Solar thermal Alkali-Metal-Thermal-to-Electric-Conversion (AMTEC) power systems potentially have several important advantages over current solar photovoltaic power systems in ultra-high-power spacecraft applications for medium-earth orbit (MEO) and geosynchronous orbit (GEO) missions. This work presents key aspects of radial AMTEC cell design to achieve high cell performance in solar AMTEC systems delivering larger than 50 kW(e) with AMTEC cell conversion efficiency larger than 22 percent. A new design parameter methodology is demonstrated establishing optimum design parameters in radial cell design to satisfy high-power mission requirements. Specific temperature-and pressure-dependent relationships define critical dependencies between key cell design parameters, particularly the impact of parasitic thermal losses on Beta Alumina Solid Electrolyte (BASE) area requirements, voltage. BASE tube number, and system power production for maximum power-per-BASE-area and optimum efficiency conditions. High-level system tradeoffs are demonstrated using the design parameter methodology to establish high-power radial cell design requirements and determine optimum radial AMTEC designs. [S0199-623(00)00102-7].