Abstract: The plume plasma potential distribution of ion thruster could deeply affect the normal work of the whole spacecraft and cause some extremely serious problems. In our experiments an emissive probe was used to measure the beam plasma potential of a 20 cm-ion thruster using Xe as propellants. Experiments were performed at axial distances from 250 mm to 900 mm downstream of the thruster′s exit plane and radial locations ranging from 0 to 450 mm. The emissive probe based on thermionic electron emission was made of 0.1 mm diameter tungsten filament with its heat current from 1.5 A to 2.5 A. The knee point of the emissive probe′s  I-V curve is clearer than that of the Langmuir probe, so the space potential distribution of our measurement results is more accurate. Emissive probe has been broadly used to diagnose plasma potential abroad, but lots of literatures indicate that there are still some divergences in the probe′s data processing methods. The probe′s working physical process of the probe was studied, and the effects of the emissive thermionic electron quantity on plasma potential diagnosis results were analyzed. Different processing methods of the probe′s  I-V curve were adopted and the advantages and disadvantages of each method were discussed. The reasons why measurement results deviate from the true plasma potential distribution were analyzed, which not only provide references for the structural reform of Emissive probe and the selection of heating current,but also give some suggestions for the plume simulation model and the improvement of thruster performance.

Key words: ion thruster, plume diagnostic, space potential, emissive probe, thermionic emission