Generally, fireballs are spherical, highly luminous regions in comparably thin surrounding background plasma, which are bounded by a double layer. Fireballs were first reported by Lehmann in the beginning of the 20th century and have so far been seen capable of producing macroscopic flow in surrounding gas. Consequent studies suggested that fast electrons that heat the neutrals were responsible for the induced aforementioned gas flows. In addition, earlier studies had a crucial assumption with regard to isothermal electrons, a simplification that was later proven not be congruent with experimental data, as it was only applicable within the fireballs and outside the double layer. It is important to note that the isothermal electrons inside the double layer have different temperatures than the ones outside. Recently, it has been established that plasma fireballs possess the ability to exert considerable force onto ions and neutrals, and hence, induce macroscopic gas flows. This attribute makes them interesting objects for fundamental scientific research. In addition, they also have the capacity to be used in space propulsion applications. Unfortunately, there is so far, not much fundamental understanding of the plasma phenomena.
Recently, researchers led by Dr. Johannes Gruenwald from the Gruenwald Laboratories in Austria conducted a study that focused on enhancing the physical knowledge of fireballs. To achieve this goal, the researchers aimed at presenting a mathematical model that could enable calculation of the force that such fireballs can provide. extend Generally, they extended the theory of force exertion by plasma fireballs to fireballs with non-thermal electrons. Their work is currently published in the research journal, Physics of Plasmas.
The authors proposed a model that described the shape of the electron density and the radial plasma potential profile fairly accurate, so as to harmonize the discrepancies existing between the assumptions of existing analytical models and the available experimental data. The researchers proceeded to show that the main plasma parameters could be derived completely with the knowledge of the potential of the electrode and the radial electron temperature profile. Furthermore, they demonstrated that the potential drop throughout the fireball was much larger than previously thought and that this larger potential drop could considerably contribute to the acceleration of ions in the double layer.
They observed that the force exerted by the fireballs with non-isothermal electrons was considerable and the predicted values for the force exertion are in excellent agreement with the available measured data. In addition, they showed that plasma fireballs were capable of producing substantial thrust at very little input power and in low mass gases such as argon. Particularly, the fireballs were noted to produce additional ions very efficiently within the rather large potential drop of the surrounding sheath.
In summary, the study by Johannes Gruenwald and colleagues demonstrated a novel mathematical model for calculation of the force that can be provided by plasma fireballs. Since this force is considerable, fireballs open up new possibilities of highly efficient and cost effective plasma thrusters for space applications. Moreover, the theory of force exertion by plasma fireballs was generalized to fireballs with non-thermal electrons. Altogether, their report presented the first ever attempt to generalize existing models to fireball dynamics, and as such offered a stepping stone for further studies that will highlight on various issues, such as the kinetic effects, that have not been considered in this work.
“This was a very exciting and intense research project for my colleagues and me as we have been working on this for some years. However, we were able to provide new insights in a field of plasma physics that is not so well understood.” Said Dr. Greunwald in a statement to Advances in Engineering. He then added “Furthermore, we demonstrated that plasma fireballs with non-isothermal electrons exert considerable force on neutrals and ions and, thus, can be applied in a new generation of low cost, high-efficiency space thrusters. As my company provides technical and scientific consulting we are more than happy to share our expertise on this field with companies that are invested in space propulsion projects. You can visit us on www.g-labs.eu or contact me via email: [email protected]. G-Labs; ‘Science is our business’
J. Gruenwald, J. Kovačič, B. Fonda, T. Gyergyek. A model for the basic plasma parameter profiles and the force exerted by fireballs with non-isothermal electrons. Physics of Plasmas, volume 25, 113508 (2018)Go To Physics of Plasmas