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Fusion is the reaction where two lighter atoms join to form a heavier one. When under the right conditions two nuclei fuse together, a remarkable thing can happen. The total mass of the resulting fused nucleus is just slightly less than the separate masses of its components, and the difference is released as energy. This makes fusion very powerful. It is the reaction that powers the sun and that occurs naturally in all stars.
The simplest fusion reaction starts with a Hydrogen nucleus, a single proton. Protons have positive electric charge and repel each other. But if you squeeze them together hard enough, the 'strong' binding force overcomes the 'weak' electrical repulsion, and the two protons fuse to form a hydrogen isotope deuterium.
A slightly more complicated fusion reaction can take place between deuterium and tritium - rare isotope of hydrogen. The fusion of deuterium with tritium is particularly efficient in the conditions we can achieve in fusion reactors. As can be seen in the illustration on the right, fusion of deuterium and tritium creates helium-4, thereby freeing a neutron, and releasing 17.59 MeV of energy.
The power that is released when fusing two nuclei has led to many experiments in the laboratory. Scientists from all over the world develop fusion power on earth, as it offers the prospect of a long-term, safe, environmentally benign energy option to meet the energy needs of a growing world population. We have now succeeded in achieving fusion reactions, but it has proven difficult to control the reaction enough to make it a practical source of electrical power.
So what is so difiicult about harvesting energy from a fusion reaction? As we mentioned before, to make fusion happen, two nuclei must come very close together. That only happens when they collide with a very high speed, which means that the temperature of the gas must be very high - so high that is becomes a Plasma.
The hot plasma needs to be confined, but the temperature of the ions is so high that there is no possibility to contain it in any normal vessel, as it would collide with the wall and cool down. To overcome this problem, people started to experiment with intricate methods of confining plasma. Read more about confinement and fusion reactors.