Fluid Modeling Of Cold Atmospheric Pressure Plasma Jet In Helium

Atmospheric pressure cold plasma jets (APCPJs) have recently been of an enormous interest due to their unique advantages and numerous potential applications. Compared with traditional atmospheric pressure non-equilibrium plasma sources, one of the most prominent features of the atmospheric pressure cold plasma jets is the spatial separation of plasma generation from their working regions, which can simultaneously achieve both high discharge stability and efficient reaction chemistry. Although APCPJs have been applied to several practical applications such as material processing, surface modification, and plasma medicine, many fundamental mechanisms still remain unknown. Therefore, further detailed investigations on these issues are needed. In this paper, the following works have been done:The plasma bullet creation, propagation and inhibition in a needle-to-plane discharge have been investigated based on a l.5D plasma fluid model. It is found that the propagation of plasma bullet is similar to the propagation of an ionizing wave. The collision ionization is the most important ionization mechanisms during the bullet propagation, while the Penning ionization can provide sufficient seed electrons ahead of it. The dynamics of the plasma jets are determined by the voltage polarity. When the plasma jets arc driven by a positive pulsed voltage, there is a net space charge domain with Gaussian-shaped distribution in the head of the plasma bullet. In this domain, the electric filed induced by space charge is strong enough, and the ionization is intensive. The bullet velocity has the same evolution as that of peak field. It starts to accelerate as soon as it is launched, and then slow down after reaching its maximum velocity at some distance. In contrast to the positive plasma jet, there are some specific features in the plasma jet driven by a negative pulse. The notable features are the existence of a narrow cathode fall region near the tip electrode and the absence of quasi-neutral ionized channels. Moreover, the propagation velocity of plasma jet in negative pulses, which is lower than that in the positive pulses, decreases all the time during its propagation.The influence of ambient air on the atmospheric pressure helium plasma jets has been studied. It is demonstrated that the electron density in the plasma jets are reduced due to the electron attachment by oxygen molecules. The Penning ionization between metastable helium atoms and air molecules facilitates the volume ionization and accelerates the propagation of the plasma jets. Without taking air diffusion into account, the maximum of streamer velocity increases with the increasing air content, and reaches its maximum when the air impurity level is increased to1%, and then decreases with the further increasing air content. Meanwhile, the plasma jet length is decreased exponentially as the air impurity level increases. In addition, the forming mechanism of ring-shaped structure on the cross-section of plasma jet is investigated by using a one-dimensional fluid model that including the air diffusion effects. It is shown that the formation of ring structure is governed mainly by the difference of direct ionization rate between helium and air.The influence of gas flow on the discharge characteristics in the dielectric barrier discharge with parallel electrodes at atmospheric pressure was investigated by a one-dimensional self-consistent kinetic model. When the discharge is driven by a sub-microsecond pulsed dc voltage, two discharge current pulses, the positive one and the negative one, are operated in a normal glow mode and a sub-normal glow mode, respectively. It is shown that the gas flow has a significant impact on the discharge characteristics, especially on the positive discharge pulse. The spatial distribution of electrons is affected by the gas flow through the convection transport mechanism. When the discharge is driven by ac voltage, the convective transport mechanism is mainly governed by positive ions. The presence of gas flow results in the formation of asymmetric discharge in a whole driving period. Associated with the surface charge, the gas flow makes the intervals between two consecutive discharge events no longer remains constant, but exhibits a alternative fashion. As the gas flow exceeds some critical value, the system will undergoes a double period bifurcation and transits into a Period2discharge pattern.The effects of discharge parameters (pulsed driving frequency, the rising time, permittivity, etc) on the atmospheric pressure dielectric barrier discharge excited by repetitive voltage pulses have been numerically studied by using a two-dimensional fluid model.It is demonstrated that both the two discharge currents decrease as the driving frequency is increased. The time delay between the igniting event and the current peak of the first discharge becomes shorter and shorter. The maximum of electron density near the instantaneous cathode at the positive discharge current peak moment decreases, and the corresponding position moves towards the instantaneous cathode when the driving frequency increases. However, although the maximum of electron density near the instantaneous cathode at the positive discharge current peak moment decreases, but the corresponding position moves towards the instantaneous anode when the driving frequency increases. As the rising time is increased, both he two discharge currents decrease. The falling time plays an important role in the second discharge. The longer the falling time is increased, the smaller current peak of second discharge becomes.Based on a self-consistent two-dimensional plasma fluid model,we investigated the propagating problems in the cold atmospheric pressure helium plasma jets that surrounded by helium itself and thin dielectric tube, respectively. The spatio-temporal distributions of electron density, ionization rate, electrical field, and electron temperature were obtained. It is found that both the radius and the permittivity of dielectric tube have an impact on the discharge characteristics. A new method of improving the electron density and plasma jet size were also proposed.