The one word to describe fusion as an energy source is “uncertain.” Scientists and engineers have been trying for controlled, earth-bound fusion for over 50 years. The primary result has been frustration. In fact there is a saying that “fusion is the energy source of the future – and always will be.” I’m not quite that pessimistic, but I do realize that the difficulties are serious.
To understand those difficulties it is useful first to understand what fusion is in terms of its potential for energy. Simply put, fusion is the merger of two atomic nuclei to form one heavier nucleus, or two different nuclei (more on the latter below). The most common fusion would be what takes place in the sun with hydrogen nuclei being fused into helium nuclei. Hydrogen comes in three forms, regular hydrogen with one proton in the nucleus, deuterium with a proton and a neutron, and tritium with a proton and two neutrons.
The difficulty of fusing two hydrogen nuclei comes from the fact that we have to get the nuclei very close together before they fuse. All nuclei contain protons which are positively charged and positive charges repel each other. Even worse, the closer together they get the stronger that repulsive force becomes. With one exception (treated below), we have to heat those atoms up to a tremendous temperature before they can get close enough to merge. They have to be so hot that the electrons are all stripped off, leaving a mixture of negative electrons and positive nuclei called a plasma. This is so hot that no material can withstand the heat. You can’t make a container to hold gas that hot because it would vaporize the container (if it didn’t cool the plasma first). The sun solves this problem by being huge and having enough gravity to hold its hydrogen together. It’s rather difficult to do that on earth so we have to find other ways to contain this stuff long enough to allow it to fuse. There are two promising ways to do this, plus one way to kind of do an end run around the problem.
The first way to contain the plasma is by magnetic confinement. Since the electrons and nuclei are electrically charged a magnetic field will turn them. The idea is to turn them away from the boundary of the “magnetic bottle,” confining them so they can have a fusion reaction. This has been achieved, but only very briefly and so far we cannot get out as much energy as it takes to do it. The problem is that the magnetic “bottles” tend to develop places where the plasma can leak out. Since plasma is so hot, it leaks out very quickly, before we can get much fusion from it.
The second way we try to achieve controlled fusion is to replace the sun’s gravity with the force from laser light shining on a fuel pellet. Essentially we try to squeeze it to hard that the fusion reaction takes place before the atoms can escape. If we ever succeed at this, fusion will consist of a series of small explosions, much like the hydrogen bomb but on a much smaller scale.
Now there is another possibility that does not require extreme temperatures. This is called muon induced fusion (or at times muon catalyzed fusion). This takes advantage of the fact that muons, relatively rare particles, have the same electric charge as electrons but are much heavier. They can replace electrons in hydrogen molecules. The hydrogen molecule consists of two hydrogen atoms and if a muon gets in there it will pull the nuclei of those two atoms very close together – in fact close enough that fusion can occur. This works and does not require extreme temperature. The problem is that muons are unstable and can only induce a few fusion reactions before they decay. We have to spend energy to create them and so far we have not been able to get more energy out than it takes to create the muons to cause the reaction.
So we have three possibilities of using fusion to generate the energy we need. All are difficult and none has yet shown itself to be practical. We can and should continue working on this, but there are no guarantees.
