An ion thruster discharge cathode assembly (DCA) erosion theory is presented based on near-DCA NSTAR plasma measurements and experimental results for propellant flow rate effects on ion number density. The plasma potential structures are utilized in an ion trajectory algorithm to determine the location and angle at the DCA keeper of bombarding ions. These results suggest that the plasma potential structure causes a chamfering of the DCA keeper orifice. Results from tests with an instrumented DCA show that increasing DC propellant flow rate causes a decrease in "keeper" orifice ion number density, most likely due to charge-exchange and elastic collisions. Combining these two results, the known wear-test and extended life test (ELT) DCA erosion profiles can be qualitatively explained. Specifically, the change in the wear profile from the DCA keeper downstream face to the keeper orifice for the ELT may be a result of the reduction in DCA propellant flow rate when the thruster operating point is changed from the TH 15 to TH 8.