WIND BACK THE CLOCK AGAIN, THIS TIME TO 1949 AND Sakhalin, a cold, remote and inhospitable island off Russia’s far eastern coast. Soviet forces captured a large part of the island from the Japanese in the closing days of World War II and for the soldiers stationed there after the war it must have been a desolate posting. But to Red Army sergeant Oleg Aleksandrovich Lavrentyev it offered what he needed: time to study. Lavrentyev had not finished school when he joined the Red Army at the age of 18, but what he had read about the fission of uranium and the possibility of a nuclear chain reaction excited him and he was determined to study physics. A veteran of battles against the Germans in the Baltic States during 1944 and 1945, he was posted to the east after the war. Working as a radiotelegraph operator, he had plenty of time to read. He got hold of physics textbooks, monographs and even subscribed to the academic journal Soviet Physics-Uspekhi. He would give reports and lectures to his officers on scientific and technical subjects. In 1949 he completed his school exams, having covered three years’ study in twelve months.
In August that year, the Soviet Union exploded its first atomic bomb, much to the pride of its citizens and to the surprise of the United States. A nuclear arms race had begun and the following January US president Harry Truman announced to Congress that the country’s nuclear scientists would accelerate efforts to develop a more powerful fusion weapon, the H-bomb. Lavrentyev realised that it was time to act. He wrote a short letter to Comrade Stalin explaining that he had worked out how to make a hydrogen bomb and also a way to control fusion reactions to generate electricity for industry. He waited. Nothing happened.
A few months later Lavrentyev wrote another letter, this time addressed to the central committee of the Communist Party of the Soviet Union. Then things started happening very fast. An officer came to interrogate him. He was then put in a guarded room and given two weeks to put his ideas down in writing. These were sent to Moscow via the Communist Party’s secret courier service on 29th July, 1950. He was then demobilised and sent off to Moscow. Stopping en route in Yuzhno-Sakhalinsk, the island’s capital, Lavrentyev was warmly welcomed by the regional Communist Party committee. Following his arrival in Moscow on 8th August, he sat the entrance exam and was enrolled as a student at Moscow State University. He had gone, in just a few weeks, from lowly radio operator in the far east to a student at the Soviet Union’s elite research university in Moscow.
In September he was summoned to see I. D. Serbin, head of the Communist Party’s department of heavy engineering industry. Serbin again asked Lavrentyev to write down his ideas about thermonuclear electricity generation, which he did in a classified security-protected room. Lavrentyev settled into the life of a Moscow university student but one evening the following January, as he arrived back at his room in the student hostel, he was told to ring a certain phone number. The person who answered was V. A. Makhnev, the Minister of Measuring Instrument Industry (the codename for the Soviet nuclear industry). Makhnev told him to come immediately to his office in the Kremlin. A man met Lavrentyev at the security gate and walked with him to Makhnev’s office. The minister then introduced Lavrentyev to his guide, Andrei Sakharov, father of the Russian H-bomb and, much later, a prominent Soviet dissident.
The previous summer, while working at Arzamas-16, one of the Soviet Union’s secret nuclear cities, Sakharov had been asked to look at the paper Lavrentyev had written in Sakhalin. In his report, Sakharov dismissed Lavrentyev’s idea for an H-bomb with a single sentence. Lavrentyev had proposed fusing hydrogen and lithium but Sakharov pointed out that this combination was not reactive enough to work in an atomic explosive (something that Lavrentyev would not have known from his readings on Sakhalin). The second suggestion, of using controlled fusion reactions to generate electricity, Sakharov found much more interesting. ‘I believe the author has formulated an extremely important and not necessarily hopeless problem,’ he wrote.
Lavrentyev’s scheme used electric fields to confine a plasma so that it would fuse. He described having two concentric spheres: the outer one would act as an ion source while the inner one, made of a metallic grid, would be put at a large negative voltage with respect to the outer one. This creates an electric field that will accelerate deuterium ions from the outer sphere towards the centre, where they form a hot plasma and fuse. The field also prevents the ions from escaping from the sphere. Sakharov pointed out a couple of reasons why the device, known as an electromagnetic trap, wouldn’t work. First, while the electric field would indeed propel ions towards the centre of the sphere, it works in the opposite direction for electrons, so they would be ejected from the device. As a result the plasma in the centre would be dominated by a positive electric charge which would stop the nuclei getting close enough together to fuse. Sakharov also thought that the low density of the plasma would mean that not enough collisions would take place. He didn’t exclude the possibility that improvements to the design could overcome these problems, but he concluded by saying, ‘It is necessary at this point not to overlook the creative initiative of the author.’ Lavrentyev, he thought, was worth cultivating.
Makhanev told the two of them that they would need to meet the chairman of the Special Committee on atomic weapons. Some days later they were again ushered into the Kremlin and to the chairman’s office. Sitting behind the desk was Lavrentiy Beria, the most feared man in the Soviet Union. Beria was in charge of Soviet security and the notorious secret police of the NKVD, the forerunner of the KGB. During the war he coordinated anti-partisan operations and ordered the execution of thousands of deserters and ‘suspected malingerers.’ He hugely expanded the Gulag slave labour camps and ordered the Katyn massacre of 22,000 Polish army and police officers. After the war he was elevated to deputy premier and took personal control of the crash programme to develop nuclear weapons. With Lavrentyev and Sakharov he was polite and formal, asking them about their families, including relatives who were in prison. Nothing nuclear was discussed. Lavrentyev got the impression that Beria was sizing them up, assessing what sort of people they were. As they left, Sakharov said to Lavrentyev that from then on everything would go smoothly and they would work together.
For Lavrentyev things did go smoothly. One evening, much to the alarm of his fellow students, darkly dressed men came and took him and his belongings away in a black limousine. His classmates feared the worst but he was back in lectures the next day having been installed in his own furnished room near the city centre. He was also given a generous scholarship, delivery of any scientific literature he asked for, and the university’s professors of physics, chemistry and mathematics tutored him personally. Soon after his interview with Beria he was visited by another official who took him to a government building where, after many security checks, he was introduced to two generals and a civilian with a copious dark beard. This time the conversation was much more technical but when it moved on to his H-bomb design Lavrentyev was unsure whether he should be talking about it. He told the three men that he had recently been to see Beria and the talk veered towards more practical matters. The bearded civilian was Igor Kurchatov, the head of the Soviet nuclear weapons programme. During the war, Kurchatov had vowed not to cut off his beard until his programme to develop a nuclear bomb for the Soviet Union had succeeded. After the first bomb was tested in 1949, Kurchatov decided to keep the beard and he often wore it trimmed into eccentric shapes. To his loyal staff, he was known simply as ‘The Beard.’
In May 1951 Lavrentyev was granted clearance to work at the Laboratory of Measuring Instruments (LIPAN), the Soviet Union’s secret nuclear research laboratory, alongside his university studies. When he arrived there, he found that a high-powered team was already working on a fusion device. To his dismay, it was not his electromagnetic trap but a magnetic design devised by Sakharov and his colleague and mentor Igor Tamm.
Despite now being at the very heart of the Soviet Union’s nuclear research effort, a place he could only have dreamed about a year earlier, Lavrentyev never fitted in there. His connection with the hated Beria made him an object of suspicion, while at the university his privileged status set him apart from the other students.
After the death of Stalin in March 1953, there was a brief struggle for power among members of the Politburo. Most of them feared and distrusted Beria and in June they had him arrested, moving a tank division and a rifle division into the city to prevent security forces loyal to Beria from rescuing him. Six months later Beria and six accomplices were accused of being in the pay of foreign intelligence agencies and of wanting to restore capitalism. They were executed days later. With his sponsor gone, Lavrentyev’s privileges evaporated and he was barred from LIPAN. Nevertheless, he finished his physics degree and went on to complete a doctorate, even without access to a laboratory. He got a job at the Physical-Technical Institute in Kharkov, Ukraine, where he continued, without success, to work on plasma confinement with an electromagnetic trap. Sakharov stayed in contact with Lavrentyev and always insisted that it was Lavrentyev’s paper written on Sakhalin that provided the spark for Russia’s controlled fusion research.
When Sakharov received that paper in the summer of 1950, he had already been thinking about controlled nuclear fusion but couldn’t figure out a way to achieve it. Although he didn’t think Lavrentyev’s electromagnetic trap would work, it did get him wondering about a magnetic trap instead. He discussed the problem with Igor Tamm. Although the two of them were working frantically on the H-bomb, they took some time out to consider the problem of controlled fusion. Following the same thought processes that Lyman Spitzer would work through in a few months’ time, they realised that charged particles could be held in place by a magnetic field because they would be forced to move in tight circles around the magnetic field lines. And by making the field lines go around in a circle inside a toroidal tube they could avoid the particles escaping off the end of the field lines – the lines would have no end. Like Spitzer, Sakharov and Tamm realised that the fact that the magnetic field was stronger on the inside of the curve than the outside would push particles towards the outer wall. But while Spitzer got around this by twisting the tube into a figure-of-8, the Russians instead twisted the magnetic field. They added a second magnetic field that did a vertical loop around the inside of the tube, and the combination of the two fields led to field lines that wound around the torus in a helical pattern. So a particle travelling around the ring would move in a tight spiral around a field line, and that field line would curve down the outer wall, say, at an angle and then across the bottom and up the inner wall. Hence the tendency of particles to drift towards the outer wall would be cancelled out by visits close to the inner wall.
In a tokamak, plasma particles still gyrate around the magnetic field lines but the lines also follow a helical path around the torus.
(Courtesy of EFDA JET)
Sakharov and Tamm enlisted the help of some theoreticians from the Lebedev Physics Institute in Moscow to flesh out the idea and then presented it to Kurchatov. Their boss was enthusiastic about the idea and began assembling a group of physicists to work on it at LIPAN. To lead the scientific effort, Kurchatov appointed one of his most able deputies: Lev Artsimovich. On 5th May, 1951 – as Spitzer was drawing up plans for his stellarator and a few months after Thonemann moved from Oxford to Harwell – the USSR Council of Ministers, with the approval of Stalin, passed a resolution launching Russia’s fusion research programme.
The researchers at LIPAN first turned their attention to how to produce a current around the torus. Starting off with a high-frequency alternating current, they soon switched to a unidirectional current pulse. In fact they found that the current moving around the torus created such a strong pinch effect that they began to wonder if the toroidal magnetic field that Sakharov had started out with was needed at all. So they began experimenting with straightforward pinch machines akin to Thonemann’s Mark I to Mark IV and Tuck’s Perhapsatron. But the researchers were having trouble getting higher temperatures from the pinch machines and although they produced some neutrons in July 1952 these turned out to be spuriously produced by instabilities.
As a result, their approach swung back towards using a combination of toroidal and poloidal magnetic fields, just as Sakharov had suggested. In 1955 they built their first tokamak-like device, although they hadn’t yet given it that name. However, like its predecessors, the new machine was not producing high temperatures. They didn’t realise it until later, but the problem was that the Russians were making their reactor vessels out of ceramics. Atoms knocked out of the vessel’s walls were polluting the plasma and radiating energy out of the plasma as UV light, so preventing it from getting hot. The Russian effort was not making progress and they were running out of ideas.
Partly as a way of injecting some new blood into his fusion teams, Kurchatov decided in 1955 that he needed to get the field declassified. He started by organising a conference of scientists from all over the Soviet Union and revealed LIPAN’s work on controlled fusion. Delegates, who didn’t even know that the programme existed, were stunned by the scale and quantity of the work already done. The following April, Kurchatov surprised western scientists by describing Soviet fusion research in his famous speech at Harwell. Now that the ice was broken, discreet connections between East and West were made at scientific conferences, even though the topic had not been officially declassified. In the autumn of 1956, for example, Artsimovich and a colleague attended an astrophysics conference in Sweden where they made the acquaintance of Princeton’s Lyman Spitzer and Harwell’s Sebastian Pease.
With their own research stalled, the Russian team was astonished to read in British newspapers in January 1958 about the success of ZETA. From the scant details and photographs in the media reports, theorists at LIPAN scrambled to figure out what sort of a machine ZETA was. From the pictures, they realised that it had to be a compact torus – more like a doughnut than a hula-hoop – but the only way they could see to contain the plasma in such a vessel was with tokamak-like magnetic fields. Once the issue of Nature arrived with the articles by the ZETA team and their US colleagues, the Russians realised they were wrong. Although the ZETA results turned out to be false, the theoretical work that the LIPAN team did trying to understand it helped them in the plans for the first large tokamak, T-3.
In Russia just as in the West, nuclear authorities decided to completely declassify Soviet fusion research just before the 1958 Geneva conference, so the Russian researchers arrived there with all their papers recently published in hefty four-volume sets, ready to share with their new-found Western colleagues. Spitzer’s stellarators were the surprise of the conference for the Russians. It was an approach, with its steady-state operation and long figure-of-8 or racetrack-shaped vessels, that had not occurred to them. Kurchatov was so taken with the idea that he ordered construction of the T-3 to stop so that a stellarator could be built in Moscow instead. LIPAN theorists compared the two approaches and argued forcefully in favour of the tokamak. Kurchatov relented and they left the stellarator to the Americans.
Russian fusion research was still having problems, but at least they were problems that could now be shared with like-minded colleagues in the West. The challenges they all faced included short confinement times, low plasma temperatures and Bohm diffusion. Following the ZETA debacle, researchers everywhere were going back to basic plasma physics and trying to understand better how plasmas work. The Soviet authorities considered the pursuit of fusion a high priority so there was no shortage of money for experiments at LIPAN. Because their understanding of plasma was poor and their instruments were rudimentary, the team of mostly young researchers there would simply build one device after another with slight variations of design to try out different ideas in the hope that one might show some improvement in performance.
One young researcher, Vladimir Mukhovatov, was trying to stop oxygen, absorbed in the walls of his device, from leaking into the plasma and contaminating it. He decided the best material to coat the inside surface would be gold. He knew that such an experiment would be hugely expensive but nonetheless he went to his team leader, Nutan Yavlinskii, and explained the idea. A week later a 2 kilogram lump of gold was sitting on his desk. Mukhovatov coated the inside of the device with gold, but for some reason it only made its performance worse: he couldn’t get a good density and it was plagued with instabilities. When he looked inside he found flakes of gold falling off the walls. He removed the gold but it was now mixed with all sorts of other material, so Mukhovatov sent the mixture off to a special workshop in the Urals to have the gold extracted. Months later he phoned the workshop and was told that they had only found traces of gold in the material he had sent, nothing like the 2 kilograms he was expecting – someone there had seen a golden opportunity and spirited the metal away. Mukhovatov nervously went to Yavlinskii to confess his mistake but his boss dismissed it with a wave of his hand, as if to say there’s more where that came from.
Three years after the Geneva conference came the IAEA’s next meeting of fusion researchers in Salzburg, Austria. It was here that Western researchers got their first blast of Lev Artsimovich. As leader of the Soviet fusion programme Artsimovich was a force of nature: he knew every device and every theory back-to-front and was constantly analysing data, assessing models and throwing out new ideas. He was the heart and soul of the fusion effort at LIPAN. In seminars he sat in a shabby oak armchair which, legend has it, once belonged to the famous quantum theorist Werner Heisenberg but had been ‘liberated’ from the Kaiser Wilhelm Institute in Berlin by Russian soldiers at the end of the war. Artsimovich would listen intently and always had the knack of identifying key points and possible weaknesses straight away. After such talks, young researchers and seasoned veterans alike would cluster around the blackboard and debate the issue vigorously with the chief. To a newcomer, used to the customary stuffy formality of Russian science, it was an exhilarating experience.
At the Salzburg conference, Artsimovich lambasted the optimistic results presented by Dick Post of Livermore, who was reporting long confinement times in a magnetic mirror machine, much longer than that reported by M. S. Ioffe of LIPAN using a similar device. ‘I want to say that Ioffe’s results are in sharp contradiction with the attractive picture of a thermonuclear Eldorado … drawn by Dr Post,’ Artsimovich taunted. It turned out that, because of a mistake in interpreting measurements, Livermore’s results weren’t nearly as good as reported and Artsimovich made sure that everyone’s attention was drawn to this mistake.
Artsimovich and his team continued to work on their tokamak design. Elsewhere, such pinch-based machines had fallen out of favour. Tuck at Los Alamos had lost interest and really only the British were continuing with pinches, although ZETA continued to be plagued by instability. But the Russian tokamak, with its pinch reinforced by a strong longitudinal magnetic field as a backbone, continued to improve. The researchers at LIPAN, now known as the Kurchatov Institute of Atomic Energy following the death of The Beard in 1960, developed new diagnostic techniques to study the plasma and ways of controlling it.
By the time of the next IAEA fusion conference in 1965, held at Culham, Artsimovich had some impressive results to report: plasma temperature of 1 million °C and confinement time between 2 and 4 milliseconds, ten times better than Bohm’s formula predicts. As we heard previously, the Russian results generated a lot of interest at the Culham conference, especially since the tokamak was almost unknown outside the Soviet Union, but Lyman Spitzer and others remained sceptical because of the indirect measurements of temperature and confinement.
Undaunted, the Russians continued to work on their tokamaks and by the time they hosted the IAEA conference at Novosibirsk in Siberia in 1968, their results could not be dismissed so easily. Now Artsimovich was boasting a temperature for the electrons in the T-3 tokamak of 10 million °C and a confinement time of 10 milliseconds, fifty times Bohm’s prediction. The meeting was buzzing with the news. There were other machines that were breaking the Bohm barrier, but it seemed that with a tokamak you could improve its performance further by making it bigger and strengthening its magnetic fields. Here, at last, seemed to be a device that would allow fusion researchers to move forward and create plasmas at thermonuclear temperatures.
There were still doubters, however, principally from Princeton. They argued that the byzantine method the Russians used to estimate the electron temperature left it open to doubt. The Russians derived the overall temperature of the plasma (ions and electrons) by measuring its magnetic properties. The ion temperature is taken from the energy of certain ions that get neutralised and then ejected from the plasma. To get the electron temperature the Kurchatov team subtracted the second measurement from the first. The Princeton researchers argued that so-called runaway electrons, of the sort that gave misleading neutron readings in ZETA and the Perhapsatron, could also be confusing the Russian measurements.
The lack of effective measurement techniques was a problem that plagued the early decades of fusion research. Not knowing what the plasma was doing made it much harder to see how to improve its properties. A few years after the Novosibirsk conference, during a debate on fusion research in the British House of Lords, one peer asked: ‘How do they measure a temperature of 300 million °C?’ The answer offered was: ‘I expect that they use a very long thermometer.’ If you did try to measure such a temperature with a standard mercury-in-glass thermometer, it would have to be around 600 kilometres long. Even if that were possible, the enormous temperature of the plasma would simply melt the glass.
The correct response to the lord’s question would have been, not a very long thermometer but a laser beam. Researchers at Culham had begun five years earlier to try to measure plasma temperature using lasers. Lasers had only been invented a few years previously in 1960 but researchers quickly realised how useful they could be. One of the key aspects of laser light is that its photons all have exactly the same frequency. This is useful because if you shine a laser beam at, say, a plasma of rapidly moving particles, some of the photons will be scattered by collisions with the particles. A photon that collides with a particle moving towards it gets a slight energy boost which increases its frequency. The faster the colliding particle, the greater is the shift in frequency. A photon colliding with a particle moving away from it loses some energy, lowering its frequency. These examples of the Doppler effect can be put to good use: if you fire a laser beam into a plasma and analyse the frequencies of the scattered photons, a small spread of frequencies suggests that the plasma particles were not moving very fast (i.e., had a low temperature) because the shifts in frequency were small; a high temperature plasma, with faster moving particles, would smear out the frequencies across a broader range. So this technique of ‘Doppler broadening’ of the scattered photons can be used to measure the plasma’s temperature.
By 1968, Culham researchers had shown that they could measure plasma temperatures in their pinch machines much more accurately than with the indirect methods used before. This capability would be invaluable to Artsimovich because he would be able to prove the achievements of his tokamak. So it was now that he proposed to Pease that the British should send a team of researchers from Culham to Moscow to settle the question. This was revolutionary. It was the height of the Cold War and some of the technology needed for the experiment could be militarily sensitive. The year 1968 was, however, a unique moment in history. At the start of the year, Czechoslovak leader Alexander Dubček began a process of liberalisation in his Eastern Bloc country, restoring freedom of speech and travel, loosening ties on the media, decentralising the economy and promoting democracy. All across Eastern Europe there was hope that the Soviet Union’s stranglehold on their countries might be loosened. Elsewhere there were similar upheavals: France was brought close to revolution by rioting students; in the US the Civil Rights Act was passed and demonstrators marched against the Vietnam War. Change and opportunity were in the air and this must have spurred on the two physicists to push for this unprecedented project.
Pease had to pull every string available to get approval, working his way up to the management of the UK Atomic Energy Authority and on to the Ministry of Technology and the Foreign Office. His ace card was the fact that the mighty Soviet Union was calling on British expertise to solve its technological problem. In the midst of this negotiation, the Soviet Union decided that the Czech experiment had gone far enough. On 20th August, Russian and other Eastern Bloc tanks rolled into Czechoslovakia, Dubček was removed and the Prague Spring was brought to an abrupt end. The familiar Cold War animosities were resumed and this delayed approval of the Culham scientists’ mission. Finally in December, an official invitation from the Soviet State Committee for Science to visit the Kurchatov Institute was received, authorised by Soviet premier Leonid Brezhnev. The necessary visas were delivered to the homes of team members at midnight, just hours before their flight, by black-clad motorcycle couriers from the Foreign Office.
For the British fusion scientists – Nicol Peacock, Michael Forrest, Peter Wilcock, and Derek Robinson – who were used to the familiar comforts of the Oxfordshire countryside around Culham, 1960s Soviet Moscow was an alien world. Driving into the city in a chauffeur-driven government limousine with -30°C temperatures outside, they passed the towering pinnacles of Stalinistera office buildings, as well as the tangled metal fortifications which helped stop Hitler’s armies from entering Moscow. This initial visit to Moscow was a fact-finding mission to see the Russian setup and decide what they would need to bring to make the measurements. The tokamak they were to study, the T-3, despite containing probably the highest temperature on Earth, was an unprepossessing sight, a tangle of pipes and wires and unfinished metal surfaces. There were all sorts of problems, including the wildly fluctuating local power supply, vibrations from the giant flywheel generators that powered the tokamak, and stray electric and magnetic fields that would affect the laser.
Back at Culham they had three months to get a suitable laser ready, build the necessary optical equipment and find the right light detectors to pick up the scattered photons. Pease put all the facilities of Culham at their disposal and by mid April 1969 they were ready to go with twenty-six cases full of equipment weighing 5 tonnes. A few items – whose descriptions were deliberately vague in the official inventory – were in fact military-grade light detectors called photomultipliers which were on a list of equipment that it was forbidden to export to communist countries. Another item – a room-sized metal cage to keep stray fields away from the equipment – was so large that the team had to travel in an adapted Boeing 707 belonging to Pakistan Airlines, the only civilian aircraft that regularly flew to Moscow that had big enough doors.
Assembling the equipment in a cramped cellar room underneath the T-3 tokamak – so they could shine the laser up through the underside of the torus – took weeks of intensive work. Artsimovich often came to check on progress, eager to see his tokamak vindicated. Living in Soviet Moscow was not easy for the team members. Local food shops were as bare as those in Britain during the war, although the Britons could supplement from the ‘Berioska’ shops, only accessible to foreigners with hard currency. Robinson’s wife Marion, who had taken leave from her own job as a chemist at Harwell to join the party, did much of the work of finding out how to survive in Moscow. They lived in the same apartment building as many of the young researchers from the Kurchatov institute and forged friendships that lasted decades. It was not all work for the Culham team: thanks to the connections of the Kurchatov representatives of the Communist Party, they were taken to see ballet at the Bolshoi Theatre, opera at the Kremlin Theatre, the czarist crown jewels in the Kremlin Armoury, and the Moscow State Circus.
Once the experimental setup was all in place, they tried for the first time to shine laser light into the plasma and measure the scattered photons. The researchers immediately found that the plasma itself was giving off so much light – much more than they had expected – that it swamped the rather faint scattered photons. For weeks they tried to tease out the signal of the scattered beam without success, with Artsimovich looking anxiously on.
In June they decided they had to implement their backup plan. They had prepared and packed a second laser that produced much shorter pulses. If they illuminated the plasma very briefly with one of these short pulses and then only opened up the detector just long enough to catch the scattered laser photons, they would not catch so much light from the plasma at the same time. They had this backup laser quickly shipped to Moscow and began making the necessary changes. Work continued into July in the sweaty heat of the Moscow summer. The new higher power laser damaged other optical components, so replacements had to be shipped out quickly in deliveries to the British Embassy to avoid delays in customs.
On 21st July, as the rest of the world watched with baited breath for Neil Armstrong to clamber down a ladder onto the surface of the Moon, the Culham team made their final adjustments. The next day, while the Apollo 11 crew were still on their way home, Robinson made a call to Pease at Culham. He said that they had seen a clear signal of scattered laser photons with the new setup and the Doppler broadening suggested that the temperature was high. Another two weeks of experiments and they were sure that the T-3 was achieving temperatures of more than 10 million °C, just as the Russians had said a year earlier in Novosibirsk. The researchers called Culham again and, now that he was sure, Pease telephoned Harold Furth, research director of the Princeton fusion lab in the United States – a series of phone calls that completely changed the course of fusion research.
In the United States, the vindication of the tokamak prompted a variety of reactions. In Princeton it caused despondency. In the offices of the fusion section at the Atomic Energy Commission (AEC) in Washington, DC, staffed danced on the tables. The problem was that the US had invested heavily in fusion. It was now funding research at four different laboratories – Princeton, Los Alamos, Livermore and Oak Ridge. Researchers were investigating a variety of devices, including stellarators, pinches, mirror machines, more exotic geometries called multipoles, and others. Some people had devoted their working lives to these machines. But none of them was performing anywhere near as well as the Russian tokamaks.
At Princeton, the Model C stellarator was not living up to its promise. There was duplication of effort between Princeton and Livermore, and Congress was looking for budget cuts to help finance the war in Vietnam. Scientists, following the example of Lyman Spitzer, were beginning to quit fusion for other fields because the prospect of rapid progress towards power-producing reactors seemed to have evaporated and there were few new ideas pointing to a way forward. The AEC’s Amasa Bishop, who now headed the fusion section, needed something to kick-start the US fusion programme, something to enthuse both Congress and his own researchers.
Bishop had visited Russia in 1967 and had been impressed by the machines at the Kurchatov Institute. When Artsimovich presented his startling tokamak results at the IAEA conference in Novosibirsk, Bishop began to think seriously about whether the US needed to start building tokamaks. Researchers from Princeton, of course, dismissed the idea. They thought the Russians were mistaken and that in reality the performance of the tokamaks was not that much better than the Model C. Bishop took their views seriously. They had devoted nearly two decades to developing the stellarator and were America’s undisputed experts on toroidal fusion devices.
Others were not so negative, however. The fusion team at Oak Ridge National Laboratory in Tennessee had been struggling for years with an unusual magnetic mirror device called the Direct Current Experiment (DCX) which heated a plasma by firing a beam of deuterium molecules (D2) into it. But after years of trying they could only get it to work at very low particle density. Put too many particles into the plasma and instabilities broke it apart. Oak Ridge’s fusion chief Herman Postma feared that, with Congress looking for savings, his lab might be cut out of the fusion programme altogether. What they needed was a new device to rally around and the tokamak seemed to offer the perfect opportunity: no one was building tokamaks in the US and Oak Ridge could become the national tokamak lab. Early in 1969 Postma’s team began designing its own Oak Ridge Tokamak, or Ormak, which would aim to both replicate the Russian results (this was before the Culham team had announced their temperature measurements) and to go beyond them to demonstrate something new. The Russian team had shown that plasma performance improved if the ratio of the radius of the torus over the radius of the plasma tube, known as the aspect ratio, was low – in other words, if the tokamak was more like a doughnut than a hula-hoop. So the Oak Ridge team designed Ormak with two interchangeable plasma vessels, one with an aspect ratio equivalent to the Russian T-3 (roughly 7) and another with the much smaller ratio of 2.
In the spring of 1969, Artsimovich came to Boston. He had been invited by two professors at the Massachusetts Institute of Technology (MIT) whom he knew and the visit was meant to be partly a holiday: he would give a few lectures and work on a book he was writing. But with US interest in tokamaks high, researchers just wouldn’t leave him alone. Some of the Oak Ridge team designing Ormak came to Boston for a private audience. Another team came from the University of Texas, where they were designing a tokamak with a novel feature. They planned to use a strong electric field to deliberately cause turbulence in the plasma in the hope that eddies would boost its temperature.
Bruno Coppi, an Italian physicist who had recently arrived at MIT, also sought out Artsimovich. Coppi had spent some years working in Princeton and came to MIT with yet another plan for a tokamak. The pinch effect in tokamaks relies on a current flowing around the toroidal chamber. That current has a beneficial side-effect: it also heats the plasma because there is resistance to the flow of current, so when the current is pushed through hard the resistance raises its temperature. It is similar to the way friction between your palms warms your hands when you rub them together. Coppi sought to design a tokamak with a low aspect ratio and a very strong toroidal magnetic field, both of which were thought to maximise this resistance effect, known as ohmic heating (the ohm is a unit of resistance). MIT had a world-class magnet laboratory and Coppi enlisted the help of its engineers to design his machine.
What all these tokamak plans lacked was money to build them, but that was soon to change. Bishop knew that the time was right to move into tokamaks. Even Congress had got interested in the Russian results and was enquiring of the AEC what it needed to catch up with the Soviets. So Bishop arranged a meeting of his standing committee of fusion advisers in Albuquerque, New Mexico, in June 1969 and called for any researchers with tokamak proposals to come and present them. The Oak Ridge team came and proposed Ormak; Texas researchers put forward the Texas Turbulent Tokamak and Coppi and his MIT colleagues suggested their compact high-field machine, known as Alcator, derived from the Latin phrase for high field torus. What Bishop wanted but didn’t get was a proposal from the Princeton Plasma Physics Laboratory, or PPPL. They were the torus experts and Bishop figured that the fastest way to duplicate the Russian results in a similar machine would be to cannibalise Princeton’s Model C stellarator. Although the Model C was racetrack shaped, if you removed the straight sections and added an electromagnet you would get a tokamak of roughly the same size as Russia’s T-3.
But Princeton wasn’t playing ball. PPPL researchers insisted at the meeting that the Russian results were mistaken and this led to heated argument. They were, however, fighting a losing battle. After days of constant pressure from the standing committee, PPPL director Mel Gottlieb finally caved in and agreed that it was vital to test the abilities of tokamaks and that Model C should be sacrificed for the purpose. The confirmation of the T-3 temperature by the Culham team a few weeks later sealed the Model C’s fate. But the AEC’s standing committee was limited by its budget and could only give two projects the green light: the converted Model C and Ormak.
Once the decision was made, the Princeton researchers didn’t waste any time. By September they had come up with a design. The plan was to increase the radius of the plasma tube – and so reduce the aspect ratio – and also to cut down the straight sections of Model C’s racetrack shape to 20 centimetres. But they learned from the Russians that having a uniform symmetrical magnetic field was important, so the design was changed again and the 20-centimetre straight sections removed altogether. This provided the proposed machine with a name: the Symmetric Tokamak, or ST. Once all the new components were ready, the Model C stellarator was switched off for the last time on 20th December, 1969 and in little more than 4 months the team transformed it into a tokamak.
Unlike the Russian machines, the ST was bristling with measuring devices. It was an unwritten law at the Princeton lab: everything has to be measured. Once they applied those instruments to the plasma in the completed ST they found that everything the Russians had said was true. The temperatures and confinement times were higher than anything yet achieved in the United States. US fusion scientists fell head over heels in love with the tokamak. In 1971, just a year after ST started operating, Oak Ridge’s Ormak was ready to go, soon followed by the Texas Turbulent Tokamak. The following year saw the completion of MIT’s first machine – Alcator A – while General Atomics in San Diego built an unusual tokamak with a kidney shaped cross-section called Doublet II, and Princeton built a second, the Adiabatic Toroidal Compressor or ATC, to test methods for heating the plasma. More joined them in the following years.
The United States was not the only country to jump on the tokamak bandwagon. Researchers at Culham were in the middle of building a new stellarator called CLEO and, as happened to Princeton’s Model C, it was rebuilt as a tokamak. France entered the fusion race by building, straight off, the top performing tokamak of the early 1970s. The Tokamak de Fontenay aux Roses (TFR) had the strongest toroidal magnetic field of the time and could generate a plasma current of 400,000 amps. Germany built a smaller machine, the Pulsator, at its fusion lab in Garching near Munich and the Italians built the small TTF in Frascati. Japan too joined in with its first tokamak, the JFT-2. And the Russians, not keen to relinquish their lead, built a string of new machines to test different ideas.
For researchers outside Russia, the arrival of the tokamak transformed the field. Throughout the 1960s Bohm diffusion had frustrated all their efforts, leeching energy and ions out of the plasma and making high temperatures impossible to reach. Their work had become an effort to understand the behaviour of plasma, rather than a race to produce a power-producing fusion reactor. But the tokamak seemed to show them a way forward to higher temperatures and longer confinement. True, they didn’t really have a thorough understanding of how it worked or why it was better than other devices but that would come, they believed, as they got to know the machine better. The important thing was that the race towards a fusion power reactor was back on.
Not that everything was plain sailing. Once researchers started to push the tokamak to the limits, its own menagerie of plasma instabilities began to make an appearance. The potentially most serious was dubbed simply a ‘disruption.’ In a disruption, the plasma current which does most of the work confining the plasma suddenly collapses to zero in a tiny fraction of a second, leading to a loss of plasma and temperature. More serious than that is the fact that the sudden loss of that huge current (in the millions of amps for a large tokamak) induces strong eddy currents in the vacuum vessel which put it under huge mechanical strain – equivalent to hundreds of tonnes for a big machine. Researchers were obviously keen to avoid such events because of the damage they could potentially inflict on their precious devices. Experiments showed that high plasma pressure, high current and impurities in the plasma, increased the chances of a disruption. They were soon able to draw up a diagram showing the boundaries of safe operation but such boundaries were a problem because high pressure and current are desirable if you want to get to the conditions needed for fusion. Finding ways to push back those boundaries became a high priority.
Another puzzling instability was discovered in Princeton’s thoroughly instrumented Symmetric Tokamak. Using an x-ray detector, the PPPL researchers detected an x-signal coming from the plasma that went up and down in a regular repeating pattern; on a plot it looked like sawteeth. The sawteeth were caused by rapid heating and then cooling of electrons in the core and they were also seen in Russia’s T-4 (an upgrade of T-3) and Princeton’s ATC. Sawtooth instability soon came to be considered a sign that a tokamak had reached respectable operating conditions and its core was good and hot. The Russian researcher Boris Kadomtsev developed a theory that seemed to explain the oscillations and, as they were relatively benign, they were considered in the 1970s as a success story of plasma theory. But as machines got bigger, so did the sawteeth to the point at which they were causing turbulence that spoiled confinement.
But perhaps the most important upshot of the boom in tokamak building during the 1970s was that it allowed researchers to develop formulas that linked plasma properties to tokamak size, known as scaling laws. Even if you don’t have a detailed knowledge of how a plasma works, scaling laws allow you to predict what sort of tokamak will give you the best results. It worked because there were so many tokamaks of different shapes and sizes that physicists could operate with different sets of conditions. By plotting the results from many machines on a graph of, say, confinement time against the major radius of the tokamak, you can plot a line that extrapolates from those results to predict what confinement you would get with an even larger tokamak. Researchers did similar plots of confinement against plasma radius, toroidal magnetic field, plasma current and electron density. Some of the results were surprising: although plasma theory predicted that confinement time would go down as the density of electrons increased, real results from tokamaks showed confinement clearly growing the more electrons there were. Overall, there was one clear message: if you wanted to get closer to the temperature and confinement time needed for a plasma to burn with fusion reactions, then the larger the volume of plasma the better. It was time for tokamaks to get big.