There’s a deep hole here starting with the speculation around Teslas ideas on the matter…https://www.sciencehistory.org/stories/magazine/the-undying-appeal-of-nikola-teslas-death-ray/

If you think about an energy system like nuclear energy where you design to prevent the cascade of exponential energy within that system, achieve critical state (ie boron rod exposure controls neutron levels such input=absorption), it’s easy to imagine how you can get as much energy density as you want in any given system. Hard part is controlling it so it doesn’t blow up.