| In inductively coupled discharge,there are two kinds of electric field distribution:axial electrostatic field and toroidal induced electric field.These two electric fields corresponds to the two discharge modes in inductively coupled discharge,namely the capacitive coupled and inductive coupling mode(E and H mode),and mode jump and hysteresis phenomenon can be observed at the same time,causing that the plasma pa-rameters is uneven and the discharge system is unstable,it seriously restricts its appli-cation in large area plasma etching and deposition industry,so this thesis focuses on the restricting factors and electronic heating mechanism of the mode transition,at the same time the generation and acceleration mechanism of double layer in the magnetic expansion plasma are also carried in detail.The collision frequency is an significant factor for the efficiency of electron heating mechanism.So we conducted the pure argon plasma experimental research by optical emission spectrum,the density of the transition point were measured under different pressure(collision frequency),the transition power points of both E→H and H→E were gradually decreased with the discharge pressure,and the width of hysteresis loop gradually increases.While the transition density shows a different variation law,as the discharge system jump into the H mode,the transition density point increases slowly with the pressure,but increases sharply in H→E mode transition.Simultaneous the ar-gon spectral line intensity of 750.4 nm and 811.5 nm was measured,the ratio of 750.4 nm spectral line intensity and working pressure was used to represent the high energy electron density and the ratio of 811.5 nm and 750.4 nm spectral line intensity was used to represent the metastable density,to investigate the electron heating mechanism and the energy efficiency,and the line distribution in the range of 400-1000nm.It was found that the high energy electron density in the two heating modes rise with input power,but the change rate was significantly different at different pressures.The ration represented metastable density will increase with input power in E mode,but in H mode it firstly in-creases,then gentle tends to be stable at high and low pressure.In E mode,high energy electron density increased with the power and make the electron-electron collision fre-quent,and the created low energy electron can effectively improve the metastable atom density,so the metastable density increased with the power,and the increase rate rise with work pressure.In H mode,the metastable density increase slowly and gradually slow down as the pressure increases,maybe decrease under high power.There are four new spectra lines in H mode:714.7nm,727.2nm,922.4nm,965.7nm.The working mixture gas will affect discharge process,then will change the heat-ing efficiency.The electron density of Ar-Hg mixed discharge was greater than argon discharge,and there exists a complex relationship between electron density and the con-tent of mercury vapor.Electron density will increase slowly and then reduce with the mercury levels,it will gradually increase after reaching a local minimum value.From Maxwell formulate and nonlocal model,we can calculate power coupling efficiency of the two heating mode as the power and collision frequency,the calculation result are consistent with the experimental results,the Penning effect occur easily and re-duce discharge threshold,so the transition power point of argon plasma is lower than mixture discharge.The additional mercury reduce the equivalent electron temperature,changed electrons-neutral particle collision frequency,changing the relationship be-tween the power absorption and electron density,the power point increase with mer-cury level,and there is a singular point when the heating temperature is 30℃,and with the pressure increase,the hysteresis loop width become wider.An abnormal hystere-sis loop appeared in high mercury level discharge,but don’t appear in argon discharge.In addition,the different antennas will change the matching network impedance,also will affect local and nonlocal electron heating modes.It is found that in different gap electrode discharge,the power points firstly and slowly decrease,finally increases with electrode gap,the changing rule of the radio frequency system inductance also showed a similar feature,the RF coupling efficiency also has a local maximum.It can be owing to when the electrode gap and electronic energy relaxation length are comparable,electron heating exist two cases:local heating and nonlocal heating,in local heating case,each coil can be regarded as an independent source,there is an enhanced effect of electron density in multiple sources,the electron density is not only the linear sum of the two souces,this phenomenon conflicts with the traditional model.It may be attributed to that the traditional theory,in order to contain the random heating in the sheath,includes linear diffusion model.The relationship between turn number and power point can be caused by sheath thickness increase and non-linear enhancement.In the Helicon discharge,there will spontaneously form a double-layer structure(DL)of "current free" in the location where is the biggest electromagnetic field di-vergence,and has a bigger potential difference,ions can be accelerated in this region.Two-dimensional ion velocity distribution function(IVDF)downstream of the double layer structure can be determined via laser induced fluorescence,and the corresponding electric potential also can be derived.By controlling the downstream LEIA pressure to study the strength of DL as the pressure change and power spectrum analysis.We can detect the high-energy ion beam(DL structure)at all work pressure,the maximum ion speed is about 10 km/s,and is inversely proportional to the downstream pressure.There are two components of ion near the downstream of DL,one comes from the ion accel-erated by upstream DL;Another comes from the source area,the high energy electron freezes on the lines of magnetic force,diffusing to the downstream area,and collisions with neutral atoms to form "U" type structure of electron density.The radial distribu-tion of the two kinds of IVDF is different in axial direction,and high energy density of ion beam is less than the background.According to the vertical direction,ion beam velocity in central region has the radial component,and near the downstream of DL,there is a high-energy ion source.Owing to the radial velocity,all the ion beam in the center will expand with the change of axial position.By calculation,we found that the axial velocity changes of ion beam cannot be simply explained by collision.At the same time,electric field and electrostatic fluctuation also been measured by the electrostatic probe.The direction of electric field will reverse in the radial axis of LEIA,and there is a smaller ion hole in the transition area,and the ion was determined by the electric field divergence,the position of ion hole gradually move to plasma center as the LEIA pressure rise.Power spectral analysis show the electrostatic wave instability caused by shear flow near the ion hole boundary,low frequency electrostatic wave is an impor-tant factor affecting the formation of double layer structure.Therefore,the pressure of plasma plume can control the jet ion velocity double layer thruster,and thus to control the impulse of thruster. |
PDF Full Text Request