There is a joke that scientists studying fusion hear with monotonous regularity: "I know about fusion -- it's the energy of the future, and always will be." But you can forgive people for believing that it is true. Fusion is the perennial bridesmaid that never quite makes it to becoming the bride -- scientists always seem to be making progress but a practical fusion reactor always seems to remain about three decades away. So what is it about fusion that makes it so hard to achieve, but still attracts scientists to work on it after six decades of trying?
Fusion is a form of nuclear energy but is entirely unlike the kind that powers the nuclear plants we have today. Today's nuclear power is produced by taking very heavy atoms, such as uranium, and splitting them apart. In the process, a small amount of the atom's mass is converted into energy and this energy is converted into electricity for our use. Fusion does the opposite: it takes very light atoms -- two different types of hydrogen known as deuterium and tritium -- and fuses them together into another larger atom, namely helium. Again a small amount of mass is converted into energy and this process is what powers the Sun and all the stars we can see in the sky. The reason why we have lots of conventional nuclear stations but no fusion ones is that atoms like uranium are really very easy to split apart, whereas it's really really hard to get deuterium and tritium to fuse.
So hard in fact, that a fusion reactor has to heat its mix of deuterium and tritium fuel to a temperature of more than 150 million degrees Celsius. The high temperature is needed so that the atoms are traveling very fast and smash together with such force that they overcome their mutual repulsion and fuse together. A gas at 150 million degrees is a nasty thing to have to deal with because it struggles to escape from its container but you can't let it touch the sides because it would burn or melt any material. The most common way of keeping the burning fuel in its place is to use powerful magnetic fields from some of the world's strongest electromagnets.
Scientists can do this, containing the fuel in a doughnut-shaped container called a tokamak. But so far it has taken more energy to heat the plasma and contain it than is produced in the fusion reactions. The reactor ends up a net consumer of energy. So why keep trying? What's the point?
The reason generations of scientist keep hammering away at this problem is that pretty much all of the things that people don't like about conventional nuclear power are absent from fusion. It's safe -- a fusion reaction can't run away uncontrollably like the reactors at Three Mile Island and Chernobyl did. It doesn't produce high-level nuclear waste -- some parts of the reactor structure will be radioactive at the end of its life but this will cool to a safe level in a few decades. And its fuel -- deuterium and tritium -- is available in almost limitless quantities from seawater and abundant minerals. In addition, a fusion reactor won't produce carbon dioxide or other greenhouse gases like a fossil fuel plant does. Imagine what a fusion-powered world would be like: it would be a cleaner and greener place, and consumers would no longer have to be dependent on the whims of unstable oil-rich states.
So why is fusion always three decades away? It's true that much was promised by earlier generations of fusion reactors only for them to not perform quite as well as expected. The previous generation to today's got close, generating 70 percent of the heat needed for break-even. Scientists are now working on a tokamak that is twice as big and much more powerful with the aim of producing 10 times as much heat as that used to heat the fuel. Called ITER, the International Thermonuclear Experimental Reactor, it is now under construction in France at a cost of around $20 billion. Meanwhile, in California, a $3.5-billion machine called the National Ignition Facility is trying to reach fusion break-even by an entirely different method using high-powered lasers.
So will these machines succeed where others have failed? Probably. Does that mean a fusion power plant will be up and running in 30 years? Who knows? There are still many hurdles to overcome between achieving break-even and an economic commercial reactor. But with its many benefits, fusion remains an enticing prospect despite the difficulty in making it work. And with projects this big, involving many thousands of people, it's no laughing matter.