The design and operation modes of a small, low power ion plasma thruster and the properties of the emitted plasma plumes are discussed. The ion beam is extracted from a primary plasma produced by a stationary low pressure electric discharge where the ion production rate is essentially determined by the discharge current. The experiments evidence that the electron neutralization current controls the space charge levels of the outgoing ion current and also influences the spatial properties of emitted plasmas. The electron plasma density increases with the discharge and the electron neutralization currents, while decreases as the plasma plume expands. However, the corresponding electron temperatures decrease when the electron neutralization currents increments. The collisional origin of this electron cooling effect is excluded because of the large collisional mean free paths involved. Then, these electron energy losses during the neutralization of the ion beam would be caused by more subtle physical mechanisms than collisions. The experimental results are compared with previous numerical simulations and similar phenomena found in other experiments.

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.

The measurement of the plasma potential of unmagnetized Maxwellian plasmas by using the floating potential of emissive Langmuir probes is discussed. The temperature of the probe was monitored in order to estimate the emitted thermoionic electron current and to determine the limits of the strong electron emission regime. Under these ideal conditions, the measurements of the plasma potential of the emissive probe are cross checked against those of collecting Langmuir probes. In agreement with previous works, the current voltage curves of emissive probes show the temperature dependent electron saturation currents. An empirical expression is suggested for the dependence of this saturation current with the probe temperature which recovers the response of collecting probes when the probe is cold. The experimental data indicate that the floating potential of the emissive probe is close to the local plasma potential only when the emitted thermoionic electron current is similar to the electron saturation current. Larger electron emission currents lead to small increments the floating potential over the local plasma potential. Our results suggest that the electron saturation current of a hot emissive probe is composed by the thermal electrons from the plasma and a returned fraction of the thermoionic emitted electrons.

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