O the Phenomenon of Onset in Magnetoplasmadynamic Thrusters
Abstract
The performance of Magnetoplasmadynamic (MPD) thrusters has been severely limited by 'onset'. The destructive onset phenomenon refers to the abrupt increase in voltage oscillations accompanied by severe electrode erosion which occurs at a critical value of the cur- rent for a given mass flow rate. This research is aimed at explaining and quantifying onset. The steady, one-dimensional, self-field flow in the MPD thruster is first analyzed for frozen flow, equilibrium flow, and non-equilibrium flow. This quasi one-dimensional theory explains the behaviour of efficiency and predicts a new mechanism for the onset phenomenon. This model predicts that smoothly accelerating supersonic flow can exist only below a critical current level because of increasing back -EMF. This limit is interpreted as onset and correlates with the experimentally observed J('2)/m onset parameter. In order to quantify electrode erosion rates, the electrode- adjacent boundary layer is analyzed using the results of the quasi one-dimensional theory. The free stream boundary conditions for the steady boundary layer flow analysis are obtained from the results of the quasi one-dimensional theory. Since the electrode wall is rela - tively cold compared to the flow, and since relatively fewer collisions are required for the heavier particles to equilibrate, an approximate two temperature boundary layer theory is developed. The two tem- perature compressible non-equilibrium ionization equations are then solved using momentum integral and energy integral methods. Wall shear stress, wall heat flux, temperature, density, and ioniza - tion fraction profiles are then calculated. These results of the two temperature theory may then be used to estimate rates of erosion by evaporation and due to sputtering. The major accomplishments of this thesis include the develop- ment of a new theory that for the first time, predicts onset due to an excessive back-EMF at high currents, the establishment of the importance of ionization and recombination, and the first study of an electrode-adjacent boundary layer in MPD flow. This thesis addresses steady flow only. Thus, only the conditions leading up to onset are considered. No attempt is made to study any phenomena beyond the limit of onset. Such a theory would have to take into consideration the unsteady nature of the flow beyond onset. However, this research provides the basis for a more comprehensive approach to understanding and quantifying the phenomenon of onset and electrode erosion in MPD thrusters. Such a theory may. prove to be valuable in the design and perhaps future design of MPD thrusters. ('1)This work supported by AFOSR-83-0033.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1986
- Bibcode:
- 1986PhDT.......147S
- Keywords:
-
- EROSION;
- BOUNDARY LAYERS;
- PROPULSION;
- Physics: Fluid and Plasma