Friday, January 1, 2016

Introduction to Ion Propulsion

In physics, ion propulsion is a type of electric propulsion used by spacecraft. As with any traditional method of rocket propulsion, ion propulsion depends on Newton's Third Law: for every action, there is an equal and opposite reaction.

A typical rocket engine uses internal mechanisms to accelerate some type of exhaust away from the rocket. Since this constitutes a force on the exhaust, the engine experiences a force in the opposite direction. Crucially, propulsion requires that mass be lost from the rocket to exhaust. Other vehicles, such as cars, use friction between wheels and road to provide a force and therefore do not need to expel mass. Operating in space or the atmosphere in which friction is minimal (there is nothing to "push off" of), rockets instead carry extra mass to accelerate. As the name suggests, ion propulsion works by accelerating ions.

The above schematic illustrates the function of a gridded electrostatic ion thruster (which is usually what is meant by "ion propulsion"). On the left side, neutral atoms of the propellant move from storage tanks (not shown) into the ionization chamber. Simultaneously, an electrode fires electrons into the chamber with high velocity. These electrons knock other electrons out of the neutral propellant atoms to create ions. As a result, the ionization chamber becomes filled with electrons and positive ions.

At the other end of the chamber are two grids. They are connected to a voltage source that maintains a static positive charge on the inner grid and an equal and opposite negative charge on the outer one. Two effects combine to remove many of the free electrons from the ionization chamber. First, the positively charged plate attracts electrons, conducting them out of the chamber. Second, the contents of the chamber are very hot. Since electrons are much lighter than the positive ions, they move faster with the same amount of thermal energy and have a greater chance of collecting on the grid. Soon, positively ionized gas (plasma) builds up in the chamber.

This closeup on a gap in the positive grid shows how positive ions eventually escape the chamber. Eventually, so many ions accumulate in the plasma layer that the repulsive force between ions exceeds the force pushing this ions away from the positive grid. Ions then pass through gaps in the grid. Once they reach the other side, the repulsive forces from both the grid and the other plasma accelerate them outward. Returning to the larger diagram, the negative grid focuses the beam of ions so that they all proceed in roughly the same direction. Finally, another electrode fires electrons at the escaping ions, preventing them from losing velocity due to the influence of the negative grid and preventing a buildup of net charge in the engine.

Typically, the type of propellant used for ion propulsion is xenon gas. The use of xenon, which is element 54 on the periodic table, has several advantages. First, it is a noble gas so it is inert and does not react chemically with other parts of the engine. Further, it is the heaviest (non-radioactive) noble gas, so the thermal effect removing electrons from the gaseous plasma is enhanced.

NASA tested the ion thruster shown above in the early 1990's. The blue glow originates from charged xenon particles.

The next post describes the applications of the ion thruster and its impact on space travel.


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