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 rev
olution 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 instabilitie
s 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.
Piece of the Sun : The Quest for Fusion Energy (9781468310412) Page 11