NOBODY IS QUITE SURE WHO HAD THE IDEA FIRST. AFTER THE whirlwind of discovery that was physics in the 1920s and 1930s, all the theoretical building blocks for generating energy by thermonuclear fusion were at hand and someone was bound to put them together. As often happens in science, the same idea popped up in several disconnected places at around the same time.
Hans Bethe, an émigré physicist from Germany who fled when the Nazis came to power in 1933 and settled at Cornell University, remembers having a conversation in Washington during 1937 with fellow émigré Leo Szilard from Hungary. Bethe had that year written a landmark paper that finally nailed down the fusion processes in stars which produce energy, so he was well placed to address the problem of creating fusion. Fritz Houtermans was another German-born physicist who fled the Nazis, but in his case to Kharkov in the Soviet Union. He is believed to have been carrying out experiments in fusion in 1937. And Peter Thonemann, an undergraduate student at the University of Melbourne in Australia, remembers working out a basic plan for a fusion reactor in 1939.
Although everything was ready for the pursuit of fusion to begin, the Second World War soon had physicists thinking of other things. After Nazi troops invaded Poland, Szilard drafted a letter alerting the US government that a fission chain reaction could be used to make a devastating bomb and warning that German scientists were working in this area. He persuaded his old friend and colleague Albert Einstein to co-sign the letter and they delivered it to President Franklin Roosevelt. That letter led eventually to the herculean Manhattan Project to develop an atomic bomb before the Nazis did. Bethe joined the Manhattan Project, heading the theory division at the top-secret Los Alamos laboratory in the New Mexico desert where many of America’s and Europe’s best physicists spent the latter years of the war.
Thonemann also joined the war effort, taking a job at the Australian government’s Munitions Supply Laboratories in 1940 and later leaving to join the research department of Amalgamated Wireless near Sydney. As the war approached its conclusion, he took up his studies again at Sydney University. In Sydney, his passion for the possibilities of fusion continued. His thesis topic was how to measure the density of electrons in a plasma. Thonemann talked endlessly about fusion and at home he melted window glass in the oven in an effort to make doughnut-shaped vessels for plasma experiments.
Thonemann came from a comfortable suburban background in Melbourne, with a stockbroker father, mother, two brothers and a sister. At university in Melbourne he had enjoyed tennis and skiing, while in Sydney he made his own surfboard to ride the waves back at Rye near his home town. He was entertaining in company and played the piano well. But he gave up this seemingly idyllic existence to go somewhere he thought would help him in his quest for fusion: Oxford University.
Stepping off a ship into the Britain of 1946 must have been a shock. Southern Australia was physically untouched by the war, so the bomb-damaged English cities, the rationing of food and clothes, and the general air of exhaustion must have made it seem an alien world. But Oxford did have what he was looking for: the Clarendon Laboratory, chock full of some of the biggest scientific names of the day – many of whom had only recently been released from war work – along with all the experimental apparatus and skilled technicians that he would need. Thonemann had been accepted to study for a doctorate in nuclear physics at a salary of £750 per year. He came armed with notebooks full of calculations describing the conditions necessary to achieve a fusion reaction. But his supervisor, Douglas Roaf, had other ideas and set Thonemann to work developing ion sources. The topics were related, however, and Thonemann was able to simultaneously carry out work on fusion ‘under the counter’ – the hunt was on.
The roots of the search for fusion stretch back a century before Thonemann began tinkering away in Oxford. At that time an argument developed between physicists, geologists and biologists over the age of the Sun. Physicists of the nineteenth century had made huge strides in understanding the world around them and, emboldened by their success, were applying their theories to ever grander problems. One pivotal achievement was the laws of thermodynamics, the principles that govern the behaviour of heat. According to the First Law of Thermodynamics, energy, or heat, cannot be created or destroyed but can only flow from one place to another and transform from one form to another. Thus the gravitational energy of a ball at the top of a slope is converted into kinetic energy as the ball rolls down the hill. Or the electrical energy of a current in a wire is converted into heat and light by a lightbulb.
Physicists found that such laws seemed to apply in every situation that they studied, and rightly concluded that they must be universal. But when these fearless physicists applied the First Law to the case of the Sun, this produced troubling conclusions. Scientists could estimate the energy being pumped out by our neighbourhood star by measuring the solar heat falling on a patch of the Earth’s surface and then extrapolating from that to the heat on the inside of a sphere with a radius equal to the Earth-Sun distance. The amount of heat was phenomenal and that then begged the question: if heat cannot be created out of nothing, where is it all coming from? The best source of energy at the time was coal, but if you had a ball of coal the size of the Sun burning to produce heat at the rate scientists had calculated, it would be reduced to ash in around 3,000 years, far too short a time for the formation of the whole Solar System.
Two of the titans of mid-nineteenth century science had a different idea: German physiologist and physicist Herman von Helmholtz proposed in 1854 that the Sun’s heat came from gravitational energy. As the Sun contracts this energy is transformed into heat and the body of the Sun glows hot, radiating light. Scottish physicist William Thompson, later Lord Kelvin, came to a similar conclusion and calculated that with this source of energy a body the size of the Sun could have been around for 30 million years.
This seemed a much more reasonable figure for the age of the Sun, but it didn’t please everyone. Charles Darwin had published his new theory of Evolution in 1859 in his book On the Origin of Species by Natural Selection where he included a rough calculation of the age of the Earth made by studying erosion processes in the part of Kent where he lived, known as the Weald. His estimate was 300 million years, and he also concluded that Evolution would need this sort of time to produce the variety of life he saw around him. As the heat of the Sun was needed for life to exist, the Sun must be at least that age. Geologists, too, required an Earth aged in the hundreds of millions of years to explain the transformations of rock that they observed. The debate over the age of the Sun and Earth raged for decades and Darwin was sufficiently troubled by Thompson’s argument that he removed any mention of timescales from his last editions of On the Origin of Species.
The solution to this mystery began to come together late in the century with a discovery entirely unrelated to astrophysics, geology or Evolution: radioactivity. French physicist Henri Becquerel first noticed that uranium salts, when he left them on top of a photographic plate wrapped in black paper, left an image of themselves when the plate was developed. The salts were emitting some sort of invisible radiation that could penetrate paper and expose the plate. Marie Curie and her husband Pierre, as well as others, continued the study of this phenomenon, which the Curies dubbed radioactivity, identifying different sorts of radiation and isolating two new radioactive elements, radium and polonium. Radium, in particular, was highly radioactive – more than a million times that of the same mass of uranium – and to those early pioneers it exhibited a fascinating property: it was hot, all the time, irrespective of the surrounding conditions. The metal appeared to be breaking the First Law of Thermodynamics. Where was all the heat coming from?
That question was answered in the following decade by Albert Einstein as a consequence of his theory of Special Relativity. His famous equation E=mc2 broadened the First Law of Thermodynamics by including matter. Energy could seem to disappear if it is converted into matter, and similarly matter can be transformed into energy. Because the speed of light (c in the equation) is a large number, a very small mass (m) of matter converts into a huge amount of energy (E).
Scientists soon realised the atoms discovered by Becquerel, the Curies and others are radioactive because they are unstable, so over time the nuclei of their atoms split apart into other, smaller nuclei. With each decay, a tiny bit of the nucleus’ mass is converted into energy, explaining the heat produced by radium and the rays that were blackening photographic plates. What those researchers didn’t know was that radiation could endanger your health. Marie Curie carried around samples in her pockets and kept them in her desk, enjoying looking at the blue-green light they gave off in the dark. She died in 1934 of aplastic anaemia, almost certainly as a result of exposure to radiation. Today, her notebooks and even her cookbook from the 1890s are considered too dangerous to handle without protective clothing and are kept in lead-lined boxes.
Scientists almost immediately began to wonder whether radio-activity was the source of the Sun’s heat. But observations of the Sun showed that it didn’t contain much radioactive material. It was mostly made of hydrogen, the smallest and lightest element which couldn’t decay into anything smaller.
The decisive clue to the source of the Sun’s energy was provided by the British chemist Francis Aston who, in 1920, was trying to prove the existence of isotopes, versions of an element that have different masses but identical chemical properties. Later he proved that isotopes do exist and, significantly, that their masses are always rough multiples of the mass of hydrogen. So the most common isotope of carbon has the mass of roughly twelve hydrogens, but there are also isotopes of carbon that weigh thirteen hydrogens and fourteen hydrogens. Although it wasn’t known at the time, this is because atomic nuclei are made up of both protons and neutrons which have roughly the same mass. The normal hydrogen nucleus is just a single proton, while the carbon nucleus has six protons plus six, seven or eight neutrons. But in 1920, as part of his search for isotopes, Aston took a number of different elements and made very precise measurements of the masses of their atoms. As everyone expected, the mass of helium, the second-smallest nucleus, was around four times the mass of hydrogen. Aston’s measurements were so accurate, however, that it was possible to conclude that while helium’s mass was close to that of four hydrogens, it wasn’t exactly the same – helium weighed slightly less than four hydrogens.
At that time, it was thought that helium really was made from four hydrogens, so the fact that the masses were slightly different was significant. One who spotted the importance of this result was Arthur Eddington of Cambridge University, one of the leading astrophysicists of the day. Eddington was an enthusiastic early advocate of the theory of Relativity and maintained contact with Einstein during the First World War when most British scientists shunned any contact with their German colleagues. Eddington was a committed pacifist and when he was called up for military service in 1918 he refused, risking prison. Prominent scientists rallied to his cause and the Astronomer Royal, Frank Watson Dyson, argued that his expertise was essential to an experiment they were to carry out to put Relativity to the test.
The scientists’ pleas won the day and in 1919 Eddington and Dyson travelled to the island of Principe off the west coast of Africa to observe the solar eclipse of 29th May. One of the predictions of Einstein’s General Theory of Relativity is that the gravity of a massive object would deflect the path of a beam of light. The object had to be very, very massive to observe this weak effect and Eddington and Dyson’s aim was to use the Sun. With the Sun’s light blocked out by the moon during the eclipse, it would be possible to see stars whose light passes close to the Sun. If gravity does indeed bend beams of light then, as the Sun moves across the sky, just before it obscures a star, the light from the star would appear to move as its path is curved by the Sun’s gravity. The pair did see stars appear to move and when they revealed their results back in Britain the news was reported around the world as the first conclusive proof of the truth of Relativity. Eddington, and Einstein, became household names.
At that time, Eddington was also working on a theoretical model of the interior conditions of stars, even though the source of their energy was still unknown. Some still adhered to Kelvin and Helmholtz’s gravitational explanation but Eddington was convinced that some kind of nuclear process was more likely. As a result, he jumped on the measurements made by Aston and in August 1920, in an address to the British Association for the Advancement of Science, proposed a new theory. He suggested that in the searing heat at the centre of the Sun hydrogen atoms are fusing to form helium atoms and, if the loss in mass in this process measured by Aston is converted into energy, this could prove to be the Sun’s energy source. Eddington estimated that if 5% of the sun’s mass is hydrogen (we now know that it is actually around 75%) and if, according to Aston, 0.8% of the hydrogen’s mass is converted into energy during fusion, then the Sun – at its current rate of heat production – will last about 15 billion years. He added, somewhat prophetically:
If, indeed, the subatomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfilment our dream of controlling this latent power for the well-being of the human race – or for its suicide.
If the Sun had indeed burned for billions of years, that gave scientists – be they evolutionists, geologists or astrophysicists – all the time they needed.
Eddington continued to try to incorporate nuclear reactions into his theory of the interior of stars but it was still beset with problems. Although Aston had shown that combining four hydrogens to produce a helium freed up mass to convert into energy, no one knew how to make that process happen. And of the nuclear reactions that could be performed in the laboratory, none of them released enough energy to power the Sun.
Another worry was that, according to the classical physics that prevailed at the time, hydrogen nuclei just would not fuse. To react hydrogen it would be necessary to strip off its outer negatively-charged electron, leaving just the tiny exposed nucleus with its positive charge. For fusion, two such nuclei must slam into each other with such force that they get so close together that it is then more advantageous for them to merge than fly apart again. It’s similar to what happens when two droplets of water are pushed together: at first they seem to try to stay separate as if surrounded by elastic skins, even though they are squashed up against each other, until eventually the best way to relieve the pressure is to merge into a single drop. The problem with two nuclei is that they carry the same positive electric charge and so repel each other – just as bringing the same poles of two magnets together creates a repulsive force – and the closer they get together, the stronger the repulsion. Classical physics predicts that it is virtually impossible to bring two nuclei close enough together to fuse.
In the 1920s, however, a new show arrived in town: quantum mechanics. In quantum mechanics there are fewer yes or no answers and more probabilities. Impossible things are allowed by quantum mechanics, they just have low probabilities of happening. A young Russian physicist called Georgii Gamow was in 1928 the first to apply quantum mechanics to nuclear reactions. He reasoned that it was not impossible for two nuclei to get close enough to fuse, and he developed a formula to find the probability of such a reaction.
Using Gamow’s formula, Fritz Houtermans, then at the University of Göttingen in Germany, and Welsh-born astronomer Robert Atkinson began to look at what might happen to nuclei knocking around together under the sort of conditions that Eddington predicted would exist in the heart of the Sun. The two scientists complemented each other perfectly for this task: Houtermans was an experimental physicist who had worked with Gamow at Göttingen and knew about the application of quantum mechanics to the nucleus but not about the interior of the Sun; Atkinson knew all about Eddington’s theory of the Sun but little about quantum mechanics. They calculated that under Eddington’s predicted conditions there would be a healthy rate of reactions between colliding hydrogen nuclei. Their 1929 paper on the topic is thought by many to be the starting point for thermonuclear fusion energy research.
It was now time for experiments to take centre stage. During a visit to Cambridge, Gamow discussed his work on the quantum mechanics of nuclei with a young physicist at the Cavendish Laboratory there called John Cockcroft. This spurred Cockcroft to develop, along with his colleague Ernest Walton, a device for accelerating hydrogen nuclei or, as they were then becoming known, protons. Protons are a constituent part of all nuclei, along with neutrons. Hydrogen, the simplest nucleus, is made up of a single proton. Cockcroft believed that if Gamow was right, his accelerator would be able to accelerate protons to a high enough speed so that, if they collided with other nuclei, some fusions might take place.
By 1932, Cockcroft and Walton had built their accelerator and used it to fire protons at a sample of the metal lithium. They found that at relatively modest energies, the protons were able to penetrate into the lithium nuclei and split each one into two helium nuclei. This was hailed as a triumph at the time: the very first ‘splitting of an atom’. Many years later, the pair would share a Nobel Prize for their achievement. It was also significant because this reaction was the first to produce a large amount of energy. The resulting pair of helium nuclei carried more than 100 times the energy of the proton that caused the reaction. The reaction was too difficult to achieve to be a practical source of energy for the Sun, but it at least showed that it was possible to liberate a lot of power from nuclei.
Back at the Cavendish Laboratory, a colleague of Cockcroft’s called Mark Oliphant made some improvements to the design of the Cockcroft-Walton accelerator so that it could separate out and accelerate deuterium nuclei. Deuterium, with its extra neutron in the nucleus, is identical to hydrogen in every way except that it is twice as heavy. This similarity makes it very hard to distinguish from hydrogen – its existence had only been confirmed a couple of years earlier. Deuterium’s discoverer, American physical chemist Gilbert Lewis, had only just managed in 1933 to separate out a usable quantity of so-called heavy water, made from oxygen and deuterium rather than hydrogen. As soon as he had enough, Lewis sent a sample over to Ernest Rutherford, the formidable director of the Cavendish Laboratory.
Rutherford was a towering figure in early twentieth-century physics, having earned a Nobel Prize in 1908 for the discovery that natural radioactivity was due to atoms disintegrating and that it produced two different sorts of radiation. In 1911 he overturned the prevailing ‘plum pudding’ model of the atom – that it was a positively charged ball peppered with negatively charged electrons – by proving that an atom has a tiny but dense nucleus and electrons orbiting around it, a description that still holds true today. Rutherford had a domineering personality, a very loud voice, and ran the Cavendish as his personal fiefdom. He had overseen the work of Cockcroft and Walton and now, with Oliphant, he was going to see what he could do with deuterium.
Rutherford and Oliphant fired deuterium nuclei, or deuterons, at lithium and a number of different elements to see what nuclear reactions they could cause. Eventually they collided deuterium with deuterium and found they produced two different reactions: one producing an isotope of helium known as helium-3 (two protons and a neutron) and the other an even heavier isotope of hydrogen (one proton and two neutrons) that would eventually be called tritium. Both of these reactions produced excess energy, roughly ten times that of the incoming deuteron. But Rutherford, for one, was not convinced that you could ever produce useful amounts of energy by this method because, although individual reactions produce energy, only around one in every 100 million accelerated protons or deuterons actually caused a reaction. So overall there was a huge energy loss. Rutherford famously said at the time:
The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine.
The reason why using an accelerator to produce fusion was so inefficient was because most of the accelerated protons or deuterons get tangled up with the electrons orbiting around the target nuclei, losing most of their energy before they reach their objective. In the heart of the Sun the situation is very different because it’s a plasma: the hydrogen atoms are stripped of their electrons and the nuclei can collide directly with each other without having to fight their way past electrons first.
Gamow, by 1938, was in the United States having defected from Russia. He decided to do so in 1932 because of Stalin’s repression but his early attempts to escape with his wife Lyubov Vokhminzeva, also a physicist, were unsuccessful. First they tried to paddle a kayak 250 kilometres across the Black Sea to Turkey but were foiled by bad weather. A later attempt to cross from Murmansk in northern Russia to Norway ended the same way. They eventually succeeded the following year, but in a far less adventurous fashion: Gamow got permission for them both to attend a physics conference in Belgium and they absconded from there. Installed at George Washington University in Washington, DC, Gamow decided that enough was then known about nuclear physics to launch a concerted effort to explain the workings of the Sun. He teamed up with another émigré physicist, Edward Teller, also at GWU at the time, and they concluded that deuterium fusion had to be the source of the Sun’s heat. Gamow felt that the time was right for a conference to debate the topic.
The star of that spring conference was Bethe who, with two colleagues, had recently completed a series of three articles summarising all that was then known about nuclear physics – work that colleagues called Bethe’s bible. Bethe arrived not having thought much about the Sun’s energy but he soon latched onto Charles Critchfield, a former student of Gamow’s, who just before the conference had proposed a series of nuclear reactions that could power the solar furnace. They worked together during the conference with Bethe helping Critchfield iron out some problems with the scheme. In their proton-proton chain, two protons collide first to produce a deuteron (one proton transforming into a neutron). The deuteron then fuses with another proton to create helium-3. And finally two helium-3s merge to create normal helium-4 plus two protons.
During the meeting, Bethe began working on another chain in which a carbon nucleus is bombarded by one proton after another, transforming it into a series of carbon, nitrogen and oxygen isotopes – hence its name, the CNO cycle – until eventually it spits out a helium-4 nucleus and returns to its original state. For six months after the conference, Bethe continued working on the problem and developed a coherent theory of energy production in stars which he published in a groundbreaking paper. The proton-proton chain was, it turned out, the dominant mechanism in smaller stars, including the Sun; larger stars favour the CNO cycle. This work would, in 1967, win Bethe the Nobel Prize for Physics, but this particular line of thought was soon put to one side as the Second World War loomed. Soon many of the world’s top physicists would be co-opted into the Manhattan Project and would turn their minds to the search for an atomic bomb.
So it was, in Oxford’s Clarendon Laboratory a decade later, that Thonemann knew he had to get deuterons to fuse if he was to have any chance of generating power. But the question was: how? Rutherford had shown that using a particle accelerator was hugely inefficient. Thonemann realised, as others soon did too, that the Sun already had the best idea. Simply heat up your fusion fuel: when you heat something up its constituent atoms move faster and if you keep heating it, eventually the atoms will be moving so fast that collisions will become fusions, releasing more heat to keep the process going and hopefully some to spare. When the Sun was forming billions of years ago from clouds of gas, that initial heating was provided by gravitational contraction – so, in a sense, Helmholtz and Kelvin had been right about the Sun’s original source of heat. But once the core reached 15 million °C, fusion ignited and from then on the outward pressure of all the heat and light produced by fusion counteracts the gravity, an equilibrium is reached and the contraction stops. Our mild-mannered local star is performing a continual balancing act: the huge crushing weight of its mass (330,000 times that of the Earth) is held perfectly in place by the slow-burning thermonuclear reactor at its heart.
But recreating a piece of the Sun on Earth is far from easy because of the extreme temperatures needed. If any plasma at that temperature were to touch the container it resides in, that container would be instantly melted or vaporised. So Thonemann had to figure out how to contain his deuterium plasma in such a way that it did not touch anything. The answer to this puzzle lay in the unique properties of plasma.
Plasma is the fourth state of matter, after solids, liquids and gases. You can turn gas into a plasma by just heating it up: at a certain temperature collisions wrench electrons free from the gas atoms. You can also make plasma with an intense electric field, which pulls the negatively-charged electrons and positive nuclei in opposite directions, eventually ripping them apart. Most flames are plasmas, as are electric sparks, lightning bolts and the glowing gases inside fluorescent tubes and low-energy light bulbs. Plasma is, in fact, by far the most common state of matter in the Universe since all stars and most of the gas between the stars are plasma. Planets like ours are rare islands of electrical neutrality in a highly-charged Cosmos.
The most noticeable difference between plasma and normal gas is that, because it is made up of charged particles, plasma is affected by electric and magnetic fields. In an electric field, plasma ions will all flow in the direction of the field and all the electrons will flow against the field (a normal gas is unaffected). What you get is an electric current in the plasma, just like the current you get from electrons flowing along a wire.
The effect of a magnetic field on plasma is more subtle. Charged particles don’t feel anything in a magnetic field if they are stationary or moving parallel to the field lines, but when they are on the move, cutting across the magnetic field, they will feel a force that is perpendicular to both their direction of travel and the field direction. So an electric current flowing along a wire from, say, west to east across the Earth’s magnetic field running from south to north will feel a force pushing it vertically upwards. This phenomenon is crucial to devices such as electric motors and actuators.
So scientists had a tool that could push plasma around, but how to fashion that into a container that can hold a plasma without it touching the sides? The germ of an answer was planted when in the first few years of the twentieth century a bolt of lightning hit the chimney of the Hartley Vale Kerosene Refinery near Lithgow, New South Wales, Australia. A Mr G. H. Clark of the refinery was so puzzled by what happened to the chimney’s lighting conductor that he sent it to J. A. Pollock, a physicist at the University of Sydney. Pollock called in a colleague, mechanical engineer S. H. Barraclough, to look at it. The short section of copper pipe appeared to have been crushed by some huge force but, as far as they knew, all that had happened to it was that a large current pulse had flowed down it from the lightning strike.
Pollock and Barraclough developed an explanation for what had crushed the tube. It was well known that an electric current flowing down a straight conductor generates a magnetic field with field lines that loop around the conductor. But if you have an electric current cutting across magnetic field lines – even if the field is created by the current – the electrons in the current will feel a force. In this case, with a straight current and a field looping around it, that force is directed inwards towards the centre of the conductor. During the Hartley Vale lightning strike, the pulse of current down the copper tube was so great that the inward force was enough to crush the copper pipe as if it were a toothpaste tube.
The phenomenon that Pollock and Barraclough discovered, soon dubbed the pinch effect, was for many years considered a scientific curiosity without much practical use. Forty years later, at that same Sydney University, Peter Thonemann learned about the pinch effect and began dreaming of fusion. He realised that if you get plasma to flow along a tube, and so produce a current, that current will generate a pinch effect and keep the plasma away from the walls of the tube. But what about the end of the tube? If it’s closed, the plasma will just accumulate there; if it’s open it will all drain out. Thonemann’s solution, which occurred to other scientists at the time, was to bend the tube around into a doughnut-shaped ring so that the plasma can keep flowing round and round as long as necessary.
In Oxford, eager to turn his ideas into reality, Thonemann wrote to the director of the Clarendon lab, Frederick Lindemann, otherwise known as Lord Cherwell, asking for aparatus to carry out experiments directed towards fusion. This was no routine matter for the young physicist since Cherwell was a powerful and well-connected man, having been a confidante of and chief scientific adviser to Winston Churchill during the war. Cherwell asked Thonemann to present his ideas in a symposium of Clarendon staff. So in January 1947, Thonemann stood up and explained his ideas for controlled thermonuclear fusion to a high-powered audience of physicists. Few queried his calculations, although there were questions about how much radiation the fusion reactions would produce. ‘You managed to stay on your horse,’ Cherwell said to Thonemann afterwards.
So, now with Cherwell’s approval, Thonemann directed the Clarendon’s in-house glassblower to make him a ring of glass tubing – a shape that mathematicians call a torus – with a diameter of around 10-20cm. The first things that Thonemann had to figure out were how to initiate the plasma – converting a neutral gas into a plasma with an electric field – and how to make a plasma current flow around the torus. Getting the current to flow is crucial, because without a current there is no pinch, but how to do it? Here Thonemann would exploit a trick known as electromagnetic induction.
Just as a wire carrying a current, when it cuts across a magnetic field, feels a force, so the reverse is also true: when a changing magnetic field cuts across a wire, the electrons in it feel a force and so start to flow as a current. Similarly, a changing magnetic field cutting across a torus filled with plasma will push the plasma around the ring. Thonemann achieved this using an electromagnet whose field is carried through the centre of the torus using a ring of iron that loops through the torus like links in a chain. Such induction only works, however, if the magnetic field is changing, so an increasing electric current in the electromagnet will create an increasing magnet field in the iron ring which will in turn induce a growing current in the torus. But it’s not feasible to keep increasing the current forever, so Thonemann used an alternating current which swings one way then the opposite way in quick repetition. With an alternating current applied to the electromagnet, the magnetic field – and hence the plasma current – is always changing, flowing first one way then the other, except during those brief instants when it changes direction.
Roaf, Thonemann’s supervisor, acquired from somewhere an alternating current generator that had been used during the war for radar work. Thonemann quickly discovered that the alternating current alone wasn’t enough to start the plasma: he had to use a static electric field to initiate it and create a conducting channel of plasma around the torus before induction could kick in and make the plasma flow. Thus began a series of meticulous studies to see how plasmas behave in magnetic fields. The physics of plasmas was an obscure and little-studied field at the time, so much of what he tried was completely new. He measured the basic conducting and magnetic properties of plasmas. He measured the strength of the pinch effect using plasmas in straight tubes. He learned that the current channel through the plasma in a torus had a tendency to expand outwards until it touched the outer wall of the torus and was extinguished.
Thonemann’s work did not go unnoticed. In December 1947, John Cockcroft, the physicist who had split the atom in Cambridge fifteen years earlier, asked Cherwell to see what Thonemann was up to. Just the year before, Cockcroft had founded Britain’s Atomic Energy Research Establishment (AERE) on a former RAF base at Harwell, just 25 kilometres from Oxford. Cockcroft met with Thonemann several times and a few months later AERE took over funding Thonemann’s work, and provided him with two assistants.
By this time, Thonemann had demonstrated the pinch effect in plasmas experimentally. Now it was time to ramp up the power to produce a much stronger plasma current and show that he could squeeze the plasma enough to generate high temperatures. Thonemann and one of his new assistants, W. T. Cowhig, worked on the theory of plasma pinches during 1948 and tested their predictions of the pinch strength on mercury plasmas in a straight tube. Because of the higher power, they would need a torus made of something stronger than glass. They also needed something to counter the tendency of the plasma current to expand towards the outer wall of the torus. Thonemann’s solution was to build a copper torus with some wires running along the inner surface of the outer wall. When the plasma current was flowing, Thonemann would pass a current flowing in the opposite direction through these wires. The oppositely flowing currents repel each other and Thonemann could use this force to keep the plasma current away from the wall.
The copper torus was built and ready to run by the summer of 1949 when Thonemann invited Cherwell and Cockcroft to come and see it in action. The torus had two small glass windows and when Thonemann powered it up you could clearly see a stable and brilliantly glowing plasma current channel in the middle of the torus tube. Cherwell and Cockcroft were clearly impressed and they began visiting Thonemann’s lab every week, usually on a Saturday morning, to keep a close eye on his progress. What they hadn’t told Thonemann was that he was not the only researcher in Britain chasing fusion.
In 1946, George Paget Thomson, a physics professor at Imperial College in London, filed a patent for a torus-shaped fusion reactor. Thomson was at the heart of Britain’s scientific establishment. His father was J. J. Thomson, a Cambridge University physicist who won a Nobel Prize for the discovery of the electron and whose name is attached to many other discoveries. The younger Thomson also won a Nobel, in 1937, for showing that electrons behaved like waves as well as like particles. Before the war he had studied plasmas alongside his father, and during the war years he focused on nuclear physics, advising the British government that a nuclear bomb was possible. With that sort of experience it was no surprise that he would begin to think about fusion.
Thomson discussed his fusion reactor idea with colleagues at Imperial and with Rudolf Peierls, a physicist from Birmingham University who had worked on the Manhattan Project during the war at Los Alamos and knew of discussions there about ways of containing a fusion plasma. Peierls was sceptical and pointed out some problems he saw with the scheme, prompting Thomson to make a few modifications. Thomson filed his patent in May 1946. It described a toroidal reactor which uses the pinch effect to contain plasma. It didn’t say how the gas would be ionised nor specify how it would be made to flow around the torus, though several methods were suggested. A torus 3m across, the patent said, would be large enough to accelerate particles and achieve fusion.
Thomson was not able to do much with his idea because he was called to New York to advise the British delegation to the United Nations Atomic Energy Commission for most of 1946. But in January 1947, Cockcroft invited him to a meeting at Harwell about the possibility of setting up a fusion programme at the new AERE laboratory. More than a dozen physicists gathered for the meeting from Imperial, Birmingham, Oxford and Harwell, including Peierls and another Los Alamos veteran, Klaus Fuchs. Thomson described his reactor and the various ways he thought electrons could be driven around the torus. Peierls again expressed his doubts and Cockcroft, although interested, didn’t think the time was yet right to build a large-scale experiment, as Thomson wanted. It was agreed at the meeting that the teams at Imperial and Birmingham should carry out further small lab experiments. Thomson set two of his students, Alan Ware and Stanley Cousins, the task of showing the pinch effect in a toroidal vessel.
But Thomson, convinced that his scheme was workable, kept up the pressure. In May he wrote to Lord Portal, the government’s Controller of Atomic Energy, arguing that he had done all the work he could on his patent and to prove the concept it was time to build a larger experiment than could be contained in a university laboratory. Thomson suggested that it could be built at the new research labs set up by the company Associated Electrical Industries (AEI) at Aldermaston Court. AEI was only too keen to take on the work, and even volunteered to pay for it. But Portal inevitably consulted Cockroft and he insisted that the work remain under Harwell’s control. At a meeting in October to discuss Thomson’s AEI proposal, Cockcroft again quashed the idea of going straight to a large reactor.
It was soon after that meeting that Cockcroft learned about Thonemann’s work at Oxford and the contrast between the two approaches would not have been lost on him. Thomson put his ideas down in a patent from the start, based on a somewhat hazy theoretical understanding, and was using all his high-level connections to move straight to a full-scale reactor. Thonemann, on the other hand, was slowly and methodically testing his ideas in the lab, working out what would work and what wouldn’t. Cockcroft made sure Thonemann had funding and staff.
Interest in fusion was starting to grow. One of Cockcroft’s deputies, H. W. B. Skinner, Harwell’s head of general physics, was called to report to a government atomic energy committee in April 1948 on activities at Imperial, Oxford and Harwell. He pointed out that physicists still had a rather tenuous theoretical understanding of plasmas and was sceptical of Thomson’s proposals for accelerating electrons around the torus, although he was keener on Thonemann’s inductive method. Skinner rightly indentified the key problem of confining the plasma with magnetic fields. ‘It is useless to do much further planning before this doubt is resolved,’ he wrote.
It was not until the following year, 1949, that Thonemann produced a pinched plasma in his copper torus and Ware and Cousins at Imperial achieved a similar feat. The stage was set for an expanded fusion research programme but events outside the world of plasmas and magnetic fields were about to intervene.
On 2nd February, 1950, Klaus Fuchs was arrested and just a month later was convicted of passing atomic secrets to the Soviet Union. Fuchs was born in Germany and became a communist as a student. He fled the Nazis in 1933 and settled in Britain. When the war broke out he was initially interned as a German national but influential professors persuaded the authorities to release Fuchs and he took British citizenship. Peierls recruited Fuchs to Britain’s atomic bomb project and the two of them were soon transferred to the Manhattan Project in the United States. Fuchs said later in his confession that after Germany invaded Russia in June 1941 and Russia became allies with the United States and Britain, he felt that the Soviet Union had a right to know what the western powers were doing in secret. Around that time, Soviet military intelligence made contact with Fuchs.
At Los Alamos, Fuchs helped work out how to implode the fission fuel in the first plutonium bomb and made numerous other contributions. He was present at the Trinity test site for the first atomic explosion in July 1945. The following year he returned to Britain to join the new AERE laboratory at Harwell. But later in 1946, US cryptanalysts, following years of effort, cracked the code used by Soviet intelligence agencies and discovered that spies had infiltrated the Manhattan Project. Decoding was still slow and difficult but the analysts eventually deciphered messages suggesting that one agent for the Soviets was a British nuclear scientist. It wasn’t until 1949 that suspicion fell on Fuchs and after being challenged by an MI5 officer he made a full confession, describing in detail how he had passed details of the Manhattan Project to his Soviet handlers since 1942. Fuchs was imprisoned until 1959 and then emigrated to East Germany.
The Fuchs case caused a near hysterical clamping down of security at British atomic facilities. Fuchs had known all about the fusion research going on in Britain and this caused great concern for Cockcroft. Although the goal of the fusion research was the controlled release of energy for power generation, not bombs, a fusion reactor would in the process produce copious amounts of neutrons, and neutrons could be used to convert the non-fissile but abundant isotope uranium-238 into plutonium. Plutonium is the key to one type of atomic bomb and at that time it was in very short supply.
Until then, the fusion research at Oxford and Imperial had been carried out openly and the researchers had published their results in academic journals. Suddenly, Thonemann and his colleagues found themselves being questioned about the implications of their work. They argued vociferously against classifying their research but to no avail: Cockcroft put strict limits on what could be published. Anything that described work on high-temperature plasmas was automatically classified, as was anything that suggested they were working towards a thermonuclear reactor.
The need for greater secrecy made it simpler for Cockcroft to do what he already knew was inevitable: it was time for fusion research to step up a gear and move to a scale that was too big for university laboratories. He decided to move Thonemann and his team from Oxford to Harwell towards the end of 1950. Six months later, Ware and a colleague moved from Imperial to AEI at Aldermaston Court to continue their work.
Thonemann moved into Hangar 7 at the former airbase. In the hangar, fusion experiments were set up inside a cage of wire mesh, soon dubbed the Birdcage, which protected them from stray electric fields of the other large machines nearby. The team grew quickly with new recruits from the universities and other government labs. One of their first tasks was to build an electrical power supply and then test it on various tori made of copper and quartz, the latter so that the researchers could see the plasma.
In the early days in Hangar 7 it became clear that there were some serious problems with Thonemann’s scheme. With the high frequency alternating current they were using to drive the plasma current around the torus there are many moments when the current stops to change direction. Ions were drifting during those brief moments and hitting the walls of the torus, causing the plasma to lose heat. The new power supply that they had built provided some improvement, but as they ramped up the power the problems got worse. After several years trying to work around the problem, one of the team’s new recruits came up with a radical proposal. Bob Carruthers had worked during the war on radar and had helped develop pulsed power supplies, ones that ramp up the current from zero to a high value in a pulse going only in one direction. Such a pulse fed through the electromagnet that linked into the torus would produce just a single burst of pinch effect, instead of the rapid beats from an alternating current, but it was worth a try.
Carruthers and a few others borrowed some components from another experiment in Hangar 7, set up a bank of capacitors – short-term stores of electric charge that would provide the current – and cobbled together a small torus by welding together two glass U-bends. Despite the Heath-Robinson nature of the experiment, the results were astonishing: they produced pinched plasmas that lasted just one ten-thousandth of a second, but the plasmas were much better contained than those produced by alternating currents. Soon the whole team was focused on pulsed plasmas and such was the improvement that in January 1954 experiments with alternating currents were abandoned altogether.
Work began to scale up. The researchers built a series of larger tori, Mark I to Mark IV, that were 1m across, and these produced ever greater plasma currents. There was still a major fly in the ointment, however. When working with a glass torus, Carruthers and a colleague took pictures of the glowing plasma current and found that it wasn’t forming a steady ring around the centre of the torus but was wriggling around the doughnut like a meandering river. This was the first time fusion scientists had encountered what they now call instabilities, in this case a kink instability. Over the following decades, as currents and power levels increased, physicists would uncover a whole zoo of instabilities and had to learn how to suppress each species. Back in the mid 1950s, they were perplexed.
It turned out that kink instabilities are a natural consequence of the pinch effect. If you think of a plasma current in a straight line, the magnetic field that it induces is like a series of rings around the current evenly spaced along it. Any slight kink in the current causes the rings on the outside of the curve to be spaced more widely apart and those on the inside more closely together. Magnetic field lines more closely packed together means a stronger magnetic field, so the force on the current that creates the pinch is unbalanced, pushing to accentuate the kink. What was needed was some sort of restoring force, pushing the kink back into line.
While the team puzzled over this, the success of the pulse transformer technique was leading to pressure for a next step to an even larger machine, one that could actually produce the temperatures necessary for fusion. Thonemann did the calculations and estimated that they would need a metal torus 3m across with the tube itself 1m in diameter. Cockcroft took the proposal to his bosses at the newly created UK Atomic Energy Authority (UKAEA) late in 1954 and they unanimously approved it. They budgeted £200,000 for the project, with the reactor itself costing £127,000.
Hangar 7 became a whirlwind of activity. Thonemann and Harwell theorists continued to hammer out the details of the design, which was completed in the spring of 1956 and a contract was then signed with the company Metropolitan-Vickers to build the machine. Nothing this big had ever been built for a fusion experiment before. The pulse transformer, the biggest that had ever been built in Britain, weighed 150 tonnes. At one point there was concern over whether Vickers would be able to get enough of the particular grade of high-quality steel needed for the transformer. Luckily, a strike in the US electrical industry meant large quantities suddenly came onto the market. Others at Harwell worked feverishly on new measuring techniques so that when the machine was working they could accurately assess the temperature of the plasma, the size of the current and the density of electrons in the plasma. In July 1955, the project was given the codename ZETA, for Zero Energy Thermonuclear Assembly – ‘zero energy’ because they did not expect it to produce surplus power.
The problem of kink instability was still a headache. Until, that was, another new recruit from the Clarendon, Roy Bickerton, suggested applying another magnetic field to the plasma, one going round the torus, parallel to the plasma current. Moving charged particles stick to magnetic field lines, spiralling round them in a corkscrew path. The field lines also have tension to them, like a rubber band, so when a kink starts to develop, this so-called toroidal field gets stretched and starts to pull the plasma back into line. Fortuitously, that correcting pull is stronger than the pinch-induced tendency to accentuate kinks once they start.
Producing a toroidal field was relatively easy: it just meant winding a wire around the torus and passing a current through it. Bickerton built a new glass torus and tested various types of winding and current values, and found that he could suppress kinks over a wide range of conditions. In 1956, the difficult decision was made to add such windings to the design of ZETA, which added significantly to its complexity and cost. Nevertheless, in August 1957 ZETA was finished, on time and on budget, ready to fuse some plasma.
Despite the scale of the project they were now working on, Hangar 7 still had the clubby atmosphere of a university physics department. Most of the scientists came from universities such as Oxford and Cambridge or from government labs where many of them had worked together during the war. Pipe-smoking was de rigueur and, as was the uniform of the day, scientists wore white lab coats and engineers brown ones. When the pressure was really on, Cockcroft, who lived on the Harwell site, would sometimes come to the hangar in the evenings with a crate of beer to give the team some light relief.
The team working on ZETA was also relatively cut off from the outside world. The scientists didn’t like it but secrecy was strictly enforced, so they couldn’t publish papers and get recognition for their work, they couldn’t give talks about fusion at conferences and they couldn’t even discuss their work with fellow scientists, family or friends. This suited Cockcroft just fine because he believed Britain had a lead in fusion technology and he wanted to keep it that way.
Britain had something to prove on the nuclear front. The Manhattan Project during the war had been a genuine collaboration between the United States, Britain and Canada. But in 1946, the US Congress passed the McMahon Act which prevented foreigners having access to American nuclear secrets and the collaboration ended. The British government then had to take stock: should it take on the vast expense of developing its own nuclear weapons or just leave the nuclear arms race to the Americans? Arguments raged behind closed government doors in London. In the end, the nuclear enthusiasts won, in part because of a desire for national prestige, but also because of the expected industrial importance of atomic energy. That decision quickly led to the creation of the Atomic Energy Research Establishment at Harwell and the Atomic Weapons Research Establishment at Aldermaston.
Britain exploded its first nuclear weapon in 1952, the third nation to do so. It also switched on the first nuclear power station – Calder Hall – to supply commercial quantities of power to the grid in 1956. Cockcroft hoped that his team in Hangar 7 would pull off a coup in nuclear fusion too. That opinion was reinforced in 1956 when he visited nuclear research facilities in the US. Although what he was allowed to see was restricted, he got the impression that the US was spending a lot on fusion but had yet to make much progress.
That same year, Harwell got a valuable, but not necessarily complete, insight into what Russian fusion researchers were doing. During April an official Russian delegation, led by Soviet premier Nikita Khrushchev, visited Britain. Among the party was Igor Kurchatov, the USSR’s leading nuclear scientist and father of the Soviet A-bomb and H-bomb. His laboratory, the Institute of Atomic Energy in Moscow, was the home of Russia’s growing fusion programme. Kurchatov contacted Cockcroft and asked if he could visit Harwell and deliver a scientific lecture. Staff from all departments at Harwell crowded into the lecture theatre to hear Kurchatov speak, along with colleagues from AEI and the atomic weapons lab at Aldermaston. Placed on every seat was a printed copy of the lecture, in Russian and English.
The Soviets visit Harwell, April 1956. Igor Kurchatov is on the far right and Nikita Khrushchev at the front, left of centre.
(Courtesy of UK Nuclear Decommissioning Authority)
Entitled On the Possibility of Producing Thermonuclear Reactions in a Gas Discharge, Kurchatov’s speech seemed daringly open to an audience forbidden from discussing their work. Although he didn’t reveal any details of exactly what Soviet scientists were working on, he did discuss the complexity of the problem and how hard it was to draw firm conclusions. In particular, he described the difficulty of determining whether or not the neutrons produced by a plasma were really the result of thermonuclear reactions – an issue that would soon come to haunt the Harwell researchers. Russian researchers in 1952, Kurchatov said, had obtained neutrons from deuterium pinched in a straight tube, but after some investigations found they had properties that were inconsistent with their coming from thermonuclear reactions.
Both Cockcroft and Thonemann suspected the lecture was a fishing expedition: Kurchatov wanted to find out – from the questions asked after his talk – what progress the British scientists had made. But Cockcroft was ready for that. He had given all the scientists attending a list of topics they were not allowed to discuss during the question and answer session. Thonemann came away from the talk with the (incorrect) conclusion that the Russians were not yet experimenting with plasma in a torus. Cockcroft, however, realised that they wouldn’t take long to catch up, judging by how quickly they developed nuclear bombs after the end of the war. He set about speeding up Britain’s fusion research.
Meanwhile, cracks were beginning to show in the secrecy surrounding fusion research. In December 1953, US president Dwight D. Eisenhower made a speech to the UN General Assembly which later became known as the ‘Atoms for Peace’ speech. In it Eisenhower pledged to make nuclear technology that didn’t have military uses freely available for the benefit of mankind. Whether his intention was quite as altruistic as it sounded historians are still debating, but it had profound implications for those toiling away in secret government labs on nuclear projects.
One result, a few years later, was the creation of the International Atomic Energy Agency, the UN nuclear watchdog which oversees civil nuclear power and tries to ensure material is not diverted into weapons production. Another outcome of the speech was the International Conference on the Peaceful Uses of Atomic Energy, which took place in Geneva in August 1955. For those who had laboured in secret through the war and the decade that followed it, the Geneva conference was an astonishing event. Previously they couldn’t even tell their own families what they were doing; now they could show it to the world and hobnob with fellow researchers from other nations whose work they knew nothing about. Scientists from western countries even exchanged notes with their opposite numbers behind the Iron Curtain.
The focus of the 1955 conference was nuclear fission, which would shortly be impacting directly on people’s lives as the first power stations came online. Fusion scientists didn’t get to join in this spirit of openness: the possibility of using a fusion reactor to produce plutonium for bombs meant that governments weren’t yet ready to let that genie out of the bottle. A passing remark at the Geneva meeting, however, did set the ball rolling for fusion declassification. The Indian physicist Homi Bhabha, president of the conference, said in his opening address:
I venture to predict that a method will be found for liberating fusion energy in a controlled manner within the next two decades. When that happens the energy problem of the world will truly have been solved forever for the fuel will be as plentiful as the heavy hydrogen in the oceans.
That teaser led to feverish press speculation about the existence of secret fusion research projects. Soon a number of governments, including the US and British, admitted the existence of their programmes, but few other details were released. That lack of information only fuelled the media’s desire to find out more.
Soon after Cockcroft’s trip to America in 1956, the US Atomic Energy Commission (AEC) formally suggested collaboration between the two countries’ fusion programmes. That didn’t lead to a flood of information back and forth across the Atlantic but the scientists did begin to visit each other’s labs and the two sides agreed to a common secrecy policy: neither would publish anything without the other’s approval. The British scientists were still agitating for more openness but the AEC took a firm line. Researchers were permitted to publish a few papers around this time which described the science of fusion in general terms but nothing about specific machines or future plans.
On 12th August, 1957, ZETA was fired up for the first time. For the first few days the researchers used plain hydrogen while they worked out the reactor’s optimum operating conditions and then switched to using deuterium. On 30th August, their detectors started to register the production of neutrons, the tell-tale signal of fusion reactions taking place. It wasn’t long before they were getting a million neutrons per pulse and Harwell was soon buzzing with excitement. Could they really have struck oil so quickly? But they didn’t want to get carried away and be fooled by a false neutron signal as the Russians had been five years earlier. It was impossible to tell at that time whether the neutrons were from thermonuclear reactions. The team didn’t even have a way of accurately measuring the plasma temperature to know if it was hot enough for fusion.
To produce power from fusion, it’s vital to have the plasma uniformly heated to a sufficiently high temperature for fusion reactions to start happening all through the core of the plasma. What the Russians saw, and the team in Hangar 7 feared being fooled by, was some other effect that wouldn’t make viable amounts of energy, such as the plasma touching the torus wall and kicking off neutrons, or some flaw in the magnetic field that caused a small part of the plasma to be accelerated to high speeds but leaving the rest too cool. So the researchers opted for caution and avoided making any public statements until they were sure they were seeing thermonuclear neutrons.
Cockcroft, however, let the excitement get the better of him. On 5th September, a Thursday, he wrote to Edwin Plowden, chairman of the Atomic Energy Authority, telling him about the neutrons but saying he wasn’t yet 100% sure they were thermonuclear. Plowden inferred from this that the probability was high, though not quite 100%, and he wrote to the Prime Minister, Harold Macmillan, the following Monday to tell him so. But by then, Macmillan could read about it in the newspapers for himself.
Britain goes for the big time: the ZETA reactor at Harwell.
(Courtesy of UK Nuclear Decommissioning Authority)
Somehow, news of the existence of ZETA had leaked out to the press along with the suggestion that something important was afoot. The day after Cockcroft wrote to Plowden there was a session on thermonuclear fusion at the annual meeting of the British Association for the Advancement of Science in Dublin and reporters were there in force expecting a big announcement of results from ZETA. Even the Irish prime minister Éamon de Valera was in the audience. Cockcroft had lined up two speakers for the session: George Thomson and John Lawson, a theorist who had worked at Harwell but had recently moved on to other things. Cockcroft had given Lawson strict instructions not to give anything away. At a press conference following the talks the pair were grilled by the press about what was happening at Harwell. Many of the questions they declined to answer because of the secrecy rules they were bound to, which annoyed the eager reporters. Thomson did concede that he thought at least fifteen years would be needed to build a power-producing reactor.
The newspapers were full of stories about fusion the next day, many extrapolating wildly about the prospects of fusion power. The Financial Times reported that ZETA had been producing neutrons since mid August, though this hadn’t been mentioned at the Dublin meeting. Others said that ZETA had reached temperatures of 2 million °C. The more fanciful reports distressed the researchers at Harwell and Thonemann argued that they had to make an official statement to set the story straight. Cockcroft agreed but his hands were tied by the agreement with the US; any official statement would have to be approved by them too. A draft press release was drawn up and then cabled to Washington.
The first reaction from the AEC was to say that they would make a parallel announcement at the same time. But after US fusion scientists expressed scepticism at the British results, the head of the AEC, Lewis Strauss, said any announcements should wait until the next Geneva Conference on the Peaceful Use of Atomic Energy, a whole year away. Plowden would have none of it and they agreed to wait at least until the next meeting of Project Sherwood, the codename for American’s fusion programme, in mid October when visiting British scientists could explain their results. Plowden told Strauss he didn’t think he could hold off the British press any longer than that.
The Harwell team knew they needed more evidence, both of the plasma temperature and the thermonuclear nature of the neutrons, but Thonemann and others suspected that the Americans had an ulterior motive: they wanted more time for their own experiments to bear fruit so it would not look like they had been beaten in the fusion race. Harwell had to accept the delay, so the team continued to run ZETA, gather more results and look for firmer evidence.
At the beginning of October, however, two momentous events occurred which would load ZETA with enormous political baggage. On 4th October the Soviet Union launched Sputnik, the world’s first artificial satellite. Less than a week later, Pile 1, a nuclear fission reactor at Windscale in Cumbria, caught fire and spread radiation across the local area. Although the Windscale pile was designed to produce plutonium for nuclear weapons – not generate electricity – the fire and its threat to the population was a major blow to the clean high-tech image of nuclear power, and the public began to question its safety. The Atomic Energy Authority needed something to distract people from the fire. ZETA, with its cleaner and safer form of nuclear power, would do the job perfectly.
On the other side of the Atlantic, the United States was reeling in shock from the launch of Sputnik. The US had always assumed it had an unassailable lead in high technology and that the Soviet Union would forever be playing catch-up. In part this was because the US had captured all Nazi Germany’s top rocket scientists at the end of the war and spirited them back to America. The conquest of space was theirs for the taking, so they thought. While the launch of Sputnik was greeted with wonder by many, for the US military it inspired terror. If the USSR could launch a satellite, it could in theory drop a nuclear weapon from space onto anywhere on US territory – something for which America had no defence. The US needed some new breakthrough to show that it was still a technological powerhouse. Fusion could provide that breakthrough but the press might portray it as a British triumph, further humiliating the US. As a result, the AEC continued to play for time.
Every day that passed was torment for the members of the Harwell team who were desperate to tell the world about their achievement; especially since inquisitive newspapers were publishing ever-more speculative stories about what they thought was going on. This particular period of history, the 1950s, is perhaps unique in that scientists – and especially physicists – were treated as heroes. Although the nuclear weapons dropped on Japan had inflicted awful damage, creating them was a tremendous technical achievement and they had brought the war to a swift end. In the postwar years physicists served up more wonders: rockets, jet planes, televisions and nuclear power. On TV and in films they were portrayed as noble knights in white coats, able to solve almost any problem. In the uncertain world of the Cold War it was good to have such people on your side. And it was into this atmosphere that the Harwell team aimed to launch their reactor, which promised clean and virtually limitless power at very little cost. They were surely unprepared for the impact their announcement would make.
At the Project Sherwood meeting in October it was agreed that both teams would publish papers describing their results and the text of a new press release was hammered out, but the AEC continued to stall, arguing that more evidence was needed. Newspaper articles were beginning to carry claims that Britain was ahead of the US in fusion technology, and that Harwell’s ‘triumph’ was being suppressed because of US-imposed security rules. Questions were asked in Britain’s House of Commons. Before anti-US sentiments got carried away, Strauss agreed to go ahead with publication but first sent a delegation of US fusion scientists to Harwell to see ZETA for themselves. Following their visit in December, the US scientists still argued for more delay but it was agreed that both sides would publish scientific reports in the journal Nature early in the following year.
So it was that the press was invited to Harwell on 23rd January, 1958 and the researchers opened up Hangar 7 to show ZETA to the world. Because of their nagging doubts about the origin of the neutrons, the researchers didn’t put anything into the papers in Nature about how the neutrons might have been produced. The press release given out to reporters, however, suggested the neutrons were probably thermonuclear. Scenting that this was a key issue, reporters repeatedly asked the Harwell scientists about the neutrons but only got evasive answers. At the press conference that day Cockcroft was similarly bombarded with questions and eventually admitted that he was 90% certain that at least some of the neutrons were thermonuclear.
The next day, ZETA was the top story across the globe and Cockcroft’s 90% certainty was reported in every story. ‘The Mighty ZETA,’ trumpeted the Daily Mail’s front page. The News Chronicle declared, ‘Britain Unveils Her Sun.’ Many described ZETA as ‘Britain’s Sputnik.’ The Hangar 7 team were suddenly celebrities, with their pictures on the front pages alongside brief pen portraits, often focusing on their comparative youth. ‘These are the names which will be linked with the controlling of the H-bomb in the same way that Rutherford, Cockcroft, Fermi and others are bracketed with atomic energy,’ said the Mail, continuing, ‘Tall, dark and bespectacled, Thonemann has dedicated all his working years to tapping nature’s biggest bank of energy.’
The Italian press gave ZETA more coverage than it had Sputnik. French papers were some of the few that pondered the 10% possibility that Cockcroft was wrong. The New York Times reported the view of one researcher that fusion reactors could power spacecraft. Newsreel cameras recorded the open day at Hangar 7 and a hastily produced TV programme explained the breakthrough to Britain’s viewing public. Despite the simultaneous papers published in Nature, few reports mentioned the work being done in the US – ZETA was a very British triumph.
ZETA and its creators continued to be feted in the months that followed, but researchers were still concerned about whether they really were seeing what they thought they were seeing. Other pinch-based machines were also starting to produce neutrons, including a torus built by Ware at AEI and America’s whimsically named Perhapsatron. But US scientists continued to be sceptical about ZETA’s results. They just didn’t believe that it could be getting up to the temperature of 5 million °C that the Harwell team was claiming, and any less than that would not be hot enough to cause thermonuclear reactions.
The questionable neutrons were about to come under close scrutiny. At the ZETA press conference in January, Basil Rose, a nuclear physicist from another section at Harwell, managed to get in to find out what all the fuss was about. Rose was in charge of Harwell’s cyclotron, a particle accelerator that shared Hangar 7 with ZETA. He quickly realised that finding out more about the neutrons was crucial, and his experiment had a detector that could measure accurately the energy and direction of neutrons. Frustratingly, he had just leant the detector, called a diffusion cloud chamber, to scientists at University College London but they hadn’t started using it yet and Rose persuaded them to ship it back to Harwell.
Peter Thonemann (left) receives an achievement award from Sir Edward Hutton (right). Sir John Cockcroft looks on.
(Courtesy of UK Nuclear Decommissioning Authority)
If ZETA’s plasma was at thermonuclear temperatures, then the deuterons would be bouncing around in random directions, colliding and fusing. The neutrons they emitted would therefore be flying out in all directions equally and with similar energies. When Rose hooked up his cloud chamber to ZETA and studied the neutrons, this was not what he found. The neutrons were mostly emitted in line with the axis of the plasma current and more strongly in one direction than the other. To prove the point, Rose got the team to run ZETA ‘backwards,’ with the plasma current flowing in the opposite direction to normal. Sure enough, the preferential direction of the neutrons was reversed. The conclusion: ZETA’s neutrons were definitely not created by thermonuclear fusion.
In mid May the team announced these results at a press conference in London and a month later Nature published the details. Press reaction was sober and relatively restrained. Perhaps newspapers were embarrassed by their own wild extrapolations a few months previously. The Manchester Guardian mused over whether the obsession with secrecy was to blame:
In a huge research project like that revolving around ZETA the day-to-day rubbing of shoulders with scientists of other specialities is the best safeguard of sound analysis and interpretation … So it will inevitably be asked whether things might not have gone differently if the members of the ZETA team had been allowed to talk freely and informally to other scientists.
Fusion scientists always maintain that ZETA was a success. It was, after all, the first machine to achieve a large, stable pinched plasma at high temperature. Scientists at Harwell continued to use it up until 1968, garnering much useful information. But in the public mind ZETA will always be remembered as a failure: British scientific hubris dashed upon the rocks of a problem more complex than foreseen. It’s true that the Harwell team were impetuous in going public with their results when they had very unreliable information about the temperature of the plasma and the nature of the neutrons, but they are not solely to blame for the mess that ensued. The need for a success after the shock of Sputnik and the Windscale fire meant that Harwell was under enormous political pressure to produce the goods.
Britain was also hungry for something to restore its national pride. Although it had emerged as one of the victorious powers at the end of the war, it was struggling to hold onto its seat at the top table. Britain had had to scramble to catch up with the superpowers in atomic power and weapons. Its economy was in tatters (rationing had only ended in 1954) and its empire, which had once spanned the globe, was being rapidly dismantled. In 1956, Britain’s adventure with the French to seize control of the Suez Canal following nationalisation by Egypt was humiliatingly squashed by a disapproving US. With so little to celebrate, the British public embraced the scientists who had given them a world lead in this wonderful new technology, and felt betrayed when it was taken from them again.
A few months after the climb-down over ZETA, British fusion scientists, along with colleagues from all over the world, gathered in Geneva for the second ‘Atoms for Peace’ conference which this time was focused on fusion. Just before it began all sides declared they would declassify their fusion research. The US and Soviet fusion programmes vied to outdo each other in displays of research activity. The US stand cost millions to put together and contained four real fusion machines. The Soviets put on a similar show. With the veil of secrecy lifted, Harwell’s researchers could see that they wouldn’t be able to keep up for long.
In 1960 Britain’s fusion researchers began moving to a new purpose-built laboratory at Culham, another former airfield some 10 kilometres from Harwell. The plan for a bigger and better ZETA 2 was abandoned, however, and the emphasis shifted to smaller machines to gain a better understanding of how plasma works. Thonemann left the programme and, in a sense, the ‘heroic age’ of British fusion was at an end. It would be some decades before another big fusion reactor was built in the UK but, elsewhere, things were just hotting up.