Experimental studies on high current arc discharges for magnetoplasmadynamic thrusters

Experimental studies at the University of Southern California (USC) in the Laboratory of Astronautical Plasma Dynamics are focused on prolonging the operational life of magnetoplasmadynamic (MPD) thruster cathodes. Optical pyrometric measurements of 2 percent thoriated tungsten cathodes have provided axial temperature profiles revealing locations of peak operating temperature. In addition, the trends between maximum temperature and varying discharge currents and mass flow rates are examined. The electron temperatures and electron densities near the active zone (the surface area of the cathode responsible for approximately 70% of the emitted current) were measured using Langmuir probing. The results will be set as boundary conditions for solid cathode models currently in development at USC. To lengthen survivability, cathodes have been constructed from thoriated tungsten due to its combined high melting temperature and low work function. Multiple trial runs varying in length and current on thoriated tungsten cathodes examined under a scanning electron microscope (SEM) have revealed typical pathways of thorium migration as well as changes in surface composition.