Study On Seed Charges And Repeatability Of Non-equilibrium Plasma Jets

Non-equilibrium Atmospheric pressure plasma jets(APPJs)have drawn lots of attention due to its advantages,especially the high repeatability.According to the streamer theory or ionization wave theory,seed charges are necessary for the propagation of gas discharge.However,due to the limitation of methods and techniques,the density of the charged is hard to be measured.In this paper,firstly,a self-designed vacuum discharge system has been used to control the gas pressure and component in order to avoid the influence of the air.The influence of seed charges on the repeatability has been researched by adjusting the frequency and oxygen content.Secondly,the effect of the visible light on the discharge breakdown delay time has been studied,and the mechanism of this effect has been studied in details.Thirdly,the nitrogen and air discharge has been studied at lower pressure by using the self-designed vacuum discharge system.The discharge is stable and has the similar dynamic behavior of plasma jets.Helium plasma jets propagate from random to repeat mode by measuring the jitter of discharge delay time td?.The frequency that the discharge from random to repeatable is defined the critical frequency-fcri.The fcri is independent of voltage amplitude,frequency and width.The fcri of three kinds of plasma jets devices have been studied,and the fcri is independent of devices.But the fcri is strongly related to the nitrogen and oxygen content.Using the self-designed vacuum discharge system,the effect of seed charges on the repeatability can be studied deeply and accurately.Firstly,the reason for the repeatability is the consistency of the ignition delay time not the change of the velocity.By changing the frequency and oxygen content,the fcri has been measured and then the decay time of the seed charges can be calculated by the fcri According to the decay curve of seed charges,the minimum seed charges density for repeatability can be calculated.And the minimum seed charges density is related to the pressure such as the density is in order of 108 cm-3 for 2×104 Pa and 107 cm-3 for 4×103 Pa.Furthermore,the minimum for atmospheric pressure has been estimated as 109 cm-3.The visible light can decrease the discharge breakdown delay time of plasma jets.Two kinds of light source have been used to pre-ionize the gas.One is the lasers with wavelength of 404?532 and 662 nm.The other light sources is the self-designed narrow band visible light with different central wavelength.The most effective position that the light illuminated on the tube is position nearby the H.V.electrode.The effect of the light is much stronger with larger inner diameter of the quartz tube.The effect of the light is inversely proportional to the wavelength of the light.The larger frequency and width can enhance the effect.These observations can be explained by the visible light-enhanced generation of free electrons before the ignition.The proposed mechanisms of free-electron generation are the optically stimulated exo-electron emission from the inner surface of the discharge tube wall and the vibrational excitation of nitrogen molecules.The stable and homogeneous non-thermal plasma plume has been achieved by using N2,O2 and air as the working gas.Using the diagnostic method of the plasma jet,the characteristics of the discharge has been studied.The plasma plume is composed of the high speed plasma bullet.And the velocity of the bullet is related to the voltage parameters and gas composition.There is a special helical shape discharge has been found by using nitrogen.The effects of discharge parameters like gas pressure,pulse repetition frequency,applied voltage,and oxygen mixing on the helical ionization waves have been extensively studied.Dynamical processes of the helical ionization waves and its mode transition have been captured by high speed ICCD camera.The helical ionization waves propagate in fast time scale dynamics with the propagation velocities of 104m/s.But the condition for the helical shape is limited and the discharge is sensitive to the parameters of discharge.The plasma plume changes from random mode to repeatable mode with the increasing of the frequency.According to the measured critical frequency,the density of the seed charges can be calculated by the decay curve as well as the helium plasma discharge.The minimum density is in the order of 106cm-3 for 1×103 Pa,107cm-3 for 3×103 Pa.It is estimated the minimum density for repeatability of air at atmospheric pressure is about in the order of 109 cm-3.