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Fusion power is supposed to save us from fossil fuels, so when is nuclear fusion going to be a viable option and why has it been so elusive?

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Humanity hasn't exactly done a great job when it comes to energy production.  I mean, don't get me wrong, we're good at producing enough electricity to meet our needs, but it's come at the cost of releasing billions of tons of carbon dioxide into the atmosphere and wreaking some pretty serious havoc on the climate, not to mention that the planet has a fairly limited supply of the main fuels we use like oil and gas.  One way of reducing our reliance on them is with nuclear fusion, releasing energy by fusing atoms together.

The idea has so much potential that researchers have been working on it for at least half a century.  The problem is, we're still a long way off.  There's a running joke among physicists that fusion power is always 30 years away, but that doesn't mean we've given up.  There are experiments in the works right now that are way more advanced than anything we've ever done, and if all goes well, the future of energy production might involve much more power and way less pollution than we could ever hope for with the technology we have today.

We already have a few options when it comes to clean energy, but they all have their drawbacks.  Things like solar and wind power are great, because they're renewable and don't cause climate change, but they can hurt the ecosystems around them and usually aren't very efficient.  They're also inconsistent and we're still trying to find practical ways to store enough power to compensate for times when say, it's super cloudy for a week, and there's also nuclear fission, which releases energy from splitting atoms, usually uranium or other heavy elements like plutonium.

Specifically, fission involves splitting the center of the atom, its nucleus, which is where the term 'nuclear energy' comes from, but supplies of the heavy elements we need for this could run out pretty easily, too.  Even worse, fission generates tons of radioactive waste from the leftover fuel.  We don't want that hanging around on the planet for a couple thousand years polluting the environment and threatening lives.

Nuclear fusion, on the other hand, avoids all of these problems.  It barely releases any CO2, doesn't produce radioactive fuels as waste, and unlike renewable sources such as solar and wind, we could control how much power we generate to keep things in line with demand, if we could figure out how to use it effectively, that is.

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To generate power, all we have to do is get nucleii to fuse and capture the energy from the particles that are released in the process.  Unfortunately, getting them to fuse involves an enormous amount of pressure and heat.  Just temperature-wise, we're talking close to 150 million degrees Celsius, at least in the conditions here on Earth.  Sure, we can get atoms to fuse in H-bombs.  Thermonuclear weapons use a fission explosion to set off a fusion explosion, but while that's a super effective way to blow something up, it isn't really a safe, useful, or efficient way of generating energy.

The problem is, it's hard to get fusion going without using the energy from a massive explosion to do it.  A few different chemical elements can undergo fusion on Earth, but scientists have pinned most of their hopes on combining deuterium and tritium.  Those are just ordinary hydrogen nucleii with one or two extra neutrons tacked on respectively.  The two nucleii are able to fuse into helium, releasing a neutron with a bunch of energy in the process.  Even better, that helium nucleus can give up some of its energy to nearby electrons, which in turn can heat up more deuterium and tritium, causing even more fusion to happen.

That process is called alpha heating and without it, a fusion reactor isn't likely to produce enough energy.  It's sort of like trying to start a fire in a cold environment.  Unless you supply enough heat in the first place, your fuel will just lose energy to the surrounding cold air instead of igniting and starting a self-sustaining burn.  When the conditions are just right for fusion to deliver more energy than the heat energy required to get it going, it's called ignition.  In thermonuclear weapons, that's what the fission explosion is for. 

In a fusion reactor, it means you need to be able to hit some seriously high temperatures, and there are two major approaches to this, ICF and MCF.  ICF, or Inertial Confinement Fusion, involves filling a tiny pellet with deuterium and tritium and getting them to fuse by imploding the pellets with high-powered laser beams.  The National Ignition Facility in California has aimed to do exactly that: firing the world's most powerful laser, split into 192 beams onto fuel pellets the size of a pinhead.  When the pellet impodes, the fuel inside undergoes fusion and releases energetic neutrons, but for full ignition, the pellet would have to collapse perfectly inward to transfer all of the energy from the lasers into the fuel and start a self-sustaining reaction.

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Focusing the lasers onto a target that small is tricky enough, but the real difficulty is making the pellet collapse in on itself in just the right way.  It has to be made as perfectly spherical as possible and pressure has to be applied equally in all directions.  So far, the most successful strategy has been to put the pellet in a pencil eraser-sized gold canister, with the lasers firing into that instead.  When the gold heats up, it emits x-rays on the inside, generating a more symmetric pressure on the pellet so it collapses in on itself.

NIF has been able to generate small amounts of energy from this type of fusion.  In 2013, the lab actually achieved a gain in energy for the first time, getting more energy from the neutrons exploding out of the pellet than the energy delivered to it from the x-rays in the canister.  Unfortunately, it wasn't more energy than the total amount going into the system from the lasers, and ultimately, that's the benchmark for turning fusion into a practical way of generating power, getting more energy out than the total we put in to get it going. 

One of the biggest problems is that when the outside of the pellet is heated to extreme temperatures, it tends to burst off in an unstable way, creating an uneven pressure on the target.  Despite those setbacks, between 2010 and 2012, NIF went from needing to produce thousands of times more energy to get to ignition to only needing tens of times more energy.  That hundredfold difference came from more effective pellet designs, but keeping up the pace of improvement has hit a wall recently.

The other approach is MCF, or Magnetic Confinement Fusion, which works in a totally different way.  It's based on a type of reactor called a tokamak, a donut-shaped chamber that produces incredibly strong magnetic fields.  Inside the reactor, there's deuterium and tritium in the form of a plasma or charged gas, basically just a bunch of nucleii and electrons just floating around.  Then you blast the plasma with microwaves or a beam of neutrons, heating it up and ultimately making it undergo fusion.  Better still, if you line the outside of the reactor with lithium, neutrons released in the fusion process will generate tritium, introducing more fuel into the reactor.  A lot of the excitement surrounding fusion has focused on the MCF approach, but the problem is that containing a plasma with magnetic fields is like trying to herd trillions of uncooperative cats all at once.  

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They just keep trying to escape!  The magnetic fields and the way they interact with the plasma leads to what's called turbulent behavior, flowing in a way that's twisted and complex on every scale you look at it.  Turbulent fluids are tricky to make certain kinds of predictions about, even with computers, and that can get even harder as you heat up the plasma to the kinds of temperatures needed for ignition, so designing and maintaining magnetic fields that can keep the fuel mixture contained becomes more of a challenge.

Like ICF, MCF experiments have managed to achieve fusion, but only a little.  The record for energy gain, that is, the amount of energy that came out compared to the energy used to heat up the plasma in the first place, is still held by an experiment in 1997 using the jet reactor in England.  On that occasion, it produced 67% of the energy that was initially put in. The reason we haven't been able to improve on it much in the past couple decades is that jet can only do so many fusion experiments with deuterium and tritium.  The neutrons released in those kinds of experiments tend to make the surrounding material radioactive, so to keep the total radioactivity levels low, experiments tend to only happen every few years. Plus, tritium isn't easy to come by and jet doesn't have a way of producing lots of it on site, so they have to be sparing with how much they use, but the record could be broken in 2019, when jet is scheduled to run a new batch of fusion experiments with deuterium and tritium.

It's also managed to achieve some seriously high temperatures, over 200 million degrees, although not for very long.  As recently as 2018, though, there were a couple promising results from some smaller MCF experiments.  For example, a Chinese experiment, called East, managed to maintain a temperature of 100 million degrees in its reactor and even sustained fusion for 10 whole seconds, but one of the biggest developments in 2018 wasn't an experimental result. 

It was a letter, signed by experts from the US National Academies for Sciences, Engineering, and Medicine.  In it, they declared their support for an experiment in Southern France that isn't even done being built yet.  It's called ITER, and when it's done, it will be the world's largest tokamak reactor.  

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35 countries have come together to build it.  It's a long road, but by 2035, ITER should be ready to start deuterium-tritium fusion experiments, and because it's so much larger than jet, the world's current biggest MCF reactor, it should be able to hold a lot more plasma in its magnetic field.  The larger volume means the plasma interacts with itself more than in a reactor like jet.  This keeps more of the heat trapped in the plasma and makes it likelier for ignition to happen.  The goal is to generate 10 times as much energy from fusion than the amount of energy supplied to it.  That would be the first viable demonstration of fusion as a power source, but increasing the size of the experiment brings along a whole set of extra challenges.  

Large magnets require lots of energy and even more work to operate the magnetic field that they generate and contain such a large amount of plasma in it.  Even if it works, there are still some challenges to tackle, like dealing with the radioactive material that gets generated in a reactor from all the parts being bombarded by neutrons.  It's not radioactive fuel waste, like in a fission reactor, so it's not as bad, but it is still some radioactive stuff that we'll have to eventually dispose of.  Plus, extracting energy from the neutrons generated by fusion wouldn't be quite as straightforward as it is in conventional power plants.

The walls of the reactor are what capture the neutrons and heat up.  In a power plant, the walls would then heat water surrounding the reactor into steam to drive turbines, but being bombarded by all those neutrons can make the walls brittle and radioactive over time.  To handle that damage for a long-term reactor, scientists need to design walls that are more resilient, but still able to efficiently transfer heat out of the reactor.  Although ITER won't be able to deliver power, researchers hope that if it's successful, those issues could be worked out in an even bigger test reactor called DEMO.

In the meantime, we'll keep working on all those other options for cleaner energy, like solar power and nuclear fission.  A lot of work is being done to make those technologies safer and more environmentally friendly, too.  As for fusion, for now, the old joke holds true.  Fusion power is still at least 30 years away, but maybe it won't be for long.

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