Overall performance of the thruster, its integrated characteristics and divergence of a stream of plasma essentially depend on the physical processes occurring in the field of an arrangement of the cathode, and, first of all, from distribution of potential of plasma space in a zone near to cathode. In the article, the analysis of optimum position of the cathode concerning a plume of plasma is resulted. Research was committed on the stationary plasma thruster М- 35 of small diameter. Cathode on cesium compound with the internal discharge and xenon as working fluid was used as neutralizer. Influence of mass flow rate through the cathode on coupling voltage with a plume of plasma and the estimation of plasma plume potential was experimentally received.