• electric propulsion
• mpd thruster
• ionization
• scaling

Ionization in MPD thrusters is a significant energy sink since power losses associated with it can indeed represent up to 50% of the total input power.
As both ionization and thrust generation are two strongly coupled processes, it is fundamental to gain more insight into the scaling mechanism of the ionization fraction.
Experimental activities carried out at Princeton University have shown that the electromagnetic scaling number ξ (that represents the value of the thruster current nondimensionalized by the current at which the kinetic power in the exhaust is equal to the power required to fully ionize the propellant) is a fundamental thrust scaling parameter for MPD thrusters.
The goal of the present work is to investigate the scaling between the ionization fraction and ξ, which has never been shown and demonstrated.
Using existing quantitative spectroscopic measurements, obtained inside the interelectrode region of a 20 kW MPD thruster, the variation of the ionization level in different operating conditions was analyzed.
The findings give evidence that ξ plays the role of fundamental scaling parameter for ionization and reveal the existence of two modes for ξ < 1 and ξ > 1: when ξ < 1, the ionization fraction scales linearly with ξ, while at ξ > 1 the process is eventually affected by erosion.
Data obtained with two different cathode configurations also show that the axial evolution of the ionization fraction is strongly influenced by either the position of the current attachment point or the chamber length.
However, when ξ increases, the ionization fraction profiles for both configurations show a very similar and constant trend in the first 60% of the cathode length.