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Fusion works by smashing atomic nuclei together to create heavier nuclei. In order to make this happen, you have to heat things up a bit—say, a few million degrees Celsius. Fusion is promising as an energy source (and potentially dangerous) because it’s exothermic—it produces more energy than it requires to start it and is therefore self-sustaining. How much energy can fusion produce? Well, our sun’s been working for several billion years just fine on fusion.
Fusion is difficult to achieve because of something called the coulomb barrier. This is the energy required to overcome the electrostatic force that repells two nuclei from each other. What makes fusion tricky is that the best fuels, the ones with the lowest coulomb barriers, are the least stable. They are isotopes of hydrogen—deuterium (2H) and tritium (3H)—so named because they have two and three neutrons in their nuclei, respectively. We are not yet able to create a fusion reaction and contain it to use as energy. In fact, the ones we’ve created that aren’t contained are called hydrogen bombs. This has physicists looking for an alternative that requires much less energy to start and contains the Holy Grail of nuclear science known as “cold fusion.”
Fission, on the other hand, is the complete opposite: Very large nuclei are split to make smaller ones, releasing energy (and a boatload of radiation) in the process. Again, the best fuels are the most unstable: Isotopes like uranium-235 or plutonium-239 don’t occur naturally. Fission is used both for nuclear power plants and nuclear bombs.
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to raise the temp and pressure ofcorse,for fusion high temp is needed since it involves overcoming of repulsive forces btw light nuclei and fusing them such high temp are possible only by using a fission bonb(atom bomb)