Abstract
The electron power absorption dynamics in radio frequency driven micro atmospheric pressure capacitive plasma jets are studied based on experimental phase resolved optical emission spectroscopy and the computational particle in cell simulations with Monte Carlo treatment of collisions. The jet is operated at 13.56 MHz in He with different admixture concentrations of N2 and at several driving voltage amplitudes. We find the spatio-temporal dynamics of the light emission of the plasma at various wavelengths to be markedly different. This is understood by revealing the population dynamics of the upper levels of selected emission lines/bands based on comparisons between experimental and simulation results. The populations of these excited states are sensitive to different parts of the electron energy distribution function and to contributions from other excited states. Mode transitions of the electron power absorption dynamics from the Ω- to the Penning-mode are found to be induced by changing the N2 admixture concentration and the driving voltage amplitude. Our numerical simulations reveal details of this mode transition and provide novel insights into the operation details of the Penning-mode. The characteristic excitation/emission maximum at the time of maximum sheath voltage at each electrode is found to be based on two mechanisms: (i) a direct channel, i.e. excitation/emission caused by electrons generated by Penning ionization inside the sheaths and (ii) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes.
Highlights
Radio-frequency (RF) driven microscopic atmospheric pressure plasma jets (μ-APPJs) are widely used as efficient sources of reactive species at low heavy particle temperatures for a broad variety of applications such as wound healing, sterilization, materials treatment/modification, and semiconductor manufacturing [1,2,3,4,5,6,7,8]
PIC/MCC simulations performed under identical conditions. Based on this comparison between experimental and computational results we improve the understanding of the electron dynamics in these discharges in three different ways: (i) we demonstrate that different spatio-temporal characteristics of the electron dynamics are obtained for different emission lines/bands and wavelength integrated PROES measurements
These differences are understood by revealing the population channels of the respective upper excited states for selected emission lines/bands and based on the simulation results. (ii) Mode transitions are observed and understood as a function of the N2 admixture concentrations and the driving voltage amplitude in the experiment and in the simulation. (iii) We demonstrate that the Penning mode is based on two mechanisms: (a) a direct channel, i.e. excitation/ionization by electrons generated by Penning ionization inside the sheaths and (b) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes
Summary
Radio-frequency (RF) driven microscopic atmospheric pressure plasma jets (μ-APPJs) are widely used as efficient sources of reactive species at low heavy particle temperatures for a broad variety of applications such as wound healing, sterilization, materials treatment/modification, and semiconductor manufacturing [1,2,3,4,5,6,7,8]. Local maxima of the ionization are observed on the bulk side of the expanding and collapsing sheath edges at both electrodes In this mode, ionization is caused by energetic electrons accelerated by a high drift electric field inside the bulk at the times of maximum current within the RF period. Based on this comparison between experimental and computational results we improve the understanding of the electron dynamics in these discharges in three different ways: (i) we demonstrate that different spatio-temporal characteristics of the electron dynamics are obtained for different emission lines/bands and wavelength integrated PROES measurements. This part is divided into three sections according to the three novel insights into the electron dynamics in μ-APPJs, i.e. the sensitivity of PROES results to different emission lines, electron power absorption mode transitions, and the physical origins of the Penning mode.
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