It is observed in the operation of the Sheet Plasma Negative Ion Source (SPNIS), that the sheet configuration of the plasma cannot be maintained at pressures above a certain limit. For instance hydrogen plasma disperses and shows spatial inhomogeneity at pressures above 0.05 Torr, while at pressures higher than 0.01 Torr the same occurs for argon plasma. The cause of the dispersion is not understood, as yet. This paper is the first of a series of attempts to find the root of the dispersing phenomenon. Focus was done on Streaming Instability as a probable cause.