Fundamental Scaling Laws for Electric Propulsion Concepts –Part 2: MPD Thrusters

As a second part of multi-phase study aimed at highlighting the fundamental scaling laws relevant to the main types of plasma thrusters, this paper focuses on magnetoplasmadynamic thrusters. The basic principles underlying the operation of different types of plasma devices are first recalled. The ideal scaling trends of self-field and applied-field MPD thrusters are outlined. Real thruster behaviour is then considered and the role of the "onset" phenomena as the principal performance-limiting factor in this class of thrusters is highlighted. Different theories on the onset are reviewed. A general stability criterion proposed by Tikhonov is discussed. Results of recent activities carried aut by the authors to obtain a better insight into the onset problem are then described. Large-scale magnetohydrodynamic instabilities have been shown to develop whenever the current rises beyond a threshold value. These instabilities have features of helical kink modes and their occurrence can be described by the Kruskal-Shafranov criterion. The growth of these modes is deemed to be the cause of the large loss of efficiency observed in MPD thrusters when operating at high current. The connection between Tikhonov's criterion and the Kruskal- Shafranov limit is discussed. This insight may suggest better ways to control or postpone the onset so as to achieve higher efficiency levels