MPD thrusters for space applications

Magneto-Plasma-Dynamic (MPD) thrusters are a particular type of electric thrusters, where the propellant is brought to the plasma state and accelerated by means of electrical energy, up to speeds which are not accessible to normal chemical thrusters. Electric propulsion is the future of space propulsion, since it allows a more efficient use of the propellant. In particular, MPD thrusters are a possible option for cases where a high power is required, such as orbit changes or interplanetary missions.

For a long time the use of MPD thrusters in real missions has been hindered by an efficiency loss observed at high power level. This phenomenon is associated to oscillations in the voltage applied between the electrodes (the thrust method is based on a high current discharge realized between a cathode and an anode, and on the use of the Lorentz force to accelerate the plasma itself) and to anode damage.

In collaboration with Centrospazio, in Pisa, we have addressed this problem by performing measurement campaigns using electric probes (Langmuir probes) and magnetic sensors in order to detect the plasma fluctuations associated to the efficiency loss.

The results have enabled us to formulate a well defined explanation for the onset of the critical regime. It has indeed been found that this regime is associated to the appearence of a mode with m=1 (where m is the azimuthal mode number) and n=1 (n is the axial mode number, where in analogy to toroidal fusion devices we consider the system as if it were closed on itself, i.e. periodic) at a frequency around 100 kHz. It has been shown that this kink-like perturbation appears only when the well known Kruskal-Shafranov criterion is violated, that is, in analogy to toroidal fusion devices, when the plasma current is high enough that the mode is resonant inside the plasma. In analogy to what is observed in fusion plasmas, the m=1/n=1 kink mode has adverse effects on the discharge performance.

Having understood the physics underlying the efficiency loss, it has been possible subsequently to invent a method to suppress the kink mode, avoiding the efficiency loss, which has been proved to be very effective. The method is based on the insertion in the thruster chamber of a dielectric plate, which intercepts the helical currents driven by the mode, thus preventing its growth. This technique has been covered by an international patent.

Essential bibliography

M. Zuin, R. Cavazzana, E. Martines, G. Serianni, V. Antoni, M. Bagatin, M. Andrenucci, F. Paganucci, P. Rossetti, Kink instability in applied-field magneto-plasma-dynamic thrusters, Physical Review Letters 92, 225003 (2004). [pdf]

M. Zuin, R. Cavazzana, E. Martines, G. Serianni, V. Antoni, M. Andrenucci, F. Paganucci, P. Rossetti, M. Signori, Kink instability suppression and improved efficiency in magneto-plasma-dynamic thrusters, Applied Physics Letters 89, 041504 (2006).

M. Andrenucci, F. Paganucci, P. Rossetti, M. Signori, V. Antoni, R. Cavazzana, E. Martines, G. Serianni, M. Zuin, Instability control system for magneto-plasma-dynamic thrusters, international patent n. WO 2006/075343 A1, published on 20 July 2006.