A semi free-piston Stirling engine alternator, called NALSEM500, has been experimentally studied since April 1994, as part of a space solar power technology program in NAL (National Aerospace Laboratory). The Stirling alternator is one of candidates for future space power applications, and will be integrated with the solar cavity receiver subsystem, having both Na heat pipes and thermal energy storage. Through a series of preliminary bench tests, it was found that the efficiency of the linear alternator was significantly low as compared to our initial design. To improve the alternator performance, some design modifications of the magnet assembly and stator core have been made. As a result, the thermodynamic efficiency of 33% and system efficiency of 20% was obtained.