Experimental Diagnostics And Numerical Simulation Of Static Sheath And Presheath Near Metallic Meshes In Weakly Collisional Plasmas

Sheath is one of the oldest problems in plasma physics. It is also one of the most important basic problems which is not only significant in basic plasma research, but also shows great importance in applications, such as plasma diagnostics, superficial treatment and magnetic-confined fusion. Though a large number of theoretical publications can be found in the literature, experimental studies of the boundary layer are rare owing to the difficulties of accurately measuring the region without significant disturbances. Previous studies were focused on the boundary structure near an absorbing wall or electrode. In laboratory plasma devices, partially transmitting electrodes (e.g. a mesh or grid) were often used to control plasma parameters or launch collective waves. The characteristics of the excited waves depend sensitively on the sheath structure near the mesh. The sheath and presheath profiles will be quite different for the completely and partially absorbing walls since their boundary conditions are different. Thus, it is of practical importance to investigate the boundary layer near the mesh.In this dissertation we mainly concentrated experimentally on the sheath and presheath structure nearby a metallic mesh in a modified double plasma device. We investigated the difference of static ion sheath structure between the metallic mesh and plate, and the potential distributions in the boundary layer near meshes with different mesh spacing. The sheath structures near the metallic meshes have also been investigated theoretically by numerical simulations.First, a negatively biased mesh has obvious influence on the nearby plasma param-eters. The electron temperature obtained by Langmuir probe is increasing observably, and the electron density is decreasing much more obvious than that in bulk plasma. The length of that obviously affected region near the mesh is close to ion mean free path, which could be considered as presheath near the mesh. These variation tendencies before-mentioned could be used to determine the boundary between bulk plasma and presheath.Using an emissive probe a complete potential distribution around an negatively biased meshes is measured. The experimental measurement of ion sheath accords with the theoretical result of Bohm sheath and Child-Langmuir sheath qualitatively. The transition region in boundary layer in weakly collisional plasma is also determined near the mesh, which accords with the theoretical result of Riemann's transition theory.Secondly, complete potential distributions around an negatively biased meshes and plate are measured by using an emissive probe. The experimental results show that the variation tendencies of potential distribution within sheath are the same, but the sheath thickness of the mesh is much smaller than that of the plate, and the average electric field near the mesh is larger. The solid metal plate can be viewed as totally absorbing wall, but the mesh is partially transparent to the ions. Since sheaths form on both sides of the negatively biased mesh, the ions moving towards the mesh in the sheath on one side can partially traverse through the mesh and enter into the sheath on the other side, adding to the ion density on that side. Thus, the ion density in the sheath regions around the mesh is higher than that around the plate. The electric field is therefore stronger in the sheath of the mesh. For the same potential drop between the bulk plasma and the wall, this results in the narrowing of the sheath region near the mesh.Thirdly, the measurements of the potential distributions in the boundary layer near meshes with different mesh spacing were conducted in weakly collisional plasmas using a emissive probe. It was shown that, because the meshes are partially transparent to ions, the sheath is thinner and the electric field is stronger for the mesh of higher transmissivity, owing to the increased ion density in the sheath contributed from the ions transmitted from the other side of the mesh. However, the potential profiles in the presheath remain almost the same for different meshes except for the shift of the sheath-presheath edge. Furthermore, the measured electric fields at the edge are close to that from the models of a transition region and independent with mesh spacing.Finally, a 2-dimensional fluid model of collisionless sheath near the metallic mesh is build and solved by finite difference method. The computed results accord with our experimental results. It is helpful in understanding the sheath structures of meshes and can make up the deficiency of experimental diagnostics. The main conclusions of numerical simulation are as follows. The equivalent potential of the mesh surface is higher than the bias voltage of the mesh, because the mesh is partially transparent to the ions, and the ions is accumulated in great numbers between fine wires of the mesh. While the surface potential of metallic plate is equal to the bias voltage. As a result, the sheath thickness of mesh is thinner than that of plate. The ion density between fine wires of the mesh is greater, when the mesh spacing becomes wider. So the mesh with the larger geometrical transparency has a higher superficial equivalent potential and a smaller sheath thickness.