This article will take the basic thrust concept of jet and 4-stroke combustion engines and give it a twist for electric propulsion. Instead of “Suck, Squeeze, Bang and Blow” it will be “Push, Zap, Suck and Blow”.
Electric propulsion is a generic name given to ion thrusters for a number of reasons, the principal one being that they all use electric fields in some configuration to produce ions that are accelerated, then neutralised, to give a resulting thrust.
Often, though, it is worthwhile to take a more basic general view of what the devices actually do, so that non-technical people and indeed other engineers can get a feel for the process and maybe decide to learn more or get involved in making them.
With this in mind, the process of making thrust can be broken up into 4 parts, using the similar easy-to-remember idea as that for combustion engines. It also aptly sums up the main elements of most ion thruster devices in a way that equations and diagrams often do not.
As was stated at the beginning, the four parts are Push, Zap, Suck and Blow.
Part 1: Push
To get an ion beam, first you need gas. Most electric propulsion devices have a chamber or a channel of some kind into which gas flows. This chamber or channel is normally called the “discharge chamber” or “discharge channel” but for the purposes of this article I will just call it the discharge region.
Gas flow is one of the parameters in the famous Tsiolkovsky rocket equation where the thrust from a device is related to the ejection speed of the exhaust and the mass flow rate of the gas used.
Therefore no matter what type of electric propulsion design you have, gas or particles of some kind must be flowing.
I use the term Push because the gas is essentially “pushed” into the discharge region; in almost all cases it comes from a tank at much higher pressure than in the discharge region and is either bled in using values or controlled with a gas feed system.
Part 2: Zap
Once the gas is flowing, the atoms need to be charged by ideally stripping away one electron. This forms a plasma, as there are now ions and electrons in equal numbers in the discharge region.
The reason for only taking away one electron is that it is the most mass efficient. When the ions are finally accelerated from the thruster, a beam current is registered which can be related to the number of ion masses that have been ejected.
If there are more double and triple charged ions in the exhaust beam then less mass is being accelerated as less thrust is being generated for the same charge. This is the case even though a double charged ion is accelerated more than a single charged ion by the same electric field.
The acceleration effect with increasing charge varies as the square root of the number of charges; the charge per ion however varies directly with the number of charges.
To change the atoms into ions there a number of methods used. Here are some of the common ones:
- A high voltage electric field can be directly applied creating an arc plasma
- An internal electron circuit (cathode and anode) can used to flow electrons through the gas. In addition a magnetic field is applying to increase the ionisation rate
- A radio or microwave frequency electromagnetic field can be applied
What all these techniques have in common is that a quite a lot of power density is dumped into the discharge region, effectively “zapping” the gas; hence the second stage where ions are created is called Zap.
Part 3: Suck
All ion thrusters have to move the ions out from the discharge region and into the exhaust region, or acceleration region. The subtlety of the Suck stage is that it is often combined with the Blow stage.
Ions in the discharge region either get accelerated within a region of the discharge plasma itself or they are extracted out by using orifices or grids with multiple holes, often called apertures.
In the case of a gridded thruster for example, a screen grid is used to bound the plasma in the discharge region and then a second grid, called an accelerator grid, is used to create the electric potential that causes the ions to be “sucked” out.
The design is quite an ingenious one. The whole discharge region is floated at high voltage, typically more than 1000 Volts while the accelerator grid is biased to a negative potential. This is done to prevent electrons from the outside streaming into the discharge region and extinguishing the plasma.
The combination of high positive screen voltage and negative accelerator voltage effectively creates a potential hill for ions to “roll” down and accelerate. There are many slight variations in the strength of the field used though; ion focusing is one of the key parameters in gridded thrusters because it impacts thruster lifetime.
As the ions are extracted, then, we get the Suck stage.
Part 4: Blow
The last stage is Blow, where the ions are ejected into space and at the same time are made neutral again, or “neutralised” as the term is referred to.
Why is this important?
The main reason is that the whole ion thrust concept is based on the ability to take a normal mass, charge it up, accelerate it to high speeds because it is charged, then make it neutral again with nothing more than its resulting fast momentum, which would require much more effort to reach if it was either heated or pushed.
However, you have to neutralise the ion beam when it leaves otherwise you build up charge at the mouth of the thruster, which very quickly stops you accelerating any ions at high electric potentials at all. It is like having a sheet of positive charge into which you want to add more positive charge; it gets harder and harder to do.
Literally then, once the ions start being sucked out, they are “blown” from the thruster. This is the case whether the thruster has grids or not. In addition there is an external electron emitting device called a Neutraliser, that “blows” electrons into the ion beam to make it neutral again.
There is an exception to the overlapping Suck and Blow stage for gridded thrusters. It is called the Dual-Stage-4-Grid thruster. It has a smaller potential Suck stage performed using the first 2 grids, then a separate Blow stage using the next 2 grids.
There you have it then: Push, Zap, Suck and Blow; a simple way to describe the complex and detailed thrust process for electric propulsion.