Application of the Multifluid Code OrCa2D for Simulations of the Multichannel Cathode and Anode Discharge in High Power Lithium Magnetoplasmadynamic Thrusters

This paper presents the first application of the 2-D axisymmetric Orificed Cathode (OrCa2D) computational code to model lithium-based magnetoplasmadynamic thrusters (MPDTs) for NASA's Space Nuclear Propulsion project. The research addresses critical modeling needs for high-power electric propulsion systems targeting 60% efficiency at specific impulses exceeding 4000 s and power levels of 0.5-1 MWe. We have enhanced OrCa2D with lithium-specific physics models, incorporating multi-state excitation and ionization processes essential for accurate lithium plasma simulations, since lithium exhibits complex multi-step ionization pathways through various excited states with excitation cross-sections approximately 20 times larger than direct ionization cross-sections. Initial single-channel simulations of a multi-channel hollow cathode (MCHC) operating at 10 kA total current and 300 mg/s flow rate demonstrate the code's capability to compute key operational parameters, showing that emitter temperatures range from approximately 1400 K for barium-infused tungsten to 3000 K for pure tungsten depending on work function values. The paper also establishes OrCa2D's theoretical suitability for full MPDT discharge simulations by demonstrating how its comprehensive multispecies transport equations naturally reduce to traditional single-fluid magnetohydrodynamic approximations while offering enhanced physics modeling capabilities including nonequilibrium ionization effects and sheath boundary conditions. The work provides essential computational tools for addressing the primary technical challenges facing high-power MPDT development, particularly the ambitious lifetime targets of 23-35 kh operation required for future Mars missions.