Laser-Induced Fluorescence Absolute OH Density and Gas Temperature Measurements in a Water Vapor Bubble Microsecond Discharge

Abstract

Despite the extensive list of laser diagnostics capable of probing gas phase discharges, electrical discharges occurring inside bubbles in liquids have to date mainly been investigated by imaging and passive optical emission spectroscopy. In this paper, we report an investigation of a bubble discharge in liquid saline water using laser-induced fluorescence (LIF). Time-resolved absolute OH density and gas temperature are measured in the plasma-filled bubble. The reproducible discharge was generated by applying a microsecond voltage pulse to a needle electrode submerged in a NaCl-water solution with a conductivity of 50 mS/cm. The needle was located between two quartz slides with a gap distance of 2 mm. Due to the formation and subsequent growth of a vapor bubble becoming as large as 8 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> in the direction of the laser beam, the bubble occupies the entire gap and expands along the inner wall of quartz windows. During this bubble expansion, a UV laser beam around 282 nm was passed through the bubble and an ICCD camera with a narrowband filter (310–320 nm) was capturing the spatially resolved LIF signal along the laser beam. Absolute OH LIF signal calibration was obtained from Rayleigh scattering measurements on the same setup in gas phase.