The Effect Of Control Parameters On The Spatial-temporal Evolution Of Nanosecond Pulsed Discharges

In this thesis,the breakdown process of the nanosecond pulsed discharges and the influence of control parameters to the discharge evolution are investigated,both experimentally and theoretically.Two kinds of nanosecond discharges generated in rare gases(helium,neon,argon,and their mixtures)are studied,i.e.the atmospheric pressure microdischarges and the fast ionization wave(FIW)discharges operated in a moderate pressure(tens of Torr).The breakdown process of the nanosecond microdischarges evolves in a form of ionization wave.Compared with the spatially-averaged electric field,the electric field in the wave front region is much enhanced(~ 1.5 times stronger),while the electric field after the pass of the wave front is much weakened.As a result,the very high energy electrons(order of 100 eV),indicated by the emission of the helium ionic line,are concentrated in the wave front region,while the moderate energy electrons(tens of eV),indicated by the atomic lines,have a high density in the region after the pass of the wave front.This result verifies that,the electric field enhancement in the wave front region results in the intensive generation of very high energy electrons.During the breakdown period of nanosecond microdischarges,the effective electron temperature of high energy electrons is much higher than that of low energy electrons(tens of eV vs.several eV).This indicates that,the electron energy distribution function(EEDF)is strongly non-Maxwellian with an elevated high energy tail,which is due to the intensive electron heating by the strong electric field.A non-local relationship between the electric field and the very high energy electrons is also observed and is verified by the Monte-Carlo simulation.The pulse repetition rate controls the initial charge density of each pulsed discharge.In the FIW discharges,with the increase of the pulse repetition rate and the initial charge density,the breakdown electric field decreases,while the residual electric field after the breakdown increases,both of which have a strong impact on the behavior of EEDF.As a result,with a low value of pulse repetition rate,the EEDF during the breakdown is strongly non-Maxwellian with an elevated high energy tail,while the EEDF after the breakdown is also non-Maxwellian with a depleted high energy tail.On the other hand,with a high value of pulse repetition rate,the EEDF is Maxwellian-like without much temporal variation during the whole discharge period.This behavior of EEDF is supported by both the measured and the model-predicted evolution of the emission intensity from the ionic and the atomic lines.The voltage rise rate has a strong impact on the electric field enhancement during the breakdown period.In the nanosecond microdischarges,a higher voltage rise rate leads to a more significant field enhancement at the ionization wave front and a higher breakdown electric field.As a result,during the breakdown,the generation of high energy electrons is enhanced and there is a higher rise rate for both the discharge current and the emission intensity.The results of this thesis improve the understanding of the evolution mechanism and the electron kinetics in high pressure pulsed dischar ges and give a guideline for the parameter-optimization in the application.