Study On The Breakdown And Randomness Of Pin-to-plane Discharge In Low Pressure Air

Plasma science has gain more and more attention in recent years for its unique property and has been wildly employed to other areas like material science and biomedical application.However,the physical mechanisms in plasma are not clear.Numerous studies indicate that both photo-ionization and background ionization can promote the propagation of plasma,in which the photo-ionization effect play a dominate role in this process.Researchers also found that plasma sometimes become unrepeatable,which means that the shape or the propagation velocity of the plasma changes at a same condition.Most studies focus on the propagation process,but neglect the ignition process.Nevertheless,the randomness of plasma aroused by two fact:one is the propagation velocity and another is ignition delay time.To explore the discharge process and its randomness,ignition delay time and develop velocity has been measured by compare photomultiplier signal and positive pulse voltage signal in this article.Detailed contents are presented as follows:1.The mean value and standard deviation of ignition delay time,ignition delay of first several discharge pulse and the time delay before discharge occurs have been measured at different frequency at a low gas pressure,the results of which prove that the background ionization level have great influence on the ignition delay time.The ignition delay time changes little when the frequency is higher than 200Hz,but increase with the decrease of the frequency when it is lower than 200Hz.Remarkable randomness can be detected when the frequency is lower than 10Hz.The relationship between first to discharge pulse is also been measured,which can prove the influence of previous discharge on next one directly.2.The influence of voltage,pulse width,gas composition and the shape of the electrode on the mean value and the jitter of the ignition delay time have been measured.The results of the experiment reveal that the ignition delay time decrease with the increase of voltage or the increase of pulse width or the decrease of radius of curvature of the pinpoint of the electrode,among which the ignition delay time is most sensitive with the change of voltage.The effect of electric field strength on ignition delay time can be proved.With the increase of the ratio of oxygen in gas,the ignition delay time decreases,which means that oxygen play an important role in the ignition process.On the other hand,there are no remarkable randomness of the ignition delay time has been observed.3.To explore the relationship between propagation velocity of plasma and each parameter,the breakdown time and its jitter has been measured by two photomultipliers at different environment.The result reveals that the propagation velocity increases with the increase of frequency when the frequency is higher than 100Hz,which remains its value at frequency lower than 100Hz.Detail analysis show that high background ionization level(>109/cm3)induced by higher frequency can promote the develop of plasma,while the photo-ionization effect domain the propagation process at lower frequency.The propagation velocity increase remarkably with the increase of voltage.The increase of the ratio of oxygen can also promote this process.However,no remarkable randomness of the breakdown time has been observed at all given parameters,which means that the propagation process is highly repeatable.4.The ignition delay time and its jitter have been analysis with the theory of gas breakdown.The result shows that the electron density is higher than 107/cm3 when frequency is higher than 200Hz,which induce a high initial electron appearance rete.And that is the reason why the ignition delay time changes little.when frequency is lower than 200Hz,the detachment reaction of negative oxygen ions is the main source of free electron.And the production rate of free electron increases with the increase of concentration of negative oxygen ions,which is the reason why the ignition delay time increase with the decrease of frequency.The electric field strength can influence the breakdown probability and the rate of detachment reaction,which will both affect the ignition delay time.Considering the randomness of ignition delay time,detail analysis shows that the breakdown probability is relatively higher in pin-to-plane discharge,which induce no significant randomness of the ignition delay time.The randomness of the ignition process is mainly aroused by low background ionization level when it is lower than 108/cm3,which induce a slow electron production process.In Summary,both ignition delay time and breakdown time are influenced by background ionization level,electrical field strength and gas composition.However,the randomness of discharge originates in the ignition process while the propagation process is highly repeatable.