AbstractThe critical technical issues, evaluation and comparison of two inertial fusion enery (IFE) reactor design concepts developed in the Prometheus studies are presented. The objectives of this study were (1) to identify and characterize the critical issues and the R&D required to resolve them, and (2) to establish a sound basis for future IFE technical and programmatic decisions by evaluating and comparing the different design concepts. Each critical issue contains several key physics and engineering issues associated with the major reactor components and impacts key aspects of feasibility, safety, and economic potential of IFE reactors. Genetic critical issues center around: (1) demonstration of moderate gain at low driver energy, (2) feasibility of direct drive targets, (3) feasibility of indirect drive targets for heavy ions, (4) feasibility of indirect drive target for laers, (5) cost reduction strategies for heavy ion drivers, (6) domonstration og higher overall laser driver efficiency, (7) tritium self-sufficiency in IFE reactors, (8) cavity clearing at IFE pulse repetition rates, (9) performance, reliability and lifetime of final laser optics, (10) viability of liquid metal film for first wall protection, (11) fabricability, reliability and lifetime of SiC composite structures, (12) validation of radiation shielding requirements, design tools, and nuclear data, (13) reliability and lifetime of laser and heavy ion drivers, (14) demonstration of large-scale non-linear optical laser driver architecture, (15) demonstration of cost effective KrF amplifiers, and (16) demonstration of low cost, high volume target production techniques. Quantitative evaluation and comparison of the two design options have been made with special focus on physics feasibility, engineering feasibility, economics, safety and environment, and research and development (R&D) requirements. Two key conclusions are made based on the overall evaluation analysis: (1) The heavy-ion driven reactors appear to have an overall advantage over laser-driven reactors; and (2) However, the differences in scores are not large and future results of R&D could change the overall ranking of the two IFE concepts.