Mass efficiency in electric rocket propulsion? Why is it more mass efficient to strip away only one electron in electric propulsion even though a double-charged ion is accelerated more than a single-charged ion? 
Please explain in layperson's terms if possible.
 A: What you want to achieve with these electric engines is  not so much the performance of chemical rockets, (a large amount of thrust that uses up fuel extremely quickly and gets you from 0 to 27,0000 km/hour in 8 minutes), as it is a far lower amount thrust sustained for a far longer period of time.

The electric engines use xenon gas, and it takes 1170 units (the units don't really matter, just the relative difference of effort) to remove the first electron compared to 2076 units to remove the second electron. When you have lots of the first level electrons removed, you don't really need to waste energy removing the next electron, especially when you are prepared wait a while (months or more) for the acceleration to accumulate to something substantial.
Ion Engines Wikipedia

The Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from a hot cathode filament and when accelerated in the electrical field of the cathode, fall to the anode. Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge without a cathode (radio frequency ion thruster).
The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system by the plasma sheath, the ions are accelerated by the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of (typically) 1–2 keV, generating the thrust.
Ion thrusters emit a beam of positive charged xenon ions only. To avoid charging up the spacecraft, another cathode is placed near the engine to emit an electron current into the ion beam with net neutral electrostatic charge.This prevents the beam of ions from being attracted (and returning) to the spacecraft, cancelling the thrust

