How does solar electric propulsion (ion propulsion) work?

Why is an ion plume blue?

An ion plume consists of a lot of electrically charged atoms streaming out of the spacecraft. They have just come from a plasma chamber and are enormously hot. The energy that heated them up also affected their electron configuration. When an atom absorbs energy, its electrons change location within the atom to a position known as a higher energy configuration. When the atom stops receiving energy (in this case moves away from the hot plasma chamber) the electrons go back to the lower energy configuration and release the energy it took to raise their energy configuration. This energy is released in the form of photons, or tiny bits of light. In this case, the light is blue. Other fuels may give off different colors.

What is an ion?
What happens to the ions once they leave the space craft?
What is energy?
What is wavelength?

What causes ions to leave DS1?
What does electrically charged mean?
What is plasma?
What's an electron?
What is electromagnetic radiation?

What makes EM radiation?