Ion Propulsion Engines

This is how an Ion Engine works and its applications

Ion Propulsion is one of the least known form to travel space unless you've read science fiction books. So as a space enthusiast that's why I have decided to write about it, so here is the brief.
Ion Propulsion have high specific impulse (Isp) requiring it to use significantly less fuel during space flight compared to its chemical counterpart. Not only it uses less fuel but it is significantly much more efficient to its chemical rocket counterpart. Ion thrusters are used to keep communication satellites in place, maintaining Low Earth Orbits (LEO), Deep Space missions, refuelling missions and numerous other varieties of missions. Ion Propulsion may seem like a wonder engine but it has several cons not allowing it to be used everywhere.

How does Ion Propulsion Work?

Source

The Process first begins with injection of the propellant (neutral) from the downstream end of the thruster to maximize ionization, the propellant (neutral) is bombarded with electrons. These electrons are generated by a hollow cathode, called the discharge cathode. The electrons flow out the discharge cathode are attracted to the discharge chamber walls, which are charged by the power supply of the thruster. When a high energy electron bombards the propellant (neutral) a pair of electrons will be knocked off the propellant, making it a positively charged ion. Magnets placed along the discharge wall redirect electrons into the discharge chamber. (1&3)
“In a gridded ion thruster, ions are accelerated by electrostatic forces. The electric fields used for this acceleration are generated by two electrodes, called ion optics or grids, at the downstream end of the thruster. The greater the voltage difference between the two grids, the faster the positive ions move toward the negative charge. Each grid has thousands of coaxial apertures (or tiny holes). The two grids are spaced close together (but not touching), and the apertures are exactly aligned with each other. Each set of apertures (opposite holes) acts like a lens to electrically focus ions through the optics.” (3)
“NASA's ion thrusters use a two-electrode system, where the upstream electrode (called the screen grid) is charged highly positive, and the downstream electrode (called the accelerator grid) is charged highly negative. Since the ions are generated in a region that is highly positive and the accelerator grid's potential is negative, the ions are attracted toward the accelerator grid and are focused out of the discharge chamber through the apertures, creating thousands of ion jets. The stream of all the ion jets together is called the ion beam. The thrust is the force that exists between the upstream ions and the accelerator grid. The exhaust velocity of the ions in the beam is based on the voltage applied to the optics. Whereas a chemical rocket's top speed is limited by the heat-producing capability of the rocket nozzle, the ion thruster's top speed is limited by the voltage that is applied to the ion optics, which is theoretically unlimited.“ (3)
I’ve taken these two extracts from NASA as I could’ve not explained better.
A second hollow cathode is called the neutralizer, it is used to push out needed electrons to neutralize the exhausted propellant. (1&3)

Different Propellants

An Ion thruster works by ionization of the propellant, although any element can be used xenon is the most commonly used, this is for its chemical properties or rather better put there lack of chemical properties. One of the questions you may be asking why not use argon’s its properties are virtually nearly the same and it’s cheaper (240x - Source 8&9) and the (Isp) is higher as the particle has less mass compared to it’s xenon counterpart. Well this may make argon look as a superior gas, as it’s cheaper and has higher (by 80%) (Isp), first we should look at the energy needed for ionization of each of the two gases, it takes 15.75eV for Argon and 12.13eV (7) for Xenon which makes Xenon 23% better, per unit atom this makes xenon 3.2 (131.293amu/40amu) times more efficient. Since the competing atoms have to go through an electric field to get accelerated to their exhaust velocity, @2500eV an Argon atom will reach ~100km/s while a xenon will reach ~55km/s. Since xenon has more mass it will generate more than twice the thrust which is a bigger and more significant advantage over having higher Isp.
From the Argon-Xenon example you may be wondering why not go one step up the periodic table and use radon, well radon is radioactive and that doesn’t quite bade well with spacecraft and sensitive electronics on it. (11)

Applications and Uses

Adjustments for drag experienced in LEO, it’s high efficiency of up to 90% compared to 35% of its chemical rocket cousin make it excellent for deep space missions with multiple destinations. Even though these engines may seem like wonder engines and we should use them for everything, I haven’t yet to mention one important statistic which is the top Ion Engines produce up to around .5 Newtons of thrust, to put it into perspective that’s equivalent of holding a few US quarters in your hand, lower powered Ion Engines produce about .07-.25 Newtons of thrust. The low thrust makes it useless at landing on anything but on asteroids. Ion Engines also tend not to work in atmospheres, (1-6) (10)

Personal Comments

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