Let’s say you invented a machine.
Here’s what it does: You put $10 into it, and in less than a second, $100 comes out. The only fuel it requires is dirt.
So ... what do you make of your machine? I’ll tell you what I make of it: All of those worries you and your family have? They are over, forever.
That’s the promise of nuclear fusion. If it ever happens, we’ll have plentiful energy, forever, with negligible carbon emissions and no long-term nuclear waste.
Do you think that’s worth pursuing?
The best nuclear fusion machine made so far (besides the Sun, of course) is the Joint European Torus (JET) in Culham, England, which has shown it can take in our proverbial $10 and crank out $6.70. That’s really not all that bad.
But it’s taken a very long time and a ton of money and effort to get to even that modest outcome. Serious research into nuclear fusion has been going on since the 1950s. Back then, you would put $10 into your fusion machine, and sometimes, if you were lucky, it would spit out a penny.
Long before JET, the British had built a reactor called ZETA, and in 1958 it seemed we had a machine that could produce real pennies, real energy from fusion. This was right after Sputnik, and boy, were the Brits pleased:
You know it’s something big when the exclamation point spans half the page!
But careful examination showed that in reality, the pennies weren’t even coming from the machine. Fusion wasn’t actually detectably happening, and this necessitated an embarrassing backpedal for the Brits and, truthfully, for the worldwide fusion effort. It was around then that fusion started to take on its unfortunate slogan in the minds of the public: “It’s the energy of the future, and always will be.”
We do have some reason to hope, though. There are new reactor designs coming online that have a shot to break the “payback” record, and I’ll get to that. But first I feel like I should talk about what makes fusion so rewarding yet so difficult.
The rewarding part
This is the part that’s pretty straightforward. If you can get two atoms to fuse together into one, a crapload of energy comes out.
The part of an atom you really want to fuse together with another one is the nucleus, which houses all the protons (positive charge) and neutrons (no charge). Electrons (negative charge) don’t fuse together, and they’re teeny, so we’re not going to worry about them right now.
The most common fusion reaction for energy generation (by Earth people, anyway) uses deuterium and tritium, which are hydrogen atoms with one and two extra neutrons, respectively:
Here’s what happens when deuterium and tritium nuclei fuse:
We get a brand-new helium nucleus (He), a neutron (n), and 17.6 mega-electron volts (MeV) of energy, manifested as kinetic energy; that is, as our two new particles zooming away really fast.
You can fuse just about any nuclei together, but deuterium and tritium are among the pairs that fuse most readily, and they are not very hard to obtain. Deuterium can be separated from ordinary seawater pretty easily, and tritium can be made by hitting regular lithium with neutrons (which themselves are produced in the fusion reaction above!)
But where does the energy in this reaction come from? If you add up the masses of the new particles (helium and the neutron), it’s less than the total mass of the old particles, by about 0.4%. That doesn’t seem like much, but when mass gets converted to energy, we have to stop by the Equations Hall of Fame and use E = mc2, multiplying our tiny lost mass by the speed of light squared (which is huge) to get the energy produced. If you do that, it really does come out to 17.6 MeV.
Here’s how I know that qualifies as a crapload of energy. If you react one gram of a deuterium-tritium mix this way, or about the weight of a raisin, you produce enough energy to heat your house (at 10,000 kilowatt-hours per year), for more than nine years. From something the size of a raisin. Or if you’re really smart, you’d use a currant…. Huh? Huhhh? ….. Man, tough crowd. Moving on, then.
The difficult part
As I’m sure you know, atoms aren’t all raisins and sunshine. The tricky part is: HOW are we supposed to smush two nuclei together? After all, nuclei have positive charges, because of their protons, so they actually repel each other. Not helping. It’s a lot like trying to shove the same end of two pretty strong magnets together. You know what that feels like. They don’t want any part of it.
But wait a second — if protons hate each other so much, how do they stay together in a nucleus? Why doesn’t a nucleus with more than one proton, like that helium nucleus, just fly apart?
Because protons and neutrons are essentially covered with Velcro. There’s something called the strong nuclear force that holds them together, but it only works at very close range. So if you had those two opposing magnets again, but you covered their ends in Velcro, you could get them to stick together if you shoved hard enough.
But we can’t just pick up atoms with our hands and press them together. We have to come up with another way to slam nuclei together, and hard.
Well, the first thing to do is strip the electrons off the nuclei. When nuclei and electrons are free of each other and are just floating around independently, that’s a plasma. We can make a plasma pretty easily. One way is to send an electric current through a gas, as in a fluorescent light bulb. Another way is to heat the gas up. Even a simple flame can be a plasma. Here’s what happens when we put a flame into an electric field, between two plates that are hooked up to a battery:
The flame — a plasma — gets pulled apart! The positive nuclei head towards the negative plate, and the negative electrons head towards the positive plate. A regular gas wouldn’t do this because its atoms are intact and electrically neutral. You might notice at the ends of the flame there are sparks, because the flame, with all of its moving charges, is also completing the circuit and conducting electricity!
Anyway, great, we’ve exposed the nuclei. Now, how will we slam them together? The two best things we can do are to compress our plasma and to heat it. Compressing the plasma gets the atoms closer together so they collide more often. Here are some helium atoms, greatly magnified but the correct relative distance apart, when the gas is compressed to about 2000 times normal air pressure:
More collisions are good, but they need to be very hard head-on collisions. We get those by increasing the temperature. After all, the very DEFINITION of the temperature of a simple gas like this is how fast its atoms are moving. At room temperature, atoms in a gas move around at about 1,000 miles per hour. But deuterium and tritium fuse most efficiently when the plasma is heated to … gulp ...180 million˚F. That corresponds to the atoms moving at around 600,000 miles per hour. Yeesh.
How are we supposed to heat ANYTHING to 180 million degrees?! Any container we put it in will melt or vaporize even at temperatures way, way below that. A frickin’ diamond will burn at 1400˚F! In fact, the highest melting point of any known material is only 7460˚F. Whoop dee doo.
Luckily, there’s a secret: the Pinch!
If you run a high current through something, it’ll induce its own magnetic field and actually pinch the current carrier together. Instead of drawing confusing electromagnetic diagrams, let’s just look at an example that’s pretty dang easy to grasp. When a lightning rod gets struck, a kazillion amps of current go through it. If it’s shaped like a tube, the pinch effect does this to it:
In fact, a lightning bolt itself is pinched together in exactly the same way. That’s why it’s a narrow bolt and not just some kind of foggy flash.
Another example is The Ant Nebula. It’s largely a giant plasma, and it may or may not be a pinch caused by moving charge, like the lightning rod. But let’s say that it is, because that gives me an excuse to insert a cool astronomy picture!
Anyway, if we can get a plasma to flow like a current, it’ll pinch itself together and not touch anything. But if we do that in a straight tube, all the plasma will just flow out the ends. If we get the plasma to flow in a circle, though, it’ll keep on going around. So, how can we get a plasma to flow?
You might remember that if you spin a magnet inside a coiled wire, you’ll induce a current in that wire. It’s the changing magnetic field that makes charged particles move. It goes the other way, too; if you run a current through a coiled wire, you can get a magnet sitting inside the coil to move; that’s a solenoid.
If we now put our plasma into a donut-shaped container, or a torus, we can run a strong current through a solenoid in the center to induce a magnetic field. If we keep ramping that up, that in turn induces a current inside the torus:
That plasma current will pinch itself together, but it’s also good to keep the plasma particles rotating in little spirals, so they always get sucked back towards the center and stay away from the walls.
The upshot is that our particles move forward not in straight lines, but a bit more like this:
The fact that we have a current in the plasma heats it up, just like a current in any wire, because there is a resistance to the current flowing. But that isn’t quite enough to heat the plasma to fusion temperatures; we need to hit it with a stream of neutral particles or fire radiation at it to heat it further. But if we get it hot enough and sustain fusion long enough, the heat generated by the newly produced high-energy particles (helium and neutrons) slamming into all the other particles keeps the plasma hot enough all by itself, and we can stop adding energy from the outside. That’s kind of the Holy Grail of fusion, and it’s called “ignition”. You need to get up around that 180-million-degree figure to achieve that.
This donut-shaped reactor is called a tokamak, which comes from a Russian phrase, and it ought to, because it was invented by Igor Tamm and Andrei Sakharov (yes, the same man that won the Nobel Peace Prize in 1975):
The term “tokamak" comes to us from a Russian acronym that stands for "toroidal chamber with magnetic coils" (тороидальная камера с магнитными катушками).
I realize that operating a tokamak already sounds kind of complicated, but you actually need to add still other magnetic fields and other physical features to keep the plasma from going unstable and totally losing it. At best, plasma instabilities mean the wall gets touched and the plasma cools and you don’t get fusion, but at worst, if the plasma current is interrupted, you might get a relativistic runaway electron avalanche (yes, it’s really called that) and blow a big hole in your reactor, and that would be highly … bad. The people at JET made a nice slide show (if you’re really interested) that illustrates that even the basic considerations of the tokamak are pretty complicated.
But think about it: what you’re basically trying to do is turn on a garden hose and then arrange a series of high-powered fans to blow the stream of water around in a nice, tight circle without splashing anything. I mean, hats off to the physicists and engineers all over the world that are working their butts off to try and do this.
Upcoming attempts at sustained fusion
I am only talking about large tokamaks here, because they seem to be the best hope that we know of for sustained fusion. But there are certainly other ways to go about it, such as firing lasers at deuterium-tritium mixes, as is being done at the National Ignition Facility at Lawrence Livermore National Laboratory and other places. There are startups like Tokamak Energy and Commonwealth Fusion Systems working on smaller tokamaks, too, but I just can’t cover everything here.
Looking over the upcoming attempts at real fusion advances, two seem to stand out. One tokamak called the HL-2M is coming online this year in China, and it’s expected that they’re going to be able to contain a plasma and heat it to as much as 360 million˚F.
China already owns the record for hottest plasma, having attained the magic number of 180 million˚F in their “EAST” reactor in November, 2018:
They won’t be using deuterium-tritium just yet in the HL-2M, because tritium is mildly radioactive. Not enough to pose a threat to the population, but enough to make cleaning up the reactor after you make a mistake an extreme pain in the butt.
But you can calculate what the return on deuterium-tritium fusion would be in your reactor once you heat up and sustain the plasma, and there is a real chance that this reactor will take in the proverbial $10 and show that it is able in principle to produce, say, $11. That would be a BFD.
The HL-2M is just one part of a much bigger collaborative effort, though. The Big Dog. That is ITER, a collaboration among 35 countries, including the U.S., to build an $80 billion (do not adjust your sets) behemoth in Cardache, France:
ITER will come online in 2025 with the goal of heating and containing a plasma to show that the return with deuterium-tritium would be 10x. That’s $10 into $100. The nature of fusion research is that the reactor IS the experiment, so we won’t know if that’s remotely true until it’s built and tested.
If this thing works, though, you can safely call it one of the biggest technical achievements in … anywhere, ever. Like the moon landing of energy.
The U.S. is sinking about $8 billion into ITER when it’s all said and done, so it’s somewhat surprising that nobody really talks about it. Fusion has a chequered history, to be sure, but we have to keep striving and keep hoping. It will happen one day.
If you want to read more, especially about the fascinating history of fusion research, but also a little more about the workings of tokamak and laser fusion, a very approachable yet appropriately deep book is “A Piece of the Sun: The Quest for Fusion Energy” by Daniel Clery (2013).
I’m leaving you with one of my favorite bands ever, Sleater-Kinney, who wrote a pretty kick-ass song about nuclear fusion (from which the title of the diary comes), because of course they did.