Still, it's interesting that in "The Galileo Seven" they talk about the ion boosters and we see the exhaust coming out of the pods.
Now, you better show me the exact quote, because you just send me on a wild goose chase.
I have severe rationalization difficulties with Scotty's amazement witnessing "ion propulsion" in Spock's Brain ("can't do that, yet").
I had tended thus far to interprete this as a colloquialism for anti-deuterium fusion power (in the antimatter fusion process - don't ask me what it could be good for, it's a wild guess - antimatter neutrons would be released which would penetrate any known confinement field and have rather adverse effects on the materials housing such reactions.
) and just thought we might be on to something, but couldn't find "ion booster" in the episode's transcript.