Diffusion Flow In The Experimental Study On Downstream Sheath Structure

We have measured the sheath potential distributions in the upstreamand the downstream sides of a di?usion plasma ?ow using a fine structuredemissive probe in the boundary region of a small mesh and a metal platewhich have two electrically insulated sides. It is shown that the sheathon the upstream and downstream sides of a ?owing plasma shows obviousasymmetry. For metal plate, the sheath region on the downstream side isthicker than that on the upstream side. The measured electric field at thesheath edge on the upstream is larger than the value on the downstreamside. While for the small mesh, the asymmetry of the sheath on theupstream and downstream sides is not obvious. At the presheath, themetal plate and the small mesh show the same feature. They both havea structure Similar to the virtual anode.The collected current of the metal plate and the small mesh wasrecorded. For the two sides metal plate, it's easy to distinguish therecorded current on the upstream and downstream side. And then, theCL sheath thickness can be calculated using the CL formula. The sheathstructures of the metal plate were also numerically calculated using one-dimensional ?uid equations including ionization and collision terms. Bothexperimental and numerical results show that the thicknesses of the Child-Langmuir (CL) sheath in the two sides of the plate satisfy a linear rela-tion, with a fitting proportionality of 1.2 between the down and upstreamthickness. The collected current of the small mesh is not distinctive onthe upstream and downstream side. For the partly absorption, the CLformula must be modified when using for the small mesh.The experimentally measured sheath thicknesses and the electric fieldswere compared with numerical results and with some theoretical modelsin the literature. The electric field at the CL sheath edge in both sides is consistent with the value given by the Godyak's model for the metal plate,and the electric field at the CL sheath edge of a small mesh is consistentwith the value given by a modified Godyak's model. At the Bohm sheathedge the electric field in the upstream side is close to Rieamann's modelbut that in the downstream side deviates from available models.In the end, for considered the in?uence of the experiment target bythe voltage bias of the separate grid, the sheath structure of the separategrid in the downstream also has been measured. With the negative biason the separate grid increases, the electron density and space potentialincreases, the electron temperature decreases.In addition, a oscillograph datareading programmed with LabVIEWhas convenient to the experiments data transfer.