Time-resolved radical species and temperature distributions in an Ar–O2–H2 mixture excited by a nanosecond pulse discharge

Time- and space-resolved temperature and absolute concentrations of OH and H are measured by UV Rayleigh scattering, Laser-Induced Fluorescence (LIF), and Two-Photon Absorption LIF (TALIF), respectively, in an Ar–O₂–H₂ (80:20:2) mixture at P=40torr and T₀=300K. The mixture is excited by a 50-pulse burst from a repetitive nanosecond pulse discharge (NSPD) in a point-to-point geometry, operated at 100kHz pulse repetition rate and 5Hz burst repetition rate. One-dimensional radial distributions of temperature and species concentrations across the discharge filament, during and after the burst, are obtained from Rayleigh scattering and fluorescence images of the laser beam. Both temperature and species concentration profiles are found to expand with time, with peak centerline values of T_peak≈1200K, [H]_peak≈6.0ṡ10¹⁵cm^‑3, and [OH]_peak≈1.0·10¹⁵cm^‑3. Secondary maxima in OH distributions are detected near the periphery of the filament after a few tens of discharge pulses. Experimental results are compared with predictions of a plasma chemical kinetics model. The model reproduces trends in temporal evolution of radial distributions of temperature, as well as OH and H concentrations. Based on a rate of species production analysis, the secondary peaks in OH radial distributions are found to be caused by radial diffusion of H atoms from the central region of the discharge filament, with subsequent formation of HO₂ and OH via reactions H+O₂+M→HO₂+M and H+HO₂→OH+OH in the low-temperature peripheral regions. The results demonstrate significant potential of the present approach for quantitative, time- and spatially-resolved studies of coupled radical reaction kinetics and diffusion over a wide range of temperatures, pressures, and mixture compositions.