One of the exotic options for spacecraft engines is the ion rocket engine. The main fundamental difference between ion rocket engines and conventional rocket engines is that they are electric engines. If gases flow out of conventional rocket engines because the pressure inside them is greater than in the surrounding atmosphere, there is no such increased pressure in ion engines.
It is well known from the physics course that equally charged electric particles repel each other and oppositely charged ones attract each other. These are the so-called electrostatic or Coulomb forces. They play a major role in technology. In particular, some kinds of "nuclear artillery," with which scientists bombard atomic nuclei and the forces acting inside the nucleus, are based on the use of these forces. Electrical forces act on electrically charged particles, such as negatively charged electrons or positively charged protons (hydrogen atomic nuclei) or alpha particles (helium atomic nuclei). As a result of such influence the particles are accelerated to enormous speeds, sometimes close to the maximum possible in nature - the speed of light in a vacuum. Thus, these particles become convenient "projectiles" for atomic bombardment.
The idea of using electric forces for jet engines was actually what came to scientists in the late 50's and early 60's of the last century, that they can easily achieve high velocities of flow, completely unattainable in conventional combustion engines.
The question is how to use electric forces to accelerate gas molecules flowing out of engines through the nozzle since these molecules have a charge and are neutral, and electric forces have practically no effect on such particles.
For their "activation" it is necessary to give the particles an electric charge, i.e. to carry out ionization, which is actually why charged molecules are called ions and for the same reason one of the upper layers of the atmosphere is called the ionosphere.
To ionize a molecule, it is sufficient, for example, to tear off one of the electrodes from its electron shell. Then the molecule will be positively charged. This is especially easy to do if one of the electrons on the electron shell is weakly bound to the nucleus, as is usually the case in metal molecules. It is possible to ionize a molecule twice, three times, etc. by depriving its electron shell of two, three, etc. electrons.
Therefore, an obligatory element of an ion engine is the so-called ionization chamber, in which ions are born from neutral molecules. For this purpose, it is sufficient, for example, to pass the molecules through an incandescent metal mesh; the electrons weakly bound to the nucleus cannot withstand the oscillations increasing due to heating and are bouncing off the molecules.
A schematic diagram of an ion engine.
The rest is simple: since there are ions, it is not difficult to accelerate them to high speeds using electrostatic forces. You can use, in particular, some gas pedal, such as those used in the laboratories of nuclear physics, although here you need incomparably lower speeds. Or you can simply pass the ions through a capacitor whose plates carry the opposite charge. If you make such plates in the form of grids or install them at an angle to each other, the capacitor will emit a stream of charged particles at high speed. So that the motor itself is not charged with electricity of the opposite sign, the electrons torn from the molecules must also be ejected outside with the same device.
Theory and experience show that in the ion engine it is not difficult to achieve expulsion velocities of 100 or more kilometers per second. This is tens or hundreds of times greater than in conventional "chemical" jet engines. Accordingly, such an engine has a greater thrust developed by each kilogram of particles (they can be, for example, cesium or rubidium metal ions).
Despite all the apparent beauty and simplicity, first of all there is a question about the source of electric current needed for such an engine. It is not possible to install a huge power plant on a spaceship, is it? As an option, it is possible to use a nuclear engine, especially with direct transformation of nuclear energy into electric energy, but such engine also does not exist and in the near future is not expected (let's be realistic). Moreover, as some calculations show, such ion engine can develop only relatively small thrust, because the number of particles flowing out of it at practically feasible power can be relatively small.
Nevertheless, for spacecraft flying in outer space it can be very profitable, although its use in combination with other engines and in superhigh-altitude aviation is not excluded, and what will be there, time will tell. There has been talk about the ion engine for more than half a century, but it's still there.