An AMTEC performance and evaluation analysis model (APEAM) was developed to predict the performance of next-generation Pluto/Express, multitube, vapor-anode alkali-metal thermal-to-electric conversion (AMTEC) cells. The model results compared well with the experimental data of PX-4C, PX-5A and PX-3A cells, tested at the Air Force Research Laboratory. The effects of various design changes on the performance of the cells were investigated. These changes were: (a) using a Creare condenser; (b) changing the number of BASE tubes and the electrode length; (c) using other electrode materials; (d) using molybdenum (Mo) structure on the hot side of the cell; and (e) using reflective rhodium coatings in the low vapor pressure cavity of the cell. The results showed that a stainless steel (SS) structure cell with improved electrodes (50% lower contact resistance than TiN, and an exchange current coefficient = 200 A.K-1/2/Pa.m(2)) could deliver 7.1 We at 3.5 V and an efficiency of 19.5%. When Mo was substituted for the SS on the hot side of the cell, the electrical power output increased to 8.8 We, and the cell efficiency increased to 20.2%, Using rhodium coatings in the Mo/SS cell increased the electrical power output to 9.7 We and the conversion efficiency to 22.5%.