Fusion is transmutation. Unlike the alchemists of old, the goal is not to turn base metals into gold, but light elements like hydrogen into heavier elements, such as helium. When this happens, the byproducts of the reaction have a lot of energy, which could be captured to produce electricity.
Fusion is extreme. Elements don’t fuse in the world around us. The electromagnetic force keeps atoms from getting close enough to merge. Atoms need a lot of energy to get close enough for fusion to happen. Unfortunately, at these high energies, the electrons surrounding the atomic nucleus tend to fly off, leaving a positively charge ion behind. Positively charged ions repel each other even more than atoms repel each other, so ions need to be even more energetic. For this reaction to be efficient, it also helps to have high densities (more elements to merge with) and for the elements to hang around long enough to collide.
Fusion is elusive. Since the start of the global fusion program in the 1950’s, there have been many approaches to getting hydrogen hot, dense, and then keeping it around for long enough to produce significant amounts of energy. Many of the approaches have been abandoned, usually due to a lack of funding after initially disappointing results. There are plenty of people and companies in the world who would like you to believe they can build a working fusion reactor, but in the absence of experimental evidence it it wise to be skeptical.
I’m interested in an approach called Magnetised Liner Inertial Fusion, or MagLIF for short. The idea is to crush a small cylinder of hydrogen inside a rapidly imploding metal can, and use a strong magnetic field to hold the hydrogen away from the walls of the metal can. Some significant experiments have already been carried out at Sandia National Laboratory in the US, but the concept is still a long way from producing electricity.