A comparative study on the spectral characteristics of nanosecond pulsed discharges in atmospheric He and a He+2.3%H_(2)O mixture

Nanosecond pulsed discharges at atmospheric pressure in a pin-to-pin electrode configuration are well reproducible in time and space, which is beneficial to the fundamentals and applications of low-temperature plasmas. In this experiment, the discharges in helium(He) and He with 2.3%water vapor(H_(2)O) are driven by a series of 10 ns overvoltage pulses(~13 k V). Special attention is paid to the spectral characteristics obtained in the center of discharges by time-resolved optical emission spectroscopy. It is found that in helium, the emission of atomic and molecular helium during the afterglow is more intense than that in the active discharge, while in the He+2.3%H_(2)O mixture, helium emission is only observed during the discharge pulse and the molecular helium emission disappears. In addition, the emissions of OH(A-X) and Hα present similar behavior that increases sharply during the falling edge of the voltage pulse as the electrons cool down rapidly. The gas temperature is set to remain low at 540 K by fitting the OH(A-X) band. A comparative study on the emission of radiative species(He, He_(2), OH and H)is performed between these two discharge cases to derive their main production mechanisms. In both cases, the dominant primary ion is He^(+) at the onset of discharges, but their He^(+) charge transfer processes are quite different. Based on these experimental data and a qualitative discussion on the discharge kinetics, with regard to the present discharge conditions, it is shown that the electron-assisted three-body recombination processes appear to be the significant sources of radiative OH and H species in high-density plasmas.

Effect of Airflows on Repetitive Nanosecond Volume Discharges

Atmospheric pressure discharges excited by repetitive nanosecond pulses have at- tracted significant attention for various applications. In this paper, a plate-plate discharge with airflows is excited by a repetitive nanosecond pulse generator. Under different experiment con- ditions, the applied voltages, discharge currents, and discharge images are recorded. The plasma images presented here indicate that the volume discharge modes vary with airflow speeds, anda diffuse and homogeneous volume discharge occurs at the speed of more than 35 m/s. The role of airflows provides different effects on the 2-stage pulse discharges. The 1st pulse currents nearly maintain consistency for different airflow speeds. However, the 2nd pulse current has a change trend of first decreasing and then rapidly increasing, and the value difference for 2nd pulse cur- rents is about 20 A under different airflows. In addition, the experimental results are discussed according to the electrical parameters and discharge images.

Spatial characteristics of nanosecond pulsed micro-discharges in atmospheric pressure He/H2O mixture by optical emission spectroscopy

Atmospheric pressure micro-discharges in helium gas with a mixture of 0.5%water vapor between two pin electrodes are generated with nanosecond overvoltage pulses.The temporal and spatial characteristics of the discharges are investigated by means of time-resolved imaging and optical emission spectroscopy with respect to the discharge morphology,gas temperature,electron density,and excited species.The evolution of micro-discharges is captured by intensified CCD camera and electrical properties.The gas temperature is diagnosed by a two-temperature fit to the ro-vibrational OH(A^(2)Σ^(+)–X^П(2),0–0)emission band and is found to remain low at 425 K during the discharge pulses.The profile of electron density performed by the Stark broadening of Ha 656.1-nm and He I 667.8-nm lines is uniform across the discharge gap at the initial of discharge and reaches as high as 10^(23)m^(-3).The excited species of He,OH,and H show different spatio-temporal behaviors from each other by the measurement of their emission intensities,which are discussed qualitatively in regard of their plasma kinetics.