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A preliminary investigation on the feasibility of a wall diagnostic using a plasma microjet has been carried out. The basic idea is to sputter the first layers of the surface with the positive ions supplied by a plasma. Neutrals coming out from the surface are excited by the plasma itself and detected by Optical Emission Spectroscopy (OES). Positive primary ions are delivered by a DC discharge with a capillary used as anode. Efforts have been paid in the first part of the work to reproduce a microdischarge first in a cathodic way and then in a anodic way. The anodic microdischarge showed the same features of the current-voltage curve for the cathodic discharge. The optical performances of the anodic discharge are superior to those of the cathodic one since more excited ions are produced for the same operating parameters. The plasma density is of the same order of magnitude it has in the cathodic case. The production of excited neutrals and ions strongly depends on the pressure, and the anode-cathode gap distance and much less on the gas flow rate. The intensity of the signal from the surface depends on the pressure, the discharge current and the gap distance. The intensity is not linearly related to the primary ion current. In fact it turned out that the signal depends on both local melting and sputtering taking place on the surface. Heat transfer to the surface is the driving factor since no signal at all was collected in samples melting at high temperature. It is finally shown that the diagnostic cannot be realized at high pressure as envisaged. However, several issues evidenced by the experiment are not yet clear and require further studies. Finally a different application of the microjet is proposed. |