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House of Math blog post authorAnton Isaksen

How a Fusor Works

Two particles colliding

A much spoken of alternative energy source for the future is fusion-energy from fusion reactors.

This energy source doesn’t pollute, can last for millions of years, and gets its fuel from water. The concept behind this energy source is a process called fusion. The very same process which supplies the sun with its energy. Fusion reactors might seem complicated, especially since they need a plasma with a temperature over 100,000,000 degrees Celsius, but doing fusion isn’t as difficult as one would think. There are several types of fusion reactors. The simplest is a so-called Inertial Electrostatic Confinement (IEC) fusor, which will be explained later. Many of the underlying subjects and concepts described here are elaborated further in high school physics classes.

What is Fusion?

Firstly, what really is fusion? The name itself is a big hint, since fusion is the process of fusing (combining) two atomic nuclei into a new nucleus. And in various instances it’s possible to gain extra energy from fusing certain nuclei. Fusion happens if two nuclei get close enough that the same force which holds the protons and neutrons in the nucleus together is able to “hook onto” the other nucleus and combine the two nuclei together. The problem is that all atomic nuclei are positively charged and repel each other like two similar magnets. So, the simplest way to get two nuclei close enough to fuse, is therefore either by pressing them tightly together or by shooting them with great speed towards each other. And this is exactly what fusion reactors do.

How Does an IEC-fusor Work?

An IEC-fusor is a type of ball-shaped particle accelerator that accelerates nuclei towards the center of a ball-shaped steel chamber where they can hit each other and fuse.

The way in which the fusor accelerates nuclei, is by using a powerful electric field. All nuclei are positively charged, as mentioned earlier, so they will be repelled from other positively charged things and attracted to negatively charged things. If one then connects the positive end of a power supply, for example a battery, to the outer walls of the chamber and the negative end to a ball-shaped hollow metal grid in the center of the chamber, an electric field will be created between the grid and the chamber walls. Which is only a fancy way of saying that positively charged particles like nuclei will be attracted/accelerated inwards to the negatively charged grid. At the same time, negatively charged particles like electrons are going to be attracted/accelerated outwards to the positively charged outer walls.

How strongly the nuclei are accelerated inwards, and subsequently how much speed they gain, depends on the voltage of the power supply connected to the grid and the chamber walls. The higher the voltage, the stronger the electric field and the more speed do the nuclei have when they arrive at the metal grid in the middle of the reactor. Inside this grid is a small empty space where nuclei from various parts of the steel chamber can collide and fuse. To get these nuclei up to a high enough speed to fuse, a voltage of several thousand volts is required. Normal batteries with a voltage between 1 and 12 volts are for that reason not strong enough, so a specialized high voltage power supply is required.

Vacuum

Another crucial factor in most particle accelerators, whether it’s the Large Hadron Collider by CERN or a fusor, is vacuum. There are particles everywhere in normal air that collide with each other all the time and move around in random directions. Such conditions make it very unpractical to accelerate particles over longer distances, as they don’t make it further than around a tenth of a micrometer before hitting another particle and bouncing away. Most of the air in the steel chamber must therefore be removed. Which is done by one or more vacuum pumps that suck out the air. With a good enough vacuum, there are so few particles present that a nucleus can be accelerated several centimeters without hitting something. At the same time, a full vacuum (no particles in the chamber) is not desired, because a few nuclei that can be accelerated and fused are required. A normal vacuum level for a fusor is around 1/100 000 of the pressure in normal air.

Fuel

It’s sadly not as easy to fuse all types of atoms. The easiest ones to fuse are the two hydrogen isotopes deuterium and tritium, which are only hydrogen atoms with respectively one and two extra neutrons in the nucleus. Ca. 0.0156% of all the hydrogen in water (H2O) is deuterium. That might seem little, but considering the fact that 70% of the earth’s surface is covered by water, it’s not too hard to get deuterium. Tritium is in contrast to deuterium way rarer and way more dangerous, so deuterium is the preferred fuel for IEC-fusors.

Sequence of Events Inside of a Fusor

A particle accelerator that fulfills all the demands requirements mentioned above can do fusion. In a ball-shaped steel chamber with a good vacuum, an inner hollow metal grid and a powerful electric field between the grid and chamber walls, the following happens when deuterium is added to the chamber:

  • The deuterium nuclei are ionized, which means that the deuterium nuclei and their belonging electrons are pulled away from each other by the electric field. Electrons are pulled towards the outer walls and nuclei towards the inner grid.
  • The deuterium nuclei are accelerated inwards towards the grid and obtain speeds of up to many millions of kilometers per hour.
  • Deuterium nuclei from around the chamber arrive inside of the grid with high speeds. An area of plasma is formed there, because of all the ions/deuterium nuclei that are flying in. A few of these nuclei hit each other and fuse, some hit the metal grid itself, and others pass right through.
  • In the case of a deuterium nuclei passing right through the inside of the grid and continuing out the other side, it will be slowed down by the electric field and pulled in towards the grid again. It can make several passes back and forth through the grid until it hits something.
  • If two deuterium nuclei hit each other and fuse, they will combine into a helium nucleus with two neutrons and two protons. This helium nucleus is very unstable immediately after the fusing, so it emits either a neutron or a proton which flies away with great speed. It has the same probability of losing a neutron as of losing a proton.
  • The proton and the remaining nucleus can hit the chamber walls and heat them up, while the neutron flies right through the walls.

What Can a Fusor Be Used For?

The largest downside of an IEC-fusor is that it cannot create enough heat to produce useful energy. It’s in fact very difficult to even observe that it has produced any. The heat from fusion in bigger and more advanced fusion reactors, like a tokamak, can at least relatively easily be observed and maybe in the future be used to produce more energy than what is needed to power them.

What can be made use of from an IEC-fusor are the neutrons created in fusion reactions. These are, as the name implies, electrically neutral. A property that lets them fly through most things if they have enough speed, including the walls of a fusor. Once outside, they can be slowed down and used to transmute elements. Transmutation is a whole different topic and is often used to make radioactive medical substances. To put it shortly, it involves bombarding a substance with neutrons so that the nuclei in the substance can absorb the neutrons and become radioactive. A bit like fusion. These new radioactive nuclei will then decay (emit radiation) and turn into new nuclei different from the original. One can for example use transmutation to make technetium-99m, which is a substance widely used in many medical examinations. The most important ingredient in making technetium-99m is a powerful and simple source of neutrons. Something fusors can do well, even though they won’t solve the worlds energy problems.