Studies On The Time-Spatial Evolution Characteristics Of Optical Emission Spectrum Of A Pulsed Spark Discharge In Water

High voltage pulsed discharges in water have attracted intensive attention for various potential applications, such as degradation of organic pollutants, water and water treatment, nanometer material synthesis. It is necessary to boost processing technique by understanding the basic processes that occur in the underwater high-voltage pulsed discharges.The optical emission spectroscopy, as a diagnostic technique to characterize plasma, provides many plasma parameters, such as the electron temperature and density, the gas temperature. In order to improve the signal-to-noise ratios, the optical emission spectroscopy of plasmas generated by the pulsed discharge in water are usually measured by accumulating many discharge pulses over hundreds of seconds. These results are insufficient to study the time-spatial evolution dynamic characteristics of plasma. So far, it is still a challenge to measure the time-spatial resolved optical emission spectroscopy of plasma generated by a single discharge pulse in water.In this study, we constructed a high speed time-spatial resolvable spectrograph for measuring the optical emission spectrum of a single electrical discharge pulse, by combining an ultrahigh-speed frame camera system with monochromator. With the time-spatial resolvable spectrograph, we measured the temporally and spatially resolved optical emission spectrum of a single spark filament in water. Temporally and spatially resolved electron densities of the spark filament were investigated. The main work and research results are shown as follow:(1) Based on the Matlab software, a spectrum program was designed to convert the spectrum image into the corresponding spectrum curve. The performance of spectrograph was tested by using a 632.8 nm He-Ne laser beam and analyzed by the spectrum program. The pixel resolution for 632.8 nm spectra is 0.013 nm. The instrument broadening for 632.8 nm spectra is 0.15 nm when the width of entrance slit is 0.2 mm, and the instrument broadenings measured by four camera channels have a good consistency. The change of exposure time of the camera has no influence on the instrument broadening.(2) A comparison of three different Abel inversion methods (Hankel-Fourier, Nestor-Olsen, Radon Inverse Transform) was completed with the different sampled datapoints and noise level. The results revealed that the Hankel-Fourier had the highest accuracy and the least sensitive to the noise.(3) Temporally and spatially resolved optical emission spectrum of Ha for nanosecond pulsed spark discharge in water was experimentally measured by the high speed time-spatial resolvable spectrograph. The electron density in the spark filament was found to be 1018/cm3 order of magnitude, using the Stark broadening of the Ha spectra.(4) By measuring the spatially resolved optical emission spectrum of Ha, along the axial direction of the spark filament, for nanosecond pulsed spark discharge in water, the spatially resolved electron density in the spark filament was investigated. The results showed that the electron density in the spark filament decreased with increasing the distance from the tip of the anode. The electron density at the tip of the anode was about three its value at the disk electrode.(5) By measuring the temporally resolved optical emission spectrum of Ha at the middle position of the spark filament, the temporally resolved electron density in the spark filament was investigated. The results revealed that the electron density decreased with time.(6) By measuring the spatially resolved optical emission spectrum of Ha along the radial direction of the middle position of the spark filament, the spatially resolved electron density in the spark filament was investigated. The electron density increased from the center to the edge of the spark filament, and its value decreased with time, keeping the radial distribution profile unchanged.