Investigation On Energy Relaxation Mechanism Of Nanosecond Pulsed Discharge

Because of its characteristics of high energy efficiency,good homogeneity,non-thermal equilibrium property et al,recent years nanosecond pulsed discharge plasma becomes the research hotspot.But,its physicochemical reaction is complex and diverse,and the time span is large,which brings difficulties to its controllable application.In this paper,optical emission spectroscopy,coherent anti-stokes Raman scattering spectroscopy are used to investigate the evolution of vibrational distribution of nitrogen molecule in atmospheric pressure nanosecond pulsed discharge plasma jets and nanosecond pulsed pin-pin spark discharge plasma.1.In this study,the spatial-temporal resolved optical emission spectroscopy and electrical characteristics are employed to study the dynamic evolution of molecules vibrational distributions,reactive species,and streamer head speed in the generation and propagation of the atmospheric pressure plasma jets.The images of discharge,waveforms of pulse peak voltage and discharge current,and spatial-temporal emission spectra of N₂（C³Π_u→B³Π_g,380.5 nm）,N₂⁺（B²Σ_u⁺→X²Σ_u⁺,391.4 nm）and He（3s³S→2p³P,706.5 nm）are recorded,the relative vibration population of N₂（C³Π_u）and the key dynamic process of each discharge pulse are discussed.The effects of pulse peak voltage on emission intensity,vibration population of N₂（C³Π_u）and APPJs speed are also investigated.The results show that the streamer head speed is about 10⁵ m/s under the pulse peak voltage of 5-9 kV,and both the streamer head speed and emission intensities increase with pulse peak voltage.The emission intensities of N₂（C³Π_u→B³Π_g,380.5 nm）rise for about ten nanoseconds but fall for several tens of nanoseconds.During the plasma generation process,the direct electron impact process is dominant in generating electronic,vibrationally and rotationally excited N₂.Several tens of nanoseconds after the generation,spontaneous emission,quenched by N₂ and O₂dominate the decay process.While the step excitation and Penning ionization extend the duration time of emission intensity.It is also found that the ratio of N₂（C,υ=1）/N₂（C,υ=0）is at high level within one microsecond.The evolution of the ratio is dominated by the direct electron impact in the initial time,and then possibly influenced by vibrational relaxation process and downward vibrational-vibrational energy transmissions process.2.In this study,vibrational coherent anti-stokes Raman spectroscopy,electrical characteristics,and temporal ICCD images are employed to study the dynamic evolution of the pin-pin discharge and its evolution of vibrational energy,under both low and atmospheric pressure.The ICCD images shows that there are two phases of the discharge,the intense phase and the relaxed phase.It is found that under the overvoltage,30%to 50%of the ground state molecules are excited to vibrational excited states at the initial time of the intense phase.After reaching its maximum at about 100 ns,population of high vibrational levels plunges to a low level,less than 1%, at about 300 ns and then varies a little in the relaxed phase.This fast cooling of vibrational energy relates to both the consumption of vibrational excited nitrogen molecules and the generation of ground state nitrogen molecules.The vibrational temperature T_v01,T_v02,T_v03,and T_v04 are calculated.The results shown that under both low and atmospheric pressure,vibrational temperatures calculated from higher vibrational levels are higher than those calculated from lower levels,which means the overpopulation of vibrational levels.Besides,vibrational temperature under low pressure are higher than those under atmospheric pressure,which is caused by the bigger number density of nitrogen molecules.