Electric probe measurements in the plume of an ion thruster

Results are presented of measurements with double and single electric probes in the beam of a UK-10 ion thruster, using xenon as the propellant gas. The probe material consisted of tungsten and secondary emission was of negligible importance. As a consequence of the large directed velocity of the ions, the ion current to a probe was essentially independent of probe voltage. Since the debye length was small compared with the probe radius, no ions or electrons were collected on the downstream part of a probe. The thruster was operated at a thrust level of 18 ± 0.3 mN. Detailed results are given for ion flux profiles and ion density in the beam, as well as for floating potential profiles and degree of nonneutralization. Ion densities reach a maximum of 1.8 ± 0.2 x 10'° cm~3 on the beam centerline at an axial distance of 15 ± 0.3 cm from the acceleration grid. The floating potential at that location is 8.5 ± 0.2 V with respect to the neutralizer, while the degree of nonneutralization there is on the order of 10~ 4. Characteristic electron energies vary from 0.5 to 2.7 eV. Evidence is obtained that the electron gas has a non-Maxwellian speed distribution function.