Determination of the Plasma Potential using emissive Probes - Implications from PIC simulations

The plasma layer in front of an emissive probe was studied with a particle-in-cell code including a Fokker-Planck model of Coulomb collisions. The results are compared with a 1-D analytic model and an experiment performed in a gas discharge plasma. At the wall in front of a strongly emitting probe a space charge double layer is formed which limits the emission current. Under floating conditions the potential in the double layer decreases towards the wall, reaches a minimum, and then rises again. The potential drop between the sheath edge and the minimum potential is limited by the development of the double layer. The rise of the potential just in front of the wall is of the order of a few kTwall. Therefore, the floating potential of an emissive probe lies (in a hydrogen plasma) up to 0.7 kTe below the presheath potential and is not, as is often assumed, at the plasma potential. An analytic model for the floating potential measured by an emissive probe was found to be in good agreement with measurements in an Ar plasma.