Fluid Simulation Of Mode Transition Characteristicses Of Atmospheric Pressure Radio Frequency Glow Discharge

In recent years,uniform atmospheric-pressure glow discharge(APGD) has attracted considerable attention because of its advantageous properties for industrial applications, including surface activation,etching,cleaning,decontamination,and thin film coating.In the last fifties,two discharge modes namedαandγmode were found in the low pressure glow discharge.And the existence of two different discharge modes has been verified in an rf (radio-frequency) APGD.It has been demonstrated that theαmode has better discharge stability,whereas theγmode produces more abundant plasma species including charged particles and metastables.The discharge inγmode turn to arc earily and this is not requisite.For continuous using APGD,particularly on an industrical scale,a through understanding of the operational characteristics of the two modes is essential.In this thesis,a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in theγmode in helium between two parallel metallic planar electrodes.The results show that as the applied voltage increases,the discharge current becomes greater and the plasma density corresponding increases,consequentially the discharge transits from theαmode into theγmode.In the first mode,referred toαmode,the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion.In the second mode,termed asγmode,the discharge current density is relatively high,the secondary electrons emitted by cathode under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region,and the electron Joule heating and the electron collisional energy loss reach their maxima in the region.The validity of the simulation is checked with the available experimental and numerical data.The discharge in pure helium and the influence of small nitrogen impurities at atmospheric pressure are investigated based on a one-dimensional self-consistent fluid model between two coaxial electrodes.The simulation of the radio-frequency(rf) discharge is based on the one-dimensional continuity equations for electrons,ions,metastable atoms and molecules, with the much simpler current conservation law replacing the Poisson equation for electric field.Through a computational study of rf atmospheric glow discharges over a wide range of current density,this thesis presents evidence of at least two glow discharge modes,namely, theαmode and theγmode.The simulation results show the asymmetry of the discharge set exercises great influence to the discharge mechanisms compared to that with parallel-plane electrodes.It is shown that that the particle densities are not uniform in the discharge region but increase gradually from the outer to the inner electrode in both modes.The contrasting dynamic behaviors of the two glows modes are studied.Secondary electron emission strongly influences gas ionization in theγmode yet matters little in theαmode.Discharge mode transition is an important issue for widespread applications of radio-frequency atmospheric pressure glow discharges(APGD).This thesis reports a study of the mode transition(α-γ) in radio-frequency APGDs using two-dimensional fluid simulation. At theα-γmode transition point,the plasma is shown to undergo transverse contraction induced by an imbalance of transverse electron drift and diffusion.While transverse contraction often leads to irreversible mode transition experimentally,numerical results suggest that it is recoverable and with increasing discharge current the plasma starts to expand transversely and resume spatial uniformity.A numerical simulation of concentric-ring discharge structures has been performed within the scope of a two-dimensional diffusion-drift model at atmospheric pressure between two parallel circular electrodes covered with thin dielectric layers.With a relative high frequency the discharge structures present different appearances of ring structures within different radii in time due to the evolvement of the filaments.The spontaneous electron density distributions help to understand the formation and development of self-organized discharge structures. During a cycle the electron avalanches are triggered by the electronic field strengthened by the feeding voltage and the residual charged particles on the barrier surface deposited in the previous discharges.The accumulation of charges is shown to play a dominant role in the generation and annihilation of the discharge structures.Besides,the filaments split and unit to bring and annihilate filaments which form a new discharge structure.