Solid Thoriated Tungsten Cathode Arc Discharges for Electrically Propelled Spacecraft

Since the early 1960s to mid-1960s, laboratory studies have demonstrated the unique ability of magnetoplasmadynamic (MPD) thrusters to impart an exceptionally high level of specific impulse and thrust at large power processing densities. These intrinsic advantages are why MPD thrusters have been identified as a prime candidate for future long-duration space missions, including piloted Mars, Mars cargo, lunar cargo, and other missions beyond the low Earth orbit. The large total impulse requirements inherent of the long-duration space missions demand the thruster to operate for a significant fraction of the mission burn time while requiring the cathodes to operate at 1-10 000 kW for 2-10 000 h. The high current levels lead to high operational temperatures and a corresponding steady depletion of the cathode material by evaporation. This mechanism has been identified as the life-limiting component of MPD thrusters. Continuing work in the Laboratory of Astronautical Plasma Dynamics at the University of Southern California (USC) is focused on extending the operational life of the cathode. In this paper, axial temperature profiles obtained under varying current levels (20-60 A) and argon gas mass flow rates (450-640 sccm) of a solid 2% thoriated tungsten cathode are presented. 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 in development at USC.