This page has a better diagram of the ion engine.
http://dawn.jpl.nasa.gov/mission/ion_prop.asp
You can see that the electrons are absorbed by the first charge plate, so that only the Xenon+ ions get accelerated by the 2 charge plates. It also shows the electron gun, called the "neutralizer cathode."
... or impractically large solar arrays.
I happen to be a believer in large solar arrays. If you had a 100m x 100m square solar array, the area would be 10,000m2 . If you are in the region between Earth and Mars, you get about 1 KW per m2 . If you head toward Venus, you might get 2 KW per m2 . So we have 10 megawatts to 20 megawatts to work with.
Now comes an issue of thrust vs efficiency, or thrust vs fuel economy (actually reaction mass economy). If you are shooting the Xenon+ ions at, say, 10% of the speed of light, you put a lot of energy into each ion and you get excellent fuel economy, but it takes a lot of energy. If you shoot out the same number of Xenon atoms at 5% of the speed of light, you get half the thrust, since P=MV and delta P = delta MV, but your energy goes down to 1/4 as much because E = MV2 . Actually you do a bit better than that because of relativity, but at 5% to 10% of the speed of light you can ignore relativity for BOTEC (Back Of The Envelope Calculations).
Now, let's use the same energy and use more fuel at lower exhaust velocity. You end up using 4 times as much fuel, but get twice the thrust. Let's plug in some real numbers. (This page http://dawn.jpl.nasa.gov/DawnClassrooms/2_ion_prop/ has all the data in lesson form, for HS physics.)
Dawn's engine grids are at +1090V and -225V, giving each ion an energy of 1315eV (1315 electron Volts). Velocity of each ion is 35,000m/s. Fuel consumption is 3.25 milligrams per second. Total power is about 10 KW. These are peak values.
I gotta go to an appointment, but that is enough numbers to work out the thrust of Dawns engines. I'm saying you could multiply the energy by 1000, and the thrust by 2000, by using larger solar arrays and reducing the exhaust velocity and the fuel economy.