A model for the basic plasma parameter profiles and the force exerted by fireballs with non-isothermal electrons

Abstract

As discovered in recent work, plasma fireballs have the ability to exert considerable force onto ions and neutrals and, hence, induce macroscopic gas flows. This property makes them interesting objects for fundamental scientific research. Furthermore, there are also the possibilities for applications in the field space propulsion. As there is a lack of fundamental understanding of these plasma phenomena, this article aims to enhance the physical knowledge of fireballs by presenting a mathematical model for the calculation of the force that can be provided by them. It will be shown that all the main plasma parameters such as the plasma potential and the electron density can be derived completely with the knowledge of the potential of the electrode and the radial electron temperature profile. The calculations show very good agreement with the experimental data if two species of electrons (i.e., fast and slow) are considered. Both electron populations have different temperature profiles as is shown with measurements. Furthermore, it will be demonstrated that the potential drop throughout the fireball is much larger than previously thought and that this larger potential drop can considerably contribute to the acceleration of ions in the double layer. This mechanism makes it more likely that the force exerted by the fireball is rather caused by heating of the neutrals via collisions with those accelerated ions and the high energetic ions themselves than by collisions between fast electrons and neutrals.