Experimental investigation of the change of the electron saturation current of a dc-heated emissive probe

We report on experimental investigations of the change of the electron saturation current of a de-heated emissive probe with the probe heating current. According to the simple theory of the emissive probe, the electron saturation current should not be affected by emission. However, in many experiments a variation of the electron saturation current with the emission current was observed. We consider two possible reasons for such variations: (a) the influence of the space charge around the probe shaft, (b) the change of the work function of the probe surface material due to heating. We tried to find sufficient experimental evidence for supporting one or the other (or both) of these two explanations. We used two different types of plasma to validate the results: a cylindrical magnetron plasma and the non-magnetized plasma of a DP machine. From our experiments follows that the electron saturation current of the emissive probe is affected by the space charge effect as well as it depends on the probe wire material.