The paper presents the results of experimental studies of the plume of low power stationary plasma thruster SPT-40 model, operating with krypton at discharge power of about 250 W, and with xenon at discharge power of about 200 W. A comparative analysis of the SPT -40 model plume characteristics was carried out that revealed significant differences in the distributions of current density of accelerated ions in the angle of deviation of the measurement directions from the thruster axis and in the distributions of average ion energies in the plume when operating with the above propellants. The flow parameters were measured by a multigrid probe – the retarding potential analyzer (RPA). This devise moved along the semicircular trajectory in a plane that included the thruster axis. The distance from the thruster exit plane to the RPA was equal to 6-7 outer diameters of the thruster-accelerating channel; at such distance, we can consider the thruster as a point source. The measurements were carried out at different operating modes in the propellant consumption and discharge voltage, and it is shown that during the SPT-40 model operation with xenon and krypton the half-angle of the plume divergence was about 70 degrees in the discharge voltage range 150-200 V and it decreased with the discharge voltage growth.

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Given paper presents the results of the propellant flow ionization peculiarities study in the discharge with crossed electric and magnetic fields burning in the different scale stationary plasma thruster (SPT) models with the ionization and acceleration layer shifted out of thruster, namely: there were determined the integral characteristics and dependences of the total current of the exhausting ions from thruster operating with Krypton under different operation modes. It is shown that the mentioned ion current and its ratio to the discharge current are typically reduced with increase of the discharge voltage and these trends are different from that ones for the standard SPT's. As result they cause the notable change of the thrust efficiency dependence on the discharge voltage. Keywords: Stationary Plasma Thruster, propellant flow ionization, gas discharge, crossed electric and magnetic fields.