Ion thrust works because the atoms in the exhaust are accelerated to near light speed through repulsion of like charges. To use ion thrust for manned missions you would need vastly more power to accelerate more fuel to a higher percentage of c. Unlikely to be obtained with solar arrays but very realistic if you can transcend the soviet era propaganda against all things radioactive and incorporate a nuclear powered electric generator…

The way I understand how any ion engine works, you can design one around ANY element that can be ionized. The MOST efficient design for use within the inner solar system would be to collect the solar wind to use as fuel and ionize it with electrical power generated by solar cells. this would not provide enough thrust for a primary drive but could be used for fine tuning a course when used in conjunction with the mass collector and the solar array configured as a solar sail. It would necessarily be used only for robotic missions as it would be a very slow boat indeed because of the tenuousness of the solar wind.

We pioneered the technology and then NASA abandoned it. Russia pushed on and used them on their satellites from almost the beginning.

I would like to see them use a more common gas for thrust fuel though.