Basic Processes of Plasma Propulsion

In order to improve performance, lifetime and reliability of plasma thrusters one has to understand and assess the fundamental processes and problem areas like nonequilibrium magneto plasma dynamics flows in electric discharges, their stability and electrode erosion effects. In order to assess the flow, pressure and density or temperature fields in plasma thruster devices several theoretical model calculations have been developed and applied to different thruster configurations. The results of these calculations are compared with corresponding experimental arc devices and both agree well with each other. Concerning the plasma stability work, a new explanation of the so called Onset Phenomenon is presented. It is based on a run-away Joule heating effect caused by the self magnetic field of a current carrying plasma channel which drastically decreases the radial heat conduction losses due to the electrons. The dependency of the electron heatflux vector and heat conduction coefficient in a transverse magnetic field has been derived by means of a perturbation approach of the Maxwell distribution. Cathode erosion measurements have been conducted by means of a fully automatically working test rig. The cathode sample is repeatedly charged by a fairly rectangular current pulse of 1400 A which lasts about 2 ms. Erosion rate measurements on thoriated (2% Thorium Oxide) cathode samples are reported on and explained by a refined cathode spot theory.