The potential distributions in the boundary regions of a mesh and solid metal plate were measured using a fine structured emissive probe and compared with theoretical models. It is shown that the sheath region near the mesh is thinner than that near the plate, owing to the fact that the mesh is partially transparent to the ions. The measured electric field at the Bohm sheath edge is in agreement with the asymptotic result in the transition region, but the field at the Child–Langmuir (CL) sheath edge is larger than the values of several models found in the literature. The potential distribution in the CL sheath region agrees with the theoretical curve in the case of the plate, but deviates from the theoretical curve in the case of the mesh.
The plasma sheath in the presence of virtual-cathode structure near a hot cathode was studied by combining the theory of thermionic emission and plasma fluid equations. Using Sagdeev potential method, the sheath solution and generalized Bohm criterion were discussed. It is shown that, different from usual Bohm sheath, the critical ion Mach number at the sheath-presheath edge is not an independent constant but depends on sheath potential drop as well as other parameters, owing to the fact that there exists large quantity of thermionic electrons near the hot cathode which has a significant influence on the whole sheath structure. The critical Mach number first increases and then decreases with the potential drop (from sheath-presheath edge to the virtual cathode), and monotonically increases with the temperature of the hot cathode. In the plane geometry, there exists an appreciable quantity of residual thermionic electrons which traverse the virtual cathode and sheath and enter into bulk plasma.
It is described that the distribution of the horizontal electric field above a striped electrode can be inferred from the trajectory of a single fine particle with known mass and diameter. The striped electrode consists of 100 segmented stainless steel strips, each electrically insulated. A traveling periodic potential profile is produced above the striped electrode by modulating the voltage signals on the strips. When the voltage modulation is on, the fine particle, which is originally levitated in the sheath region above the striped electrode, experiences a periodic oscillation along both the vertical and the horizontal directions because of the periodic electric force arising from the modulation voltages. Tracking the motion of the fine particles, the electric force is obtained from the momentum equation including the gravity and the neutral gas friction. With the particle charge estimated by the vertical oscillation method, the electric field can be derived. The horizontal electric field obtained by this method is in agreement with the result predicted by a collisional particle-in-cell simulation.
The measurements of the potential distributions in the boundary layer near meshes with different mesh spacing were conducted in weakly collisional plasmas using a fine-structured emissive probe and the results of the sheath thickness and electric field at the sheath-presheath edge were compared with theoretical models of collisional presheath and collisionless sheath. 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. The thickness of the sheath decreases while the electric field at the edge increases with the increase of the neutral gas pressure. Furthermore, depending on the pressure, the measured electric fields at the edge are close to that from the models of a transition region.
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