How they work: More detail

The last article was more of an overview of an ion thruster. Now I'll tell you how they really work, at the molecular scale.
First, definitions:
    Ionization: "Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons to form ions, often in conjunction with other chemical changes." ~Wikipedia, see sources, https://en.wikipedia.org/wiki/Ionization

    Plasma: Ionized (see above) gas.


In an ion thruster, the high voltage between the positive and negative electrodes (see previous post for the diagram) rips the electrons off the gas atoms (usually xenon, a gas similar in properties to neon), ionizing them. Gas that is ionized this way has a positive charge, since the electrons that were ripped off were negative. If something is charged it acts like a magnet, and that is exactly what the gas molecules do, pushing each other apart and causing the gas to expand.
Since magnetic fields can be used to move ions, electromagnets are used to direct and accelerate the plasma out the back of the thruster. This is also helped along by the fact that the backward electrode on the thruster is negative, and therefore attracts the positively ionized plasma (like magnets).