If Mana is a fifth force besides Strong/Weak force, Electromagnetism and Gravity, wouldn't spellcasting be the process to use some of this energy, convert/transform/shift it somehow and use it affect the other four forces and matter?
So "unlimited spellcasting because of fusion reactor" might not work unless you can transform electrical energy into Mana.
So "unlimited spellcasting because of fusion reactor" might not work unless you can transform electrical energy into Mana.