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 o
f 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.
Piece of the Sun : The Quest for Fusion Energy (9781468310412) Page 5