Ion thrusters are a form of electric propulsion. They use electrical forces to throw ions (atoms or molecules whose total number of electrons does not match their total number of protons thus giving them an electrical charge) out the back of the engine and thereby generate thrust via Newton's 3rd law.
Hardtech ion thrusters do exist, and have been used on at least 9 probes and craft built by the mundane space agencies. However, hardtech ion drives have one major limitation. They take forever to get up to what the Fen consider a useful speed.
Because the ions they use for reaction mass are so tiny compared to the craft they are propelling the acceleration they provide is also tiny. As such Hardtech ion thrusters have extremely low accelerations. NASA et al make up for this by allowing for really long flight times to get their craft up to speed. Ion thrusters are however very efficient and frugal in the use of propellant which is why the 'Danes like them.
There are two general categories of hardtech ion thrusters, those which use electromagnetic force (Lorentz Force) to accelerate the ions and those which use electrostatic force (Coulomb Force) to accelerate them. The term 'ion thruster' is often only applied to electrostatic or gridded ion thrusters.
Wavetech ion thrusters have much, much better acceleration than hardtech ones, but use the same general principles. They are also often speed drives which frustrates 'Dane scientists to no end. 'Waved ion thrusters, while not as economical as their hardtech counterparts are still very efficient explaining their popularity among Fen. After all the less remass you need the more you can stuff into your vehicle, or the longer you can go for.
- + if there are more protons, - if there are more electrons.