# A Practical Method for Controlling the Asymmetric Mode of Atmospheric Dielectric Barrier Discharges
## 1 Introduction  
Atmospheric dielectric barrier discharges (DBDs), due to their advantages in producing low-temperature plasmas without a precise vacuum chamber, have been widely used in a lot of industrial applications such as surface modification, energy transformation, biomedical sterilization, and pollution abatement [1; 2; 3; 4; 5]. Such pervasive applications have already made DBDs a prospective and an active research topic for many years. Being a nonequilibrium dissipative system, DBDs can exhibit an abundance of nonlinear characteristics or phenomena which are extremely sensitive to the operational environment and controlled parameters [6]. Typically, it is well known that in most cases atmospheric DBDs appear as a symmetric period-one discharge (henceforth called SP1), that is, the shape and amplitude of positive and negative current pulses are basically identical and repeat every one applied voltage cycle [7]. However, under certain conditions, a lot of simulation and experimental research has reported that DBDs can also work in various nonlinear modes such as asymmetric period-one (henceforth called AP1), multi-period, quasi-period and chaos through different evolution routes [8; 9; 10; 11]. Since different discharge modes can influence the stability of DBDs in terms of the density and composition of active species in discharge plasmas and then impact their efficacy in related applications [12; 13], the investigation on the control of nonlinear modes is valuable and imperative both in theoretical and practical aspects. It is worth noting that before bifurcating into other nonlinear modes from SP1, thedischarge always first transforms into the asymmetric mode as a transition phase [14]. Therefore, investigating the mechanisms related to the asymmetric discharge mode could serve as a stepping-stone for the interpretation of other, more complex nonlinear behaviors in DBDs and deserve more attention. The initiation mechanism regarding the SP1-AP1 transition was first reported by [PERSON] et al. in 2003 [15]. Through the numerical simulation of a homogeneous DBD, they observed the SP1-AP1 transition at both lower and higher concentrations of nitrogen impurities, and they concluded that such a mode transition is contributed by the residual quasi-neutral (plasma) region. Our group has also made some effort on this aspect. According to our previous works, the SP1-AP1 transition in shorter gaps might be attributed to the discordance of the evolutionary spaces between electrons and ions [16], while, in longer gaps, the rise in seed electron level induced by the electron backflow is the main reason for the generation of AP1 discharge [17], and the traditional explanations such as \"residual positive column\" [7] or \"instantaneous anode\" [18] in essence are special forms of the \"electron backflow region\". Furthermore, we also proposed a preliminary state-controlling method which can adjust the discharge mode from AP1 to SP1 by applying a first-peak-leveled driving voltage from the beginning of the discharge [19]. Despite all those discoveries, there are still many obstacles that hinder the application of mode controlling in practical industrial applications. For example, the most favored condition in practical applications is that the discharge mode needs to switch to SP1 mode right after asymmetric discharges occur, which was not achieved in previous studies. In addition, the existing method needs to compute the initial moment of the flat-top stage, which is inconvenient for practical devices. Note that the frequency of the applied voltage can also influence the distribution of seed electrons as the amplitude of the voltage does, and changing the frequency of the power supply is much easier to implement for practical power supply devices. Therefore, in this paper, we try to use a more practical method to switch AP1 discharge into SP1 discharge by changing the driving frequency.