How does solar electric propulsion (ion propulsion) work?

What causes the ions to leave DS1?

There are two electrically charged grids at the back of the thruster. They form, in a way, a system to propel atoms between the plasma chamber and the vastness of space. These two grids are a strongly positive one on the inside and a more weakly negative one on the outside.

When the positive ions in the plasma get between these grids, they are repulsed strongly by the positive grid and attracted less strongly to the negative one. This causes them to fly away from the positive grid towards the negative one. This is called electrostatic acceleration.

The ions fly away from the positive grid towards the negative one, but the attractive force of the negative grid is not sufficiently strong to overcome the repulsive power of the positive grid. The ions are given direction by the pull of the negative grid, but the pull is not enough to keep them in between the grids. They shoot out past the second grid and out of the thruster entirely. The force of these atoms going backwards makes thrust to propel the ship forward.

How does DS1's engine work?
How could something as small as an atom move a space craft?
What is an ion?
How does propulsion work?

How fast does DS1 go?
How are ions accelerated?
What is plasma?
What happens to the ions once they leave the space craft?
Are ion engines radioactive?
Are ion engines nuclear?

Why do ion engines need a vacuum to work?