Why is it so hard to make a nuclear fusion reactor efficient?

Question

In other words, why is it so hard to get the Q value above 1 (ration between energy output and input)

In: 179

Answer 1

For fusion to occur, two positively charged nuclei (all nuclei are positively charged) have to be brought very close together.

ELI5, this can be done by a combination of a) make those nuclei move VERY fast and hope that they “collide” b) “press” those nuclei closer together to increase the likelihood of “collision”. The first essentially means high temperature and the other is high pressure.

The sun achieves fusion because the pressure is very high due to gravity and the sun’s mass. We cannot achieve those pressures realistically so instead we rely on much higher temperatures and somewhat less pressure.

So this becomes a very complicated engineering problem. Temperatures this high will melt any known material and therefore it is necessary to manipulate the nuclei (in a state called plasma) using magnetic confinement with super high magnetic fields. This takes a lot of electromagnets and the only (reasonable) way to get this is through supercooled magnets which takes up a lot of energy. Then there is the problem of getting the plasma to that high temperature – which again takes up a lot of energy. So there is this problem of needing very cold temperatures to get the magnetic field and very high temperatures in the plasma at the same time.

Now imagine trying to do this, not with micrograms but realistically several grams of material continuously. When fusion occurs, it releases energy and that energy must be captured and directed quickly so that it does not disrupt the magnetic fields and also rapidly converted to electrical energy. None of this happens without even more devices around the fusion area which then also have to be protected from radiation and heat etc etc.

Ultimately a huge amount of energy and equipment is needed simply to initiate and sustain fusion for more than a brief microsecond for more than just a few micrograms (something relatively easily achievable in a lab) So the energy payback or Q is very very very hard to achieve.