Very vague indeed. Back when I was in college (the 80s) at least one of our physics professors had a home brew particle accelerator -- which will let you do fus

Very vague indeed. Back when I was in college (the 80s) at least one of our physics professors had a home brew particle accelerator -- which will let you do fusion very inefficiently. I suspect we are talking about some device that can do fusion very inefficiently and can measure the energy output relative to input rather than in any way being a stepping stone to a workable design (e.g. a spark generator probably qualifies).