Ion Thruster

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Ion Thruster

In very simple words, an ion thruster, or ion engine, uses electricity to ionize a neutral gas into positive ions, which are then accelerated by an electric field to produce thrust for spacecraft propulsion. These high-efficiency, low-thrust engines are ideal for long-duration missions, though they can also be used in atmospheric applications to create a neutral wind, such as in ion-propelled aircraft.

I was already writing about alternative fuels for rockets, such as hydrogen and methane. But it is amazing to learn about such an interesting and advanced propulsion, which is already used for many decades. 

NASA is focusing on liquid hydrogen as the most efficient fuel in use. SpaceX is working on methane as a fuel of their StarShip, with a hope to produce methane on Mars.

However long interplanetary distances are still currently a huge problem, and therefore an ion thruster may be the right answer to the problem.

The ion thruster used on the Deep Space 1 spacecraft. It was able to propel ions at speeds of up to 146000 km/h into space. Source: NASA.

Ion thrusters are in use on the International Space Station, for keeping geosynchronous satellites in their orbits, and for deep space missions like the Dawn spacecraft. While ion thrusters produce very small amounts of thrust, they are extremely fuel-efficient, requiring only a tiny amount of propellant to produce a large change in velocity over long periods of time. 

How Ion Thrusters work?

As mentioned earlier, an ion thruster or drive is a form of electric propulsion used by spacecraft in space vacuum. It works by creating positively charged atoms or ions, which are accelerated at very high velocities through electrically charged grids at the back of the engine producing thrust.

At their core, ion thrusters use electrical energy, usually from solar panels to ionize a propellant gas such as xenon. Ionization process means removing electrons from atoms to create positively charged ions. These ions are then accelerated and ejected at very high speeds up to 30 to 50 km/s, producing thrust in opposite direction, as known from Newton's third law of motion.

While the thrust is very small (about 0.05 to 0.5N), compared to other types of engines, the efficiency is extremely high (specific impulse Isp about 3000 to 10000 s) compared to about 400 s for chemical rockets. 

Ion thrusters are be divided into two main types based on how they accelerate the ions: as electrostatic or electromagnetic. 
Gridded Ion Thruster; source: Wiki

Electrostatic Ion Thrusters use electric field to accelerate ions. Ions are accelerated by the Coulomb force along the electric field direction. Temporarily stored electrons are reinjected by a neutralizer in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster.

Some key factors:
  • Exhaust velocity: 20 to 50 km/s
  • Thrust: 0.1 N
  • Efficiency: around 60-70%
  • Example: NASA's NEXT and ESA's T6 ion engine.
Advantages:
  • Very high efficiency and specific impulse.
  • Suitable for long duration missions, such as Dawn, Deep Space 1, BepiColombo.
Disadvantages:
  • Require precise grid alignment.
  • Grid erosion limits lifetime. 
Electromagnetic Ion Thrusters use magnetic field, and sometimes electric field too, to accelerate ions or plasma. Ions are accelerated by the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.


Schematic of Electromagnetic Ion Thruster; source: NASA.

Some key factors:
  • Exhaust velocity: 15 to 40 km/s
  • Thrust: 1-2N
  • Efficiency: 50-65%
  • Example: Hall-effect thrusters, magnetoplasmadynamic thrusters, VASIMIR (Variable Specific Impulse Magnetoplasma Rocket)
    • Starlink satellites is small Hall-effect thrusters for orbital adjustments. 

Advantages:
  • Simpler structure.
  • Can handle higher power levels, which gives more thrust. 
  • No accelerating grids to erode. 

Disadvantages:
  • Lower efficiency.
  • Magnetic field design is complex. 

What gas is used for ion thrusters?

Xenon is the most common gas used in ion thrusters because it has many benefits compared to other gases. Other propellants, such as krypton and even iodine and bismuth, are also used or are being developed for specific applications to reduce costs or improve performance. 

Xenon is a noble gas, meaning that it does not react with any other element under almost any circumstances, which means that it is very stable as it does not affect other components of the engine. At room temperature, it remains a gas, making it easy to handle and store.

Xenon also has high molecular mass (131.293 u), which makes it easier to ionize since removing an electron from an atom with a higher molecular mass requires less energy, which is quite important quality. But there is another benefit of higher molecular mass. As ions are being accelerated through an electric field, the higher mass of Xenon atoms allows for more momentum to be imparted to the ion, which, for a given exhaust velocity, results in more thrust from the spacecraft, making it a desirable propellant for ion engines. 

Xenon's high atomic mass, combined with its inertia and high storage density, makes it an efficient choice for producing a significant amount of thrust when accelerated through an electric field. 
This is part reason why Xenon is chosen over other noble gases like Helium, Neon, Argon, and Radon.

How it works, using Xenon:

The mechanism of Xenon ionization in an ion thruster is following: Xenon gas is bombarded by electrons in a chamber, where a neutral Xenon atom Xe collides with an energetic electron e⁻, ejecting an electron and forming a positively charged Xenon ion and two free electrons. The general reaction can be written as: 
Xe+e−→Xe++2e−cap X e plus e raised to the exponent negative end-exponent right arrow cap X e raised to the exponent positive end-exponent plus 2 e raised to the exponent negative end-exponent

Acceleration part:
Positive Xenon ions are accelerated through a voltage difference between two grids: a positively charged screen grid and a negatively charged accelerator grid. This potential difference creates an electric field that exerts a strong force on the ions, accelerating them at high speed out of the thruster to create thrust, up to 40 km/s.

Neutralization part:
As the high-speed ion beam exits the thruster, a separate source injects electrons into it (Neutralizer). 
This process neutralizes the ion beam, preventing a buildup of positive charge on the spacecraft and ensuring the ion flow is balanced. 

Ion engine mechanism and structure; source: Matsuda


Reference:
Wiki
Matsuda
SpaceflightNow
Electric Propulsion – Spaceflight Now
Astronautix

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