Just for the "fun" of it, I wondered what a "nuclear fusion reactor" TOS style might look like.
I came across this webpage
, which contains as hand drawing of a colliding beam fusion reactor (almost 1960's style) that reminds me of these large GNDN engine room props
Usually theoretical fusion reactors have either a ring shaped accelerator tube
(tokamak) or are spherical in nature (i.e. several lasers in the inner skin of the sphere focus on one spot in the center to ignite the fuel). Maybe that could be a clue to these enigmatic soccer balls
sitting on the TOS energizers from Season Two on? (and an explanation why these large GNDN props literally moved into the background or went missing)