Generation of High-Density Pulsed Gas–Liquid Discharge Plasma Using Floating Electrode Configuration at Atmospheric Pressure

In this paper, a high-density gas–liquid discharge plasma is obtained combined with nanosecond pulse voltage and a floating electrode. The discharge images, the waveforms of pulse voltage and discharge current, and the optical emission spectra are recorded. Gas temperature and electron density are calculated by the optical emission spectra of N2 (C3Πu → B3Πg) and the Stark broadening of Hα, respectively. The emission intensities of N2 (C3Πu → B3Πg), N2+ (B2Σ → X2Π), OH (A2Σ → X2Π), O (3p5P → 3s5S0), He (3d3D → 3p3P20), gas temperature, and electron density are acquired by optical emission spectra to discuss plasma characteristics varying with spatial distribution, discharge gap, and gas flow rate. The spatial distributions of discharge characteristics, including gas temperature, electron density, and emission intensities of N2 (C3Πu → B3Πg), N2+ (B2Σ → X2Π), OH (A2Σ → X2Π), O (3p5P → 3s5S0), and He (3d3D → 3p3P20), are presented. It is found that a high-density discharge plasma with the electron density of 2.2 × 1015 cm−3 and low gas temperature close to room temperature is generated. While setting the discharge gap distance at 10 mm, the discharge area over liquid surface has the largest diameter of 20 mm; under the same conditions, electron density is in the order of 1015 cm−3, and gas temperature is approximately 330 K. In addition, the discharge plasma characteristics are not kept consistent in the axial direction, in which the emission intensities of N2+ (B2Σ → X2Π), N2 (C3Πu → B3Πg), OH (A2Σ → X2Π), and gas temperature increased near the liquid surface. As the discharge gap is enlarged, the gas temperature increases, whereas the electron density remains almost constant. Moreover, as the gas flow rate was turned up, the electron density increased and the gas temperature was kept constant at 320 K.