Why does nuclear fusion require so much heat and pressure to occur?
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The energy from fusion comes from the nuclear strong force, and this really kicks in within a few femtometers of the nucleus. This is well within the electron shells. Outside of this range, the dominant force is the electrostatic force, where the positive charges of the nuclei repel one another, the negative electron clouds around them also refuse to share the same space. This means that, if it weren’t for the incredibly strong nuclear force, fusion would actually require exorbitant amounts of energy to bring about and not produce anything, since you have to push a proton ‘uphill’ so far to get fusion to occur.
With the strong nuclear force, while it still requires all of that energy to overcome the electrostatic repulsion, it releases *even more energy* once that’s done.
To provide the amount of energy to each individual particle to overcome this barrier, some combination of heat, pressure, and linear motion is used.
To cause a nuclear reaction you need to make atomic nuclei collide. This takes tremendously more energy and concentration of matter than making entire atoms (electron shells) collide, thus requiring a lot of heat and pressure.
In order to fuse hydrogen into helium, you need to overcome two atoms mutual repulsion of each other due to the electromagnetic force. To do that, you need energy, in the form of heat and pressure. The sun gets both for free as a side effect of its mass. Being so massive, it has a lot of gravity, which puts a lot of pressure on the core which generates a lot of heat. Stars are in a perpetual state of blowing itself apart, but it’s gravity keeps it together, and the two opposing forces reach a point of stasis.
On Earth, we don’t have the advantage of mass and gravity, so we have to apply much more heat and pressure on what is a plasma of hydrogen that isn’t very dense.
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