Underwater discharges in critical electric fields: Emission spectrum and spatial-temporal distribution characteristics of streamer plasmas

Underwater subsonic streamers exhibit a double-layer structure characterized by a plasma core and gas sheath. The distribution characteristics of the plasma are critical factors that influence the development of subsonic streamers. This paper investigates the development of subsonic streamers in critical electric fields, focusing on the emission spectroscopy and spatial-temporal distribution of plasmas. The research results indicate that streamer discharge undergo three distinct stages: initial weak ionization, strong ionization, and morphological transformation. Positive streamer plasmas have weak luminescence, with point-to-spherical morphologies, featuring characteristic lines of hydrogen (Hα 656 nm), oxygen (O 777 nm), and hydroxide (OH 309 nm). In contrast, negative streamer plasmas are brighter, with additional characteristic lines of tungsten (W 407 nm, 429 nm) and sodium (Na 589 nm), and display stratified morphology: nonmetallic particles form bifurcated filaments, while metallic particles initially appear as point-like structure, rapidly expanding into plume-like structure. During the strong ionization stage, the emission spectrum of plasmas exhibits rich line emission and continuous radiation. Further analysis reveals that continuous radiation is primarily due to black-body radiation and recombination radiation. During morphological transformation stage, the plasma evolves into a channel-like structure, accompanied by a weakening of the emission spectrum, with Hα emerging as the dominant spectral line. Finally, this paper provides a detailed explanation of the spatial-temporal evolution of streamer plasmas from the perspectives of magnetic field forces (magnetic pinch effect, Ampère force) and space charge effect.