Simulation On Temporal-spatial Nonlinear Behaviors In Atmospheric-pressure Dielectric Barrier Discharges

Recently, there has been growing interest in atmospheric-pressure dielectric barrier discharges (DBDs). With the advantages of nonvacuum plasma operation, low gas temperature, mild discharge, high yield of active species, etc., atmospheric-pressure DBDs have shown immense potentials for numerous industrial applications, such as surface modification of materials, biological and chemical decontamination of media, material deposition, etching, etc. As a spatially extended dissipated system, atmospheric-pressure DBDs with a large number of freedom degrees possess rich and complex temporal nonlinear behaviors including period doubling bifurcation, quasi-periodic behaviors, chaos, etc., and complex spatial nonlinear behaviors including filamentary discharge, self-organized pattern, spatial chaos, etc. A deep understanding of these nonlinear behaviors not only can offer valuable guidance to effective control of the stability and uniformity of atmospheric-pressure DBDs, but also makes it possible to lock atmospheric-pressure DBD operation in a preferred manner to meet the specific application requirements. At present, though the mechanism and characteristics of atmospheric-pressure DBDs have been studied widely, their complex temporal-spatial nonlinear dynamics behaviors are not understood completely. In this paper, a one-dimensional and a two-dimensional fluid models are developed to study the temporal-spatial nonlinear behaviors in atmospheric-pressure DBDs.Radio-frequency atmospheric-pressure discharge has more merits due to the low breakdown voltage and high yield of charged particles. In Chapter2, a one-dimensional fluid model is developed to study the temporal nonlinear behaviors in a radio-frequency atmospheric-pressure DBD. Numerical results show that when the discharge operates at the y-mode with relatively larger negative differential conductivity, the discharge behaviors become complex and various periodic states and chaotic discharge can be observed. In y-mode, the gas ionization mainly occurs near the cathode sheath region. Hence the sheath characteristics should be responsible for these complex temporal nonlinear behaviors. When period multiplication and chaos take place, the electric field in the sheath bifurcates to cause the regular or irregular fluctuation in sheath thickness, and thus results in the period multiplication or chaotic behavior of the discharge current density. Unlike DBD operating at the frequency of kilohertz, the discharge current amplitude fluctuates very little for rf DBD. Nanosecond or submicrosecond pulse-excitation DBDs not only can produce uniform nonequilibrium plasma, but also can increase electrical efficiency and plasma density. In Chapter3, a one-dimensional model is developed to simulate the temporal nonlinear behaviors in the atmospheric-pressure DBD driven by unbipolor pulsed voltage. In the pulsed DBDs, there exist two discharges during a pulsed period, and the secondary discharge occurring near the falling flank is mainly generated by the surface charge on the dielectric layers. Therefore, the parameters such as frequency and pulsed width could influence the discharge by changing the surface charge density, especially when the nonlinear behaviors occur. In the simulation, by increasing the falling time or frequency, a clear period-doubling bifurcation cascade to chaos can be observed, and when further increasing the parameter, discharge would return to the periodic state. With proper parameters, stable period-2discharges can sustain with increasing duty cycle. The positive discharges weaken gradually, and the negative discharges enhance gradually.In order to further study the radial structure of temporal nonlinear behaviors, a two-dimensional model is developed to simulate the atmospheric-pressure DBDs driven by sinusoidal voltage with kilohertz frequency. Results show the period-2n (n=1,2...) and chaotic discharges perform nonuniform radial structure. In period-2n or chaos, not only the shape of current pulses doesn’t remains exactly the same from one cycle to another, but also the radial structures, such as spatial evolution process of charged particles and the strongest breakdown region, are different in each neighboring discharge event. In chaos, the various evolution processes with different spatial structures occur randomly. The nonuniform discharge structure is closely related to the local plasma resistance.Besides the period-doubling route, atmospheric-pressure DBD can be driven to chaos through other routes. In Chapter5a quasi-periodic torus route to chaos has been observed. By studying the current and voltage trajectory in the phase space, we find that with the increasing excitation frequency the discharge system first bifurcates to a quasiperiodic torus from a stable single periodic state, and then torus and phase-locking periodic state appears and disappears alternately. In the meantime, the torus becomes increasingly wrinkling and stretching, and gradually approaches a fractal structure with the nonpositive largest Lyapunov exponent, i.e., a strange nonchaotic attractor. After that, the discharge system enters into chaotic state. The open area inside the annulus in the Poincare section is gradually filled and finally engulfed.Self-organized pattern is one of the important spatial nonlinear behaviors in atmospheric-pressure DBDs. Its complex macroscopic symmetry and regularity has attracted much attention. In Chapter6, the formation and evolution of the one-dimensional self-organized patterned discharge have been studied. When the applied voltage is increased, various structures of patterned discharge including three channels pattern, five channels pattern, four channels pattern, and three wide channels pattern could appear. Not only the arrangement of discharge channels in these patterns but also the discharge modes are different, even the evolution of discharge channels of one pattern in positive and negative half period could be different. When the applied voltage is raised high enough, discharge enters into spatial chaos, which also performs random behaviors temporally.In Chapter7, the spatial nonlinear behaviors in the DBD system with a single dielectric barrier in He-N2mixtures are studied using a two-dimensional fluid model. Results show that under the simulation parameters, one or more discharge channels appear in the discharge space. It can be seen the discharge channels extend outwards near dielectric layer and shrink inwards near the naked electrode. And the positive and negative discharges on the current and the discharge spatial distribution are asymmetric in such electrode structure. By increasing the voltage amplitude, enlarging the electrode width, or increasing the nitrogen content, the number of the discharge channel increases. It should be noted that in a certain electrode width the maximum number of discharge channel is fixed and when the nitrogen content is high enough, the discharge could become relatively uniform.