As a child, I read Friday by Robert A Heinlein, which portrayed a future in which energy needs are addressed by energy storage devices called "Shipstones", which are described as a way to pack more kilowatt-hours into a smaller space and a smaller mass than any other engineer had ever dreamed of. To call it an "improved storage battery" (as some early accounts did) is like calling an H-bomb an "improved firecracker."…
The challenge for fusion is that it's so phenomenally difficult to tap that gradient (without annoying the neighbors) that it may well prove impossible forever.