Please note, textbooks lack any information on the Fusion Fuel Cell. Rampant speculation works here. Besides, if "rest mass" possessed credibility, Einstein's postulates crash. He said mass equals energy. And special relativity requires the conservation of energy, therefore, mass. If objects become heavier during near luminal speeds, where does the extra mass originate? Furthermore, photon capture imparts extra mass to the target exactly equal to the photon mass. Then "rest mass" becomes real and quantifiable.
A re-write of the Fusion Fuel Cell with comments above integrated, further comments welcome: