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

Abstract

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.

Propagator integral method for plasma physics kinetic equations with practical applications

“…The density of particles reaches a higher value for negative positions, while is reduced for x > 0.1, respect to the case without force. This effect is qualitatively similar to what happens in the acceleration grid of an ion thruster [99,100], where, due to the intense electric field, generated by means of polarised grids, the ions are exhausted at high speed to generate thrust and the electrons are slowed down to keep them inside the discharge chamber and contribute to the ionization process.…”

“…This reference voltage can be easily modified before starting the measurement process. Even when this circuit is not unique and other options exist in the literature [137], this scheme has been tested in previous works [99,100,141], providing good results and its verification or possible improvements are out of this thesis scope.…”

“…In the experimental setup employed to obtain the results presented below in this section, the plasma was generated by means of a ring cusp ion thruster, operated with an argon gas flow, as that described in [99,100]. The density and temperature ranges of the electrons in the cold plasmas involved were n e = 10 7 -10 8 cm −3 and T e = (0.75-1.25) eV, respectively.…”

“…This type of plasma is widely used for surface treatment [5,6], electric propulsion [8] and medicine [57]. Its dynamics is of main interest in this dissertation to develop and solve kinetic models that improve the understanding of plasmas generated in the low temperature plasma laboratory of the UPM PlasmaLab 1 , along with the improvement of the ALPHIE electric thruster [100,127].…”

“…Its study is of mayor importance, e.g., to the analysis of an electric thruster [100] and its evolution should be included in the models if the computational resources allow it. Furthermore, its dynamics affects the evolution of a plasma as a whole, specially the self-consistent electric field when a charge separation appears [101].…”