Cover image credit: “WEST fish-eye lens” by Christopher Roux/EUROfusion. Cropped and recolored from the original. License CC BY 4.0.
Many an ancient philosopher once gazed at the sky and wondered how the Sun got its seemingly limitless energy. Every day, quite reliably, the Sun shone with a brilliance that no Earthly thing could match. As you might expect, this was baffling. A common idea was that the Sun was on fire, but how on Earth — or, well, how out of Earth — could the fire find enough fuel to last so long? In fact, as recently as the mid‐19th century, the most realistic explanation that the world’s top scientists could muster was that the heat was generated by constant impacts by space objects, just like the fiery storm that would be kicked up by a large meteorite ramming into the Earth. By this account, the Sun could only possibly last 300 000 years longer before sputtering out and banishing the Solar System into the afterlife.1
In a sense, that final theory was right. Obviously, the Sun will live for much longer than 300 000 years, but it is powered by collisions — not of meteorites, but of individual atoms. The process is known as nuclear fusion, and in some ways it seems too good to be true. Its fuel is hydrogen, the most abundant substance in the Universe. It is ludicrously efficient. The waste it produces has no long-lasting harmful effects. To replicate it — to create, as it were, a miniature Sun on Earth — would herald clean, almost limitless energy for all. There is just one problem. As the old joke goes: “nuclear fusion is just 30 years away — and always will be.”
Nonetheless, there have been no shortage of attempts to attain it. And, at the risk of getting my hopes too high, it is possible that we might just be getting closer.
1Elephant in the Room
The Sun tends to be pretty hard to ignore. That made it all the more confusing that it could shine, quite literally, day in and day out, for a longer period of time than any fire on Earth. Even so, fire was seemingly the only terrestrial phenomenon that resembled the Sun, so it was one of the earliest theories about the source of the Sun’s energy. There was only one problem — fire requires fuel, and a measurement of the size of the Sun reveals that it could not possibly hold enough fuel to keep burning for longer than a few thousand years.2 The most elementary history and archaeology could quickly prove that the Earth — and thus the Sun — had been around for quite a bit longer than that.
Thus the idea of impacts from space debris. On the face of it, this idea seemed almost reasonable. There is tons of debris in space, and an impact can cause enormous amounts of damage. In 1994, for instance, comet Shoemaker–Levy 9 crashed into Jupiter, leaving visible scars — some as large as planet Earth.3
The large, brownish mark was an impact plume created by the largest fragment of Shoemaker–Levy 9, captured here by the Hubble Space Telescope. It remained visible for weeks.
Credit: NASA. Public domain.
There was, however, one problem. For the Sun to shine continuously, the Sun would have to be a veritable celestial punching bag, impacted so often that it would rapidly grow heavier from the additional mass of all the incoming debris. It would grow so much heavier, in fact, that the orbits of the planets would shift due to the Sun’s increasing gravitational pull — and this simply wasn’t happening.2 The Sun must have some other way of shining. So what was it?
There were many ideas, some more exotic than others. Perhaps the Sun was collapsing in on itself, making its gas glow fiery white in its free‐fall.2 Perhaps the Sun was so radioactive that it glowed red‐hot.4 Perhaps the Sun wasn’t actually bright and hot at all — instead, it had an enormous electric field that warmed up the Earth’s atmosphere at a distance.5 There were theories upon delusional theories, but in the end a convincing explanation of the Sun’s power was still notably absent. For the scientific community, the Sun was a massive elephant in the room, except that the elephant was about 92 million miles away from the room and glowing mysteriously and also really, really big. Ah, well — c’est la vie.
2The Unbearable Lightness of Helium
It was many years later before the light of an answer rose over the horizon of the scientific world. It began with none other than Albert Einstein.
“Mass–energy equivalence” — the idea that mass and energy are just different ways of looking at the same fundamental concept — was first introduced with the famous equation E = mc2. A while later still, it was observed that, rather curiously, helium atoms are lighter than the sum of their parts. In 1920, Sir Arthur Eddington made an intriguing connection: if you were to assemble helium atoms from smaller parts — hydrogen atoms, say — the only way for the helium atom to be lighter would be if the missing mass were converted into energy. His proposal, then, was that the Sun — a massive object known to have hydrogen and helium in abundance — got energy by fusing together hydrogen atoms into helium atoms.6
It was a crazy idea, but it was just the right amount of crazy. Even if the Sun could only fuse a fraction of its atoms, the energy released from this process could let the Sun burn bright for a very long time. Unlikely as it seemed, it was the only viable option.
Since we’re well into the weeds of crazy ideas, why not toss in another one: if the Sun can pull off fusion, why can’t we? It turns out that it’s very hard to fuse together the nuclei of atoms — for the most part, they do not want anything to do with each other, and must be coërced into getting close enough to actually fuse. The Sun can use its incredibly high temperatures and the crushing weight of its gravity. For us Earthlings, however, it’s a different story entirely.
3What If Donuts Were on Fire?
One of the first attempts at a fusion reactor was shrouded in secrecy. Not in the sense that the government didn’t allow the public to know about it, no — it was, in fact, the opposite.
Arthur Kantrowitz and Eastman Jacobs worked in aerodynamics at NACA (the National Advisory Committee for Aeronautics), which would later evolve into today’s NASA. They would later become quite accomplished in this field, reaching lofty research positions and pioneering new aerodynamic wing designs, among other things.7; 8 In 1938, though, they wanted to try something different: nuclear power.
NACA’s Langley Memorial Aeronautical Laboratory in Virginia, today known as the NASA Langley Research Center, was home to Kantrowitz and Jacob’s fusion experiments.
Credit: NASA. Public domain.
Fusion, at this point, had become a somewhat mainstream idea, and Kantrowitz and Jacobs believed that they could recreate it if only they could get hydrogen hot enough. They planned to do this with very high‐energy radio waves that would excite the hydrogen atoms into a plasma with temperatures in the tens of millions of degrees Celsius. With those temperatures achieved, they then had to be sustained, while maintaining enough pressure for the hydrogen atoms to be forced to fuse. No physical material could possibly contain temperature and pressure that well, so the only option was to forgo a physical container altogether. Instead, they’d use water‐cooled wires to generate a magnetic field. Since the plasma was electrically charged, it would feel the force of this magnetic field, and thus it could be effectively contained without physically touching the magnets. The result, in theory, would be a donut‐shaped mass of superhot plasma that could undergo fusion.9
It turned out that the operative words were “in theory.” In practice, the containment … didn’t work.
There was another containment that also failed around this time. Being aerodynamicists, Kantrowitz and Jacobs were tasked with working on, well, aerodynamics. This entire fusion idea hadn’t gone through any kind of formal approval whatsoever, and the duo was certain that NACA’s bureaucracy wouldn’t allow such a risky project. At the very least, they definitely wouldn’t take kindly to the misappropriation of their resources to run such a project without any kind of vetting. So, running on this suspicion, they decided to run the project in secret, under the intentionally nonsensical codename of “Diffusion Inhibitor.”9
Their suspicion turned out to be correct. As they grappled with the plasma containment problem, the containment of their little secret was breached. NACA administrators discovered and unceremoniously shut down the project.9
For a very long while afterwards, fusion remained mostly unexplored. Research into it only began in earnest decades later, due to the carnal human desire to blow stuff up. The atomic bomb was, at the time, the newest exciting way to accomplish this goal, but it seemed that these bombs could be made even more explosive by adding the power of fusion.
Controlling fusion, however, was a far more difficult matter than building a bomb in which it could proceed with abandon. One solution, rediscovered in the US, the UK, and the USSR, was to use magnetic fields to trap the fusing hydrogen — just as the long‐forgotten Diffusion Inhibitor had done. There were many ideas on how to do this optimally, with all kinds of fancy names: z‐pinches, stellarators, and — perhaps most influentially — a design that confined the hydrogen in a donut shape, known as the tokamak.
4ITER Persequendum
The Cold War was well underway. Nixon and Brezhnev had, well, a couple of disagreements. In 1973, seeking to alleviate the mounting distrust between their respective countries in matters relating to nuclear weapons, they agreed, finally, to collaborate on achieving fusion.10
The result was INTOR, a name that referred to both the collaborative project and the reactor it was trying to build. It followed the tokamak model, and as such, just like all other so‐called magnetic confinement systems, it needed a powerful magnetic field. But INTOR, which even official documents called a “very large device,”11: 19 needed a very large magnetic field to match. For that, they needed some fancy materials: superconductors.
One of the easiest ways to get a strong magnetic field is to use an electromagnet — generating a magnetic field using an electric current, no bulky permanent magnets required. But to carry that electric current, you of course need wires, and ordinary wires would have been no match for the stringent demands of INTOR. Those demands would settle for nothing less than perfect — perfect conductivity, that is. Most ordinary materials cannot conduct electricity perfectly, losing at least some of the electricity’s energy to heat (which is why electronics heat up). This limits the amount of electrical current that can actually pass through such ordinary wires, which in turn limits the magnetic field they could generate, and INTOR could accept no such limits. No, it required a magnetic field so strong that it could only be generated by a material that carried electrical current perfectly, without losing a scrap of its energy — a superconductor.11: 26
Using superconductors in electromagnets isn’t all that uncommon — it’s the technique used in the majority of MRI machines, for instance.12 INTOR, however, had to deal with a few problems. Aside from the sheer scale of the thing, there was the matter of temperature. Superconductors work due to a phenomenon rooted in quantum mechanics that generally requires the material to be in a minimum‐energy state — in other words, a cold temperature. Exactly how cold depends on the superconductor, but the ones that were deemed optimal for INTOR — Nb3Sn and NbTi — would only work below around −426 °F (−254 °C)13: Table 9.3, which is to say quite cold indeed. In itself, this is not entirely out of the question, since, after all, MRI machines regularly make use of such supercold superconductors. But consider the amount of superconductor that would need to be cooled for INTOR, and how it would have to remain this cold despite being right next to fusing hydrogen plasma reaching millions of degrees, and you might start to see the problem.
Today, INTOR is no more, but its legacy is carried on by its successor, ITER. Though the leadership structure and the exact technologies are different, ITER’s goal is the same — to build the world’s biggest tokamak under international leadership.
A cross‐section of the superconducting Nb3Sn wires used in ITER.
Credit: “ITER wire” by Charlie Sanabria. Unmodified from the original. License CC BY 4.0.
That’s not the only problem, though — fuel, surprisingly, is a daunting issue. Though hydrogen is the most abundant element in the Universe, not all hydrogen is created equal — in nature, there are three distinct isotopes, including not only the overwhelmingly common protium but also deuterium and tritium. ITER — along with many other fusion reactors of all different kinds — is designed to fuse a mixture of deuterium and tritium, since this particular reaction is easier to start than other possible fusion reactions. Deuterium, while rarer than protium, isn’t terribly hard to come by — but finding tritium is a different story. On Earth, tritium is extraordinarily rare — and, worse, it decays over time. The solution that ITER uses, much like many other planned reactors, is to generate tritium using a so‐called breeding blanket. Once the fusion reaction gets started with a small amount of tritium, the breeding blanket will then be able to “breed” more tritium using captured particles escaping the reaction.14 To that end, one of the goals of the ITER project is to test various breeder blanket designs.14; 15
That may end up being one of ITER’s more important contributions, whether for better or for worse. Due to the importance of tritium breeding, its findings on this topic will doubtlessly be key pieces of information for future fusion endeavors. But in the process of carrying out the fusion needed to test the different breeding blanket designs, ITER will use up a significant portion of the world’s existing tritium reserves. All fusion research afterwards will have to tread on the thin ice of a miniscule supply of this vital fuel, at least until the technology is mature enough to consistently produce more tritium.14
Meanwhile, there are other challengers racing to outdo ITER. China’s EAST tokamak has achieved several records for the longest amount of time a fusion reaction has run continuously.16; 17 Commonwealth Fusion Systems, a startup spun off from MIT, is building the SPARC tokamak using the superconductor YBCO, which works at far higher temperatures than the Nb3Sn used by ITER. Calculations predict that, upon its scheduled completion in 2025, SPARC should be able to produce a whopping 11 times more energy than it consumes, blowing competing tokamaks out of the water — they, after all, have not yet produced enough energy to even match what they consume.18, 19
In fact, the competition goes even further — tokamaks are no longer the only game in town.
5Orange Dawn O’er the Horizon
When you have a lot of nuclear bombs and no particular desire to use them to blow up the world, it’s natural to wonder what to do with those bombs. This was the thought process American physicists found themselves in during the 1950s, and the result was an idea to generate electricity from detonating these bombs — including, perhaps, fusion‐based hydrogen bombs.20
Eventually, they decided that blowing up an entire hydrogen bomb was probably a bit excessive. Perhaps a much smaller detonation could be used to produce energy. Today, that concept has evolved into inertial confinement, an alternative to the tokamak and other systems that use magnetic fields to confine fusing plasma. Consider the National Ignition Facility in Livermore, California. The fuel in the NIF reactor is located in a small pellet only 2 mm wide, cooled to hundreds of degrees below zero. The pellet relaxes happily in this state until it is bombarded by powerful lasers that hit it from all angles at the exact same time, with nanosecond‐level precision. So powerful are these lasers, in fact, that within an instant they heat the pellet to hundreds of millions of degrees. The plasma doesn’t even get time to escape before these high temperatures and high pressures make fusion begin.21
A few of the NIF’s laser amplifiers, used to increase the amount of energy funneled into the fuel pellet.
Credit: “Preamplifier at the National Ignition Facility” by Damien Jemison/Lawrence Livermore National Laboratory. Unmodified from the original. License CC BY‐SA 3.0.
Using this system, the NIF announced a breakthrough on : they had become the very first fusion reactor to produce a net gain of energy.22 Several months later, in case there were any doubts, they did it again.23
Meanwhile, on the tokamak front, the past few years have been very eventful for superconductors. Ever since their discovery, the race has been on to find superconductors that can function at ever higher temperatures. In March of 2020, Ranga Dias of the University of Rochester made the extraordinary and groundbreaking claim that his team had discovered a superconductor that worked at room temperature. It took the scientific world by storm, reaching a coveted publication spot in Nature … before it was revealed by independent investigation to be almost certainly fake. In 2023, his team once again reported to have found a room‐temperature superconductor, but, for obvious reasons, doubts remain.24 Not to be outdone, Lee Sukbae and Kim Ji‐Hoon of Korea University unveiled their own alleged room‐temperature superconductor later that year. It was a rather unorthodox material, bearing little resemblance to other known superconductors — and further independent research reached the tentative conclusion that it didn’t look like a superconductor for the very simple reason that it wasn’t a superconductor to begin with.25
And that’s part of the thing about fusion — the prospect of energy as boundless as the Sun, and all the mystical things associated with it, tends to get people excited. Excited enough that they will see patterns that don’t exist, delude themselves into false hope, or even just lie for fun and profit. Such stories are as old as fusion itself, and will continue for decades to come. So, as always, one must take good news about fusion with quite a few grains of salt — in a time when moving away from fossil fuels is crucial, we cannot afford to invest solely in fusion power at the expense of other sources of energy like renewables or nuclear, and similarly we cannot afford to hedge all our fusion bets on one particular reactor. A cautious approach like this lends itself well to small reactors like the aforementioned SPARC tokamak, which can be constructed and adapted relatively quickly.
With technology moving as fast as it is, it’s beginning to look like a well‐rounded, multifaceted approach could crack the secret of fusion in the near future. It’s an idea that would’ve sounded impossible until quite recently — but the impossible is only impossible until it isn’t.
DDedication
This article is dedicated to Andrei Dmitriyevich Sakharov (1921–1989), whose pioneering work in the peaceful application of fusion technology earned him both praise from the world and censure from the Soviet regime. He was born in Moscow, living with his extended family and learning from his father, a widely respected physics writer and teacher. He quickly developed a great talent in physics and, after working as a lumberjack and an engineer, landed a doctorate supervised by Igor Tamm of the Soviet Academy of Sciences. He then followed Tamm to a daunting endeavor: building a hydrogen bomb.26; 27
Their belief at the time was that designing weapons of mass destruction was a necessary evil to keep the United States in check, so that neither party would have free reign to start a nuclear war without worrying about the consequences of potential retaliation. Even so, Sakharov began to feel increasingly weary of the whole “building a murder machine” thing, and as he rose through the ranks of academia he resolved to use his growing influence for good.26; 27 In 1950, he began to air the idea of using a magnetic field to control fusing plasma and generate power from it — an idea that he would later develop into the tokamak.28 Aside from this invention, he also began voicing opposition to nuclear tests that risked harming civilians with radiation. He was one of the few who dared to oppose Trofim Lysenko, a corrupt pseudoscientist who believed genetics was fake, persecuted scientists who refused to support him, and promoted disastrous agricultural policies that led to millions dying from starvation. Most audaciously of all, Sakharov criticized the Soviet economic system itself.26 Soviet officials were, of course, not exactly renowned for open‐mindedness, so they sent Sakharov into exile, expunged records of his vital contribution to the invention of the tokamak, and even barred him from receiving the Nobel Peace Prize he won for his activism.26; 28 But in a sense, Sakharov got the last laugh — today, it is thanks to his work that countries and corporations the world over can dream of using fusion for peace.
SSources
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“Committee Reviews Progress on Test Blanket Modules” by Luciano Giancarli. Iss. 127 from: ITER Newsline. Published by ITER (Saint‐Paul‐lès‐Durance, FR) on . URL: https://www.iter.org/newsline/-/2572.
“Early Steps Toward Inertial Fusion Energy: 1952–1962” by John H Nuckolls. Published by the Lawrence Livermore National Laboratory (Livermore, CA, US) on . URL: https://www.osti.gov/servlets/purl/658936; DOI: 10.2172/658936.
“Andrei Sakharov: Biographical” by Andrei Sakharov; edited by Tore Frängsmyr, Irwin Abrams. Published by Nobel Prize Outreach AB (Stockholm, SE). Originally published by World Scientific Publishing Co. (Singapore, SG) in . URL: https://www.nobelprize.org/prizes/peace/1975/sakharov/biographical/.
“Sakharov’s Scientific Legacy” by Zhores A Medvedev. Vol. 319, iss. 6049, p. 93 from: Nature. Published by Nature Publishing Group (New York City, NY, US) on 1986 Jan 09. URL: https://www.nature.com/articles/319093d0.pdf; DOI: 10.1038/319093a0.
XDisclaimer
I am not a scientist or professional in any field. The content of this article merely expresses my personal views, opinions, and visions for the future. This content is not intended for use as professional advice on any matter.
