Princeton Satellite Systems was awarded its first patent in Japan, “Method to produce high specific impulse and moderate thrust from a fusion-powered rocket engine”. This technology was licensed from Princeton University’s Princeton Plasma Physics Laboratory. It is for a compact, low-neutron, nuclear fusion reactor that can be used as a rocket engine or as a power generator. The reactor can be built in sizes from 1 to 10 MW. A typical robotic spacecraft would use two engines. A human mission to Mars or the outer planet might use six 5 MW engines.
Here is the Japanese patent certificate.