Numerical Investigation On The Energy Spectrum Of Electrons In The Bullet In The Atmospheric-pressure Argon/Oxygen Plasma Jet And Its Oxygen Concentration Effect

Low-temperature atmospheric-pressure plasma jets have been widely applied in many fields such as surface treatment of materials,biomedicine,and thin film deposition,owing to theiradvantages of generating more active particles,stable discharge operation,convenient operation,and economical rationality.Inert gas,such as argon and helium,is commonly used as a working gas in producing the plasma jets.With the use of an addition of oxygen to working gas,more types and numbers of active particles can be generated.Electrons are important particles in the plasma jets because of their abundance and significant role in the interactions of the plasma jets with the aqueous solutions on the surface of living cells.Whether the generation of active particles in the plasma jets or the interaction of the plasma jet with the aqueous solutions,these two aspects closely correlate with electron energy.When applying a positive pulsedvoltage to a needle-plate discharge structure,the produced plasma jets are in a typical pattern appearing as a series of bullet-like plasma volumes,i.e.,so called the plasma bullet.Due to the above,it is of essential importance to investigate the energy spectrum of electrons in the jet and its oxygen concentration effects in the atmospheric-pressure argon/oxygen plasma jetsThis thesis has performed a systematical numerical investigation on the energy spectrum of electrons(ESE)in the atmospheric-pressure argon/oxygenplasma jets,based on a typical needle-plate discharge configuration with a positive applied pulsed voltage and on a one-dimensional Particle-in-cell Monte-Carlo collision model.The investigation includes not only the spatiotemporal evolutions of the ESEs in the entire discharge space and in three characteristic areas but also the effects of both the applied voltage and the oxygen concentration on the evolutions.Three characteristic areasrefer to the plasma bullet(the bullet),the dark channel behind the bullet,and the dim area in front of the bullet along the propagation direction of the bullet.This thesis obtains the following new results:1.Spatiotemporal evolutions of the bullet in the atmospheric-pressure argon/oxygenplasma jet(1)The velocity of the bullet increases and then decrease as the bullet propagates,but basically it is of the order of magnitude of 105 m/s.(2)In the temporal evolution of ESE in the jet,i.e.,in entire discharge space,there is a characteristic time,before which the peak value of ESE decreases,the peak position shifts towards high energy,and the distribution of ESE becomes wider and wider,but after which the reverse is true.(3)As the bullet propagates along the jet axis,there is no characteristic time for the evolution of ESE in each characteristic area.In the three characteristic areas,the ESE in the bullet is of the lowest peak as well as the broadest distribution range which means the largest ratio of high-energy electrons,and the ESE in the dim area has the highest peak and the narrowest distribution range,indicating the smallest ratio of high-energy electrons.(4)In the entire discharge space and the three characteristic areas,the evolution characteristics of ESEs are not changed when increasing the applied voltage amplitude,but the peaks of ESEs get low and their distribution ranges become broad,thus inducing an increasing average energy of electrons in the three areas.2.Oxygen concentration effect of ESE in the atmospheric-pressure argon/oxygenplasma jet(1)In the oxygen concentration range of 1%-10%,the largest velocity of the bullet decreases with the increase in oxygen concentration,but it basically remains onthe level of 105 m/s.(2)In the oxygen concentration range of 1%-5%,changing the oxygen concentration does not affect the evolution characteristics of ESE in the entire discharge space and has very slight effect on the space electric field.Increasing the oxygen concentration,all the ESEs in theentire discharge space and in the three characteristic areas present the same evolution,i.e.,rising peak value,peak position shifting towards low energy,and narrowing distribution,thus the resultant average energies of electrons get small.