Numerical And Experimental Investigations On Energy And Charges Deposition In A Needle-Like Electron-beam Plasma

Electron beam plasma(EBP)has a wide variety of industry,airborne,and military applications,mostly due to its important advantages including high ionization efficiency,low electron temperature and easy control on density and area.The electron beam with energy above several keV propagating in gas at a pressure from several Torr to several atmosphere results in significant energy and charge deposition,which have a great influence on the properties of the generated plasma.Numerical and experimental investigations on a continuous needle-like EBP were presented in this thesis with the focus on the influcene of the energy and charge deposition on the physical properties.The numerical investigation was divided into two parts.In the first part,fluid-Possion equations coupled with the Monte Carlo method were used to simulate the spatio-temporary behavior of a needle-like EBP.The Monte Carlo method was used to describe the beam propagation in gas and provided the values of the energy and charge deposition on discrete grids for fluid-Possion equations.The results indicated that the time evolution of the spatial potential was influenced by the presence of charge depostion whereas the potential in quasi-equilibrium was mainly determined from the spatial distribution of the secondary electron,which was a result of ambipolar diffusion.The potential in quasi-equilibrium was positive near the beam entrance and most negative along the tip of the beam range.When the enclosing boundary surfaces were moved within the beam range,the potential was nearly positive everywhere.The calculation on the diffusion-drift flux indicated that the net current of the secondary electrons flowing back to the incident plane in quasi-equilibrium balanced the incident beam current,which was the so-called return current in three-dimensional space.In the second part,general properties of a needle-like EBP under the influence of the self-similar behavior of the energy and charge deposition were studied.The Monte Carlo simulation indicated that the energy and charge deposited by a needle-like electron beam with different initial energy at different pressures can be expressed in a self-similar form,that is,the energy and charge deposition can be expressed using a universal dimensionless shape function given in term of the beam range multiplied by a normalized coefficient,respectively.Analytical relations between the energy and charge deposition with the initial beam energy,beam current,and the discharge pressure were built,respectively.Meanwhile,with a start from the coupled model describing an EBP and some appropriate assuputions,indespendent equations controlling the time evolution of the plasma density and space charge density were built,respectively.Finally,analytical relations between the plasma density,space charge,spatial potential(electric field)and P?T0?Ib were obtained,respectively.These relations were given by us for the first time and quantitatively described the behavior of an EBP in the three-parameter space,which were totally different from the single-point description of the numerical simulation.Experimental investigations were divided into three parts.The first part was about the improvement on the EBP apparatus and the properties of the beam current.The main improvement was on the conductance-limiting pipe.Based on the gas flow theory in viscous tube given by Santeler,it was founded that the limiting effect of the short pipe on the gas flow was almost equivalent to that of the orifice under some certain conditions.The experiments on the new designed conductance-limiting pipe indicated that it could achieve the almost same pressure difference as the original one.Meanwhile,the new one was hard to burn but easy to locate.In the second part,experimental investigations on plasma density and spatial potential were presented.The discharge image and the ion current obtained from the electrostatic probe illustrated the spatial distribution of the plasma density,which was qualitatively consistent with that energy deposition distribution computed from the Monte Carlo simulation.The maximum electron current and the positive floating potential found in the current-voltage curves of the boundary surfaces indicated the ion motion was limited,which was an important feature of the plasma at medium and high pressure and had an important effect on the plasma properties.It was also observed that the positive floating potential increased with the increase of the discharge pressure,which was in agreement with the theoretical results given in the third chapter.In the third part,a justifiable kinetic model in an argon EBP at medium and high pressure and small beam current was established based on the experimental and numerical investigation on the time correlation between the electron beam current and the plasma density.By using the characteristic frequency in the electron beam waveform,the time correlation between the ion current obtained from the electrostatic probe and the electron beam waveform,and the change of the residual density with the change of the pressure and density were obtained experimentally.The comparison between the numerical simulation and the experimental results indicated that the influence of the excited particles on the plasma density was very small,which significantly simplified the kinetic model.