Studies On The Modulated Strategy And Characteristics Of Atmospheric Pressure Dielectric Barrier Discharge

Dielectric barrier discharges(DBD)as a common method of generating high density plasmas with abundant active particles and plasma species,have been widely investigated in the past few decades.As a non-equilibrium discharge system,DBD possesses rich and diverse discharge phenomena,and exhibit huge promise and potential in industrial applications.Some of these applications include(but are not limited to)material processing,thin film deposition,vapor deposition,environmental science and management through treatment of air and water.Additionally,it is worth noting the increasing prominence of plasma applications in the biomedical field,where precise control of generated species show unprecedented technological superiority over conventional treatment methods,and is indicative of plasmas irreplaceable role in medicine in the near future.Although the DBD possesses huge potential superiority in applications as compared with other plasma sources,many key issues have yet to be resolved in order to truly realize industrial skewed operator cantered applications.Some of these issues that have to be tackled include the lack of the capacity for directional optimization,and selective modulation of key plasma parameters under the atmospheric barrier discharge system.The focus of this doctoral thesis revolves around the study of DBD systems through the adoption of two kinds of modulated waveforms.These waveforms include dual-frequency waveforms and tailored voltage waveforms.Through the employment of the forementioned waveforms,detailed research and analysis would be presented that discusses the resulting key plasma parameters and associated mechanisms under the different DBD modulation conditions.An one-dimensional fluid model with a semi-kinetic treatment method has been adopted for this work to arrive at numerically simulated discharges.The discharge evolutions based on the two kinds of modulated waveforms,were rigorously analyzed to investigate their effects on the plasma discharges under different modulation conditions.As such,the analysis of the discharge evolutions revealed discharge mechanisms during parametric modulation under different conditions based on the kinetic behavior of electrons in th e discharges;which includes electron impact ionization,electron heating mechanisms and the electron energy distribution function.The results from numerical simulations were then verified and validated by experiments.When driven under a dual-frequency condition,the results show that: an extra low frequency component applied on the high frequency component has significant improvement on increasing the plasma density through the nonlinear coupling effect.As compared with single frequency discharge systems,the gas temperature also exhibits no obvious increment.Moreover,it is also observed that dual-frequency DBD systems have lower breakdown voltages,and ? mode can be maintained over a wider operational range of voltages.Further studies utilizing frequ ency spectrum scanning reveals that the gain in plasma density due to the synergistic coupling effect of the dual-frequencies is more prominent when the frequencies are of a comparable range,as opposed to when the frequencies are vastly different.Based o n the results from numerical simulations,a series of experiments were performed under similar conditions for experimental verification.The experimental results exhibit good consistency with the results from simulations.This therefore demonstrates that dual-frequency systems provide a feasible method to independently optimize and modulate key plasma parameters selectively according to application specific demands,in a DBD system.In an attempt to realize selective independent control of plasma parameters under a dual-frequency driven condition,an analysis on the effect of the modulation of tailored voltage waveforms in driving DBD systems was carried out.The results indicate that an electrical asymmetry effect(EAE)can be realized accordingly that couples the fundamental wave with its harmonic wave.The EAE can be classified into slope asymmetry effects(SAE),and amplitude asymmetry effects(AAE)accordingly.Exploitation of SAE results in an effective change in the sheath voltage amplitude for both po wered and grounded electrodes,which ultimately modulates the space distribution of both electrons and ions.On the contrary,utilizing the AAE results in an effective change in the distribution of the average electron energy and ion flux.This allows for precise control over desired electron density and electron energy in a given DBD discharge.Moreover,it is shown that the fundamental frequency has an effect of modulation of the electron density at the ground electrode,and that the number of harmonics i s able to modulate the electron energy at the powered electrode.This illustrates the ability of control and modulation of plasma parameters through selective modification of both the fundamental frequency applied as well as the number of harmonics.Collective knowledge and understanding of the mechanisms involved during the presented parametric investigations provides a comprehensive overview of a novel method to realize independent control of plasma parameters in an atmospheric pressure DBD system.