Sest, Inc. proposes to perform a detailed evaluation at the both convertor and component levels of a small, low electrical output power (50 to 500 mW) Stirling cycle based convertor utilizing light weight radioisotope heater units (LWRHU) as the heat source. The proposed milliwatt radioisotope Stirling convertor (MRSC) will be optimized for output power with source temperatures in the range of 480 to 560 oK so as to provide electric power at convertor efficiencies in the range of 15 to 20% and sized to make maximum use of the existing LWRHU heat source. These efficiency levels are on the order of 2 to 4 times those of thermoelectric conversion systems. The evaluation processes will incorporate an in-depth evaluation of the structural materials and fabrications techniques required to maximize convertor specific power while at the same time insuring high reliability and long operating life. Due to the unique operating characteristics of convertors of this size a proof of concept linear alternator will be developed and tested during Phase I. At the completion of the proposed Phase I effort a specific final configuration for the detailed hardware design and fabrication in the Phase II effort will be fully defined. In addition a scaling study will be performed identify optimal configurations over the entire power range of interest.