| Atmospheric radio-frequency (rf) plasmas have attracted an increasing amount of attention due to its unparalleled capability for production of chemically reactive species. In this paper we explored a one-dimensional fluid model, incorporating16species and65key reactions, to investigate the generation mechanism of ROS in atmospheric He/O2rf discharges. From the computational data the atomic oxygen density is almost linearly dependent on the power density and its largest value was observed at the oxygen admixture of0.6%while increasing the oxygen impurity at a constant power density, and an optimal oxygen impurity level of0.3%is also found to produce the largest density of single delta oxygen (SDO) at a given power density. The dominated production and destruction reactions of ROS are also discussed based on the simulation results.To speed up the computation, after considering the importance of every reaction, only17species and48reactions selected from the old reaction set are used to conform the key reactions related to the ROS and discuss the variation of ROS depending on the oxygen admixture at a given applied voltage. By increasing the applied voltage, the evolutions of ROS based on the simulation results are also given. The simulation data agree well with the experimental measurements.Several experimental and computational studies have shown that increasing frequency and decreasing the discharge gap can effectively enhance the discharge stability in atmospheric radio-frequency (rf) discharges, but the frequency and discharge gap effects on the reactivity of rf discharges, represented by the densities of reactive oxygen species (ROS), are still far from fully understood. In this paper, a one-dimensional fluid model is used to explore the influences of the driving frequency and discharge gap on the production and destruction of ROS in atmospheric rf helium-oxygen discharges. From the computational results, with an increase in the frequency the densities of ROS decrease always at a constant power density, however in the relatively higher frequency discharges the densities of ROS can be effectively improved by increasing the input power density with an expanded oxygen admixture range while the discharges operate in the a mode, and the numerical data also show the optimal oxygen admixture for ground state atomic oxygen, at which the peak atomic oxygen density can be obtained, increases with the driving frequency. Moreover, under the same power density, with the increase of discharge gap, when the oxygen admixture is small, the max time-averaged electron density and temperature increase, when the oxygen admixture is more enough, those trends change, the max time-averaged electron density gets peak value when the discharge gap is1.2mm, but the max time-averaged electron temperature decrease with the discharge gap.When the discharge gap increases, the spatial curve becomes more flat in the discharge region. After an average in time and space, the densities of ROS all increase with the discharge gap.At last, this paper also presents an experimental and computational study on the optimization of the pulse modulation frequency radio-frequency (rf) discharges at atmospheric pressure. Based on the measured and simulated data, when the gas is helium, to improve the electron density and electron temperature in a pulse modulation rf discharge, a lower modulation frequency smaller than50kHz with a higher duty cycle usually larger than30%should be used. But to reduce the power consumption and lower the gas heating, the duty cycle should be smaller than60%. On the other side, to gain a quasi-continuous rf plasma in a whole modulation cycle, a higher modulation frequency larger than100kHz is preferred. When the gas is the admixture of helium and oxygen, based on the previous analysis, the effects of pulse modulation on ROS are also discussed. |
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