Thank you for the input! I was going to write a paragraph about the fuel but then it just escaped me.
The purpose for the regular rocket diagram was to describe the basic process of a rocket operation, essentially conservation of mass and momentum, but in more simple terms. The plasma thruster operates by kicking out low mass high velocity particles (similar to an ion engine) from the nozzle. I must have done a poor job getting the comparison in there so I apologize for that!
The big point I missed was the fuel. So the reason that the walls are open and in a coil shape is that it uses residual gases as the thrusting medium as well as electrons. Electrons are stuck in the low potential region within the plasma and then once they build up enough pressure, which occurs extremely quickly, they then jet out the nozzle at very high velocities and take some gas molecules with them. I could not get a good picture of it, but there is a dull green glowing zone where the nozzle is pointing on the inside of the chamber wall. At lower pressures(where it comes out like a beam), you essentially are only firing electrons out of the rocket and they behave much more like electron gun than a thruster. So the propellant is a mix of electrons, water molecules, hydrogen, nitrogen, oxygen, and a few trace gases. Its just leftover air.
Hopefully this clarifies some things!
RE: Plasma Thruster!