Orbital Arc is developing a new type of ion engine. Great, but sounds pretty sci-fi. What is an ion engine, anyway? For that matter, what is an ion?
Let’s get back to basics.
(NASA test of a Hall Effect Thruster (HET) for Gateway lunar space station; HETs are one major type of ion engine. The blue glow is from Xenon ions.)
So, first, an ion engine is just a type of thruster; it’s a device that pushes a fluid or a gas out one end to push a machine in the opposite direction, just like a standard rocket engine, or jet engines on airplanes. Push stuff out the back to move forward. The difference with an ion engine is just how it does the pushing. A jet engine uses a rotating turbine to push air. A rocket engine combusts fuel and uses a nozzle to push out the resulting hot gases. An ion engine uses an electric field to push ions.
An ion is just a charged particle. It can be an atom, or a molecule, and it can have positive charge or negative charge. Basically, this just means that one or more extra electrons have been added or removed from the atom or molecule, so that it has more or less electric charge than it would in its neutral state.
For example, say you have hydrogen. Hydrogen (H) normally has one proton (positive charge, +) and one electron (negative charge, -). If I remove the electron, I get a positively charged hydrogen ion (H+) because now I have one proton, and no electrons, so the total charge is +1. If I add an electron, then I get a negatively charged hydrogen ion (H-), because now I have two electrons and only one proton, so that’s -2 and +1, for a total of -1 charge. Wikipedia draws this out nicely below.
The fun thing about ions is that they are very electrically active. Neutral atoms and molecules don’t really do much in an electric field most of the time, at least not until the electric field becomes very extreme. But if I put an ion in an electric field, it will zip off in one direction or the other based on its charge and the field direction. The field can push the atom or molecule, and the atom or molecule pushes back on whatever materials are generating the field.
And, that’s the basis of ion thrusters: you make ions, you use electric fields to push them out the back of a spaceship, and that push back gives you some thrust.
That’s great, but how do you get ions? Anyone who remembers their high school science classes knows that electrons don’t like to be pulled off of their atoms, or stuck on to other atoms that are already neutral. So how do you prevent the ions from just becoming neutral again?
Well, there are a lot of ways, actually. Probably the most common way to get ions is chemically; certain atoms or molecules have a high affinity for extra electrons, or a high affinity for losing electrons; this is mostly because they have unfilled valence electron orbital slots, or else have one or two extra electrons beyond a full valence orbital. This concept is a bit complicated, but you can think of the valence orbital as an egg carton and electrons as eggs. If the egg carton has 6 slots for eggs, but only 5 eggs in it, the particle is pretty okay with picking up an extra egg. But, if the egg carton has 6 slots, and the particle has 7 eggs, it’s annoying to carry that extra egg around outside the carton, so the particle is okay giving it away, or maybe sharing it with some other particle.
To strain the analogy, if a particle with an open egg slot meets up with a particle that has a spare egg, they may become friends, get married, and bond together as a larger molecule that shares eggs happily between their cartons. Or, the two particles may just have a short fling, hand off a spare egg to fill a spare slot, and go their separate ways – in the latter case, they both end up as ions.
The other ways to make ions are a bit more extreme. First, you can make ions by making a plasma. Plasma can be thought of as just “ion soup” where most or all of the particles in the soup are ionized. Basically, when you heat up a gas, the particles in the gas move faster, and bump into each other harder. If you heat the gas up hot enough, then some of those collisions can start to knock electrons off of some of the particles; this is called impact ionization. If you heat the gas up really, really hot, then the impacts between particles that knock electrons off start happening faster than the reaction where the electrons join back up to their atomic nuclei; once that happens, it won’t take long for basically all the particles in the gas to be ionized, at which point you have a plasma.
You can accelerate the plasma process by adding an electric field component to it. You still need some kind of impact to start the reaction, but then you can accelerate those electrons that got knocked loose using the electric field, and get them going extremely fast. When the electrons hit other particles, they hit hard enough to knock off even more electrons, which fly off and knock off more electrons, causing a chain reaction. In a directional electric field (like between a positive and negative charged wire), this chain reaction propagates only one direction, with electrons flowing toward the positively charged conductor, causing an electrical arc. In an oscillating electric field (say, one that switches polarity very quickly, like an few million times each second), the field switches direction so fast that the free electrons are jerked back and forth and slammed into the gas particles over and over without most of them reaching a conductor; this is a pretty efficient approach and is probably the most common way to making and maintaining a plasma. These high frequency polarity oscillations are called Radio Frequency or RF for short; the radio in your car works by using an antenna to detect electromagnetic waves emitted across space from conductors oscillating at these frequencies.
(Plasma inside a spherical tokamak fusion reactor from MAST. The plasma is the glowy part. The rest is the electromagnetic containment system to try to keep the plasma going without letting it melt the container.)
Plasma is messy. Most of the molecules in plasmas will break apart due to all the collisions. Plasmas are so hot that there are no materials that can survive in direct contact with them for long, so they need to be contained and handled mostly using electric and magnetic fields. We aren’t very good at this kind of electromagnetic containment yet, so we can’t maintain plasmas at very high densities (which is one of the problems we need to solve before we can build useful fusion reactors). Inevitably there is some level of damage or erosion of systems that generate plasmas. The plasma atoms also like to react with everything they touch, since they are basically carrying around completely empty egg cartons most of the time. These atoms gobble up the free electrons needed to sustain the plasma too (a process called “recombination”), which means you must continuously add more energy to the system, just to keep it ionized.
This takes us to the third way to make ions: Field Effect Ionization. Field effect ionization is even more extreme than RF plasma generation in terms of the forces involved, but it is also cleaner and more efficient, for reasons we will see shortly. Field effect ionization relies on creating an extremely intense electric field over a small area, which is so strong that it just rips electrons right off particles that enter that area. When I say extreme electric field, I’m talking 10-50 gigavolts per meter. At such high fields, the field stress alone can cause metal protrusions on the electrodes to melt or even evaporate. But, that’s what it takes to yank electrons off of passing particles.
But, once you have this powerful electric field, the rest of the ionization process is quite clean. The electrons don’t end up as free electrons, they actually quantum tunnel straight into the electrode conductors. The ions that result tend to have such uniform charge and energy that they flow through the field without really bumping into each other much. This means ionization with almost no impacts involved, so most of the time molecules don’t break apart in the process. And, since all the resulting ions are positively charged, with very few free electrons in the system at all, the ion flow can be managed much more easily with electromagnetic fields than a mixed plasma, where every time you pull one way on one set of charges, the opposite charges fly the other way.
An astute reader will clearly recognize which way of making ions I like best. Orbital Arc’s ion thruster design is built around field effect ionization of a gas in steady-state flow. It is a challenging thing to set up. But if successful, it opens up use of molecular fuels in ion thrusters, and makes ion thrusters much more scalable and efficient.