Accelerating the ‘plain air’ isn’t really necessary to the propulsion. Once you’ve accelerated the ions you’ve already accomplished a momentum transfer - if tho

Accelerating the ‘plain air’ isn’t really necessary to the propulsion. Once you’ve accelerated the ions you’ve already accomplished a momentum transfer - if those ions then go and share that momentum with some non ionized air molecules that’s their own business. I guess maybe the non ionized air acts to keep the ions around between the two charged grids for longer, which means that for each ion you create you can maybe exploit its attractive/repulsive force for longer?