Coils outside the chamber create a magnetic field, which causes electrons from the cathode to spiral and become trapped at the mouth of the chamber.
The build-up of negatively-charged electrons at the mouth of the chamber attracts the positively-charged ions, accelerating them out of the chamber.
The stream of accelerated ions leaving the chamber thrusts the spacecraft forward. Although the force is small, over time it creates great speed in the frictionless environment of space.
