What Are Ion Thrusters and How Do They Work?
Everyone has seen huge rockets blasting off into space. They use extreme chemical reactions to generate a huge amount of energy quickly and overcome the earth’s gravity. The Merlin engine used by SpaceX creates over 190 000 pounds of thrust. The problem arises when we start talking about long-range missions. That type of propulsion system is extremely inefficient, with a fuel efficiency of 35%. There’s no question that chemical propulsion is (currently) the best way of getting into space, but once you’re there, it’s the ion thruster you want.

What are Ion Thrusters?
To explain how ion thrusters work, you have to first think about how propulsion works. It’s all based on Newton’s third law of motion: for every action, there is an equal and opposite reaction. That means if you were to throw a rock your hand would be pushed back, just as much as the rock was pushed forward. Thrusters work in the same way. They shoot out particles at high speeds to give themselves a push in return. Chemical rockets burn fuel creating large amounts of heat, which creates pressure and eventually launches gases out of the bottom of the rocket. This approach results in a large number of particles being launched. The ion thruster utilizes electricity to accelerate a much smaller number of particles, but at much larger speeds. That high-speed results in a 90% fuel efficiency, which is way better for long missions than the 35% of chemical rockets.
Design of the Gridded Ion Thruster

Inside the ion thruster, the first step is to release Xenon into a chamber. Xenon is used because of its large weight, which makes it give a bigger push onto the spacecraft, and because it is somewhat easy to ionize. Once in the chamber, the xenon is bombarded with electrons. When the electrons run into the xenon, they knock one of its electrons, resulting in 2 electrons and a positively charged Xenon ion. The electrons are attracted to the sides of the chamber, where they are absorbed. Because of the large pressure created by the xenon gas, the ions are pushed through a special grate called the screen grid, which has a positive charge. The screen grid is followed by the accelerator grid which has a negative charge. These two grids create an electromagnetic field, which accelerates the Xenon ion to a very large velocity and launches it out from the spacecraft. However, there is one more step necessary for the ion thruster to function. The positive xenon ion tends to attract to the spacecraft and fly back toward it while pulling the space ship to itself and getting rid of any thrust is created. To stop that from happening, the beam of Xenon has to be neutralized. This is done by shooting out equal amounts of electrons as xenon atoms, from the back of the ship. The negatively charged electrons combine with the positive xenon atoms and become neutral.
Design of the Hall-effect thruster
The ion engine I just described is in use by NASA today. However, Russia has been using a different design for quite some time, called the Hall-effect thruster, and NASA might transition to using them soon.

The Hall effect thruster is much more complicated than the ion thruster described before. It uses some very complicated electromagnetic fields and interactions. I will do my best to explain it, but I had to read multiple different explanations, several times, to actually grasp the concept completely. First, the Xenon atoms are injected into a chamber, from an anode(positive). At the same time, electrons are being injected from a cathode(negative) at the other end of the chamber. Some of these electrons are sent into the chamber, while the rest go to ionize the xenon ion beam. The anode and cathode interact to create an electromagnetic field, labeled “E” on the diagram. There are also 5 cylindrical objects on the diagram, these are all electromagnetic solenoids, which basically means they create an electromagnetic field between them, labeled “B” on the diagram. These two fields interact to create the Hall current. This current traps electrons in a circular path around the chamber. On the diagram, it looks like a bunch of loop de loops. The spinning electrons eventually run into the Xenon atoms ionizing them. Then the field between the cathode and anode (“E”) accelerates the Xenon ions out of the spacecraft. Finally, they are neutralized by the electrons released from the cathode.
Now, you might be wondering why NASA would begin using something so much more complicated when they have a perfectly functional design. It is simply because ion thruster has a limited amount of thrust it can produce, and the Hall effect thruster can create more than NASA’s design.
Conclusion
Ion thrusters are an amazing technology that will constantly be used in the coming years. Their efficiency allows them to use less fuel. The fact that they use solar panels to create electricity and generate thrust, means they can function for years. Many missions have already used them, for example, the deep space 1 mission, and many missions will continue using them.
