measured τ_effH(n=3) and the local air entrainment into the jet as indicated by their comparison with calculated effective lifetimes based on published quenching rates. Furthermore, operating the μAPPJ in the burst mode allows for the estimation of the residence time (t_res) of ground state H–atoms in the helium gas channel, which is larger for Q_He=0.3 slm (t_res≈1.2 ms) compared to Q_He=1 slm (t_res≈0.5 ms). H-atoms consumption in the gas channel can be affected by diffusion and mechanisms involving neutral ground-state and metastable species among others. Furthermore, based on an error propagation analysis, density uncertainty as high as 64% (depending on the operating condition) is revealed. This mainly originates in the ratio of the two-photon absorption cross sections of Kr and H atoms ( σ(2)_Kr / σ(2)_H ) which has not yet been measured in the picosecond regime. Nevertheless, the combined utilisation of ps–TALIF and streak camera diagnostics demonstrates high sensitivity and temporal resolution for directly probing key reactive species in atmospheric pressure microplasmas.

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Dates et versions

hal-05150015 , version 1 (15-07-2025)

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Dimitrios Stefas, Yanis Agha, Laurent Invernizzi, João Santos Sousa, Swaminathan Prasanna, et al.. Ultrafast ps–TALIF and streak camera diagnostics of atomic hydrogen in a helium microplasma jet. 2025. ⟨hal-05150015⟩
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