Why isn’t nuclear fusion possible yet?
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The short answer is that it is; it’s just not yet possible to do on a large scale without putting more energy in than we get out.
The main problem is that, for nuclear fusion to happen, you have to make two atomic nuclei collide. They both have a positive charge, so they repel each other, like trying to make two magnetic north poles touch. This means the nuclei have to be moving very fast, which means the fusion material has to be at an extremely high temperature. That takes a lot of energy.
We now have the problem of confining the hot plasma. You can just put it in a big vat – even if the walls are made of something that won’t melt at that temperature, the plasma would cool down from touching them. So you have to use magnetic fields to confine it. That takes a lot of energy too.
The final problem is that atomic nuclei are tiny (100,000 times smaller than atoms), so unless the pressure is high, the chances of them actually colliding are small.
In the stars, these problems are solved by the weight of the star compressing its core, making its pressure and temperature increase until fusion can happen. But we simply can’t use that process on earth, because it requires too large a scale (and leaves the problem of how to contain the mini-star).
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