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 o
f 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.
Piece of the Sun : The Quest for Fusion Energy (9781468310412) Page 4