Characterization of the Ion Beam Neutralization of Plasma Thrusters Using Collecting and Emissive Langmuir Probes

In gridded electrostatic plasma thrusters, an ion current is extracted from a primary plasma and later accelerated to impart thrust. The operation modes are strongly influenced by the neutralization of the ion current which otherwise becomes space charge limited. This neutralization process is investigated in a small diameter ring cusp ion thruster by using both collecting and emissive Langmuir probes. The ion beam was neutralized by a DC current heated wire placed at the outer section of the thruster and the probes were located between 11 and 24 cm along the axial axis of symmetry. The outgoing ion current is determined by both the magnitude of the discharge current and the ion extraction and the acceleration potentials. The levels of thrust calculated from the emitted ion current are equivalent to similar devices but requiring lower electric power levels. The experimental results evidence that the electron neutralization current not only controls the space charge level of extracted ion currents, but it also affects the spatial properties of the emitted plasmas plumes. The electron temperature of the plasma decreases along the axial distance when the neutralization of the ion beam becomes more effective. On the contrary, higher neutralizer electron currents increase the outgoing plasma density. These experimental results suggest that subtle physical mechanisms are involved in the ion beam electron neutralization process (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)