In other words, why is it so hard to get the Q value above 1 (ration between energy output and input)
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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.
Nuclear fusion takes a lot of input energy to get it going. It also takes a lot of energy to keep a fusion reaction going. But fusion reactions also give off a lot of energy.
The difficulties with efficient reactors are getting enough energy focused into one spot in the first place, and then getting enough of the output energy fed back into the reaction to sustain it (while providing new fuel, and extracting the excess energy in a useful way).
Stars are the standard nuclear fusion reactors. For them, gravity provides the input energy; the gravity of the Sun is enough to squish hydrogen together hard enough to get it to fuse. That is a lot of energy, though.
Jupiter has 300 times the mass of the Earth but isn’t massive enough to get nuclear fusion to start.
Without gravity nuclear fusion usually involves focusing a lot of energy into a single spot. The US’s National Ignition Facility, for example, achieves nuclear fusion by [focusing a whole bunch of giant lasers](https://upload.wikimedia.org/wikipedia/commons/3/3d/NIF_building_layout.png) onto a single spot (the circle in the bottom right of that image). NIF ran from 2009 to 2012 but even with all those lasers wasn’t able to focus enough energy in a small enough spot to get nuclear fusion to start.
It was redesigned and started up again in 2021, and managed to achieve nuclear fusion.
It is really difficult to focus all that energy in one place to start the reaction. The reactor then has to achieve the same effect with the random energy the reaction throws out. It is much easier to focus energy you are in control of (from the lasers) than from the nuclear explosion you just created. Think of it as the difference between lighting a candle with a match and lighting it with a bomb. In theory it could work, but the bomb energy is much harder to control and focus.
It’s easy to do, provided you have a solar mass of hydrogen and the space to get it close enough to fuse without incinerating the stuff you want to power with it.
What do you mean by efficient?
Neutron efficiency? Thermal efficiency?
The worlds investment into nuclear fusion research is so relatively small that we can barely build a small test reactor every few decades. So research advances veeery slowly .
Even the sun would have a hard time trying to fuse atoms if it wasnt for some quantum tunneling effect. Trying to overcome the force of two pluscharged particles are quite hard.
Really Really simply put, to initiate a fusion reaction takes a huge amount of energy to smash the fusing atoms together enough to fuse them.
Enough energy, in fact, that we have a hard time creating that much energy outside of a fission detonation (a nuclear explosion).
The issue Really isn’t so much the difficulty of initiating the fusion, because we do it every time we detonate a thermonuclear bomb. It’s been done literally thousands of times.
The issue is controlling the reaction after you’ve initiated it by pumping in a nuclear bombs worth of energy, and having a container strong enough to contain it.
The Youtube channel Thunderf00t does a really good job of explaining it. YouTube search Thunderf00t fusion and watch.
Essentially to make hydrogen fuse into helium (and release the massive amount of energy that makes it worthwhile) you need to apply enormous heat and pressure.
You can apply a truly ridiculous amount of heat, and a bit of pressure.
Or you could apply a little heat and a whole lot of pressure.
The Sun actually works off the second one.
Its sheer mass is what induces fusion. The force of gravity squeezes the hydrogen so tight it actually merges into Helium. That and some heat to excite the atoms and make them a bit more amenable to merging, and you get a star.
The problem is that we don’t have a star-sized amount of mass to squeeze the atoms together.
So we have to work with what we’ve got.
Turns out if you heat it up more, you don’t need as much pressure.
Gravity is pretty much free, the sun fuses just by sitting there.
Pouring lasers and whatever other methods of heating we can get into a mass of hydrogen (or other fuseable gas) costs a lot of energy, as does containing and squeezing it all with magnetic fields so it doesn’t melt the reactor.
The challenge is basically to find a means of heating and squeezing the fuel that takes less energy than we release by merging the atoms together.
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