In the tranquillity of Aspen, Spitzer had plenty of time to think. Like others before him, he realised that you would need a very hot plasma so that nuclei collide with enough force to fuse, that it would need to be relatively dense so that enough collisions take place, and that the best way of keeping it away from the walls of its container was with magnetic fields. Spitzer didn’t, as Peter Thonemann and George Thomson had done, conclude that the pinch effect was the answer. Instead, while riding the long ski lifts up the mountain, he imagined a straight tube with magnetic field lines – imaginary lines that show the direction of the field at any place – running straight and parallel along its interior. The effect of this uniform field on the charge particles of the plasma – which would normally be moving in straight lines with random directions – would be to exert a force on each particle pulling it perpendicularly towards the magnetic field lines. This turns their straight line motions into tight little spirals orbiting around the field lines. The particles can move freely along the length of the tube, but are prevented from moving across it, towards the walls, by being bound to orbiting the field lines. As long as the field avoids the walls, Spitzer reasoned, so will the particles.
Particles in plasmas, like in gases, naturally fly around in random directions. But apply a magnetic field and plasma particles become locked in spirals around the magnetic field lines.
(Courtesy of EFDA JET)
Creating a uniform magnetic field in a tube is easy: simply wind a length of wire round and round the tube along its full length and pass an electric current through it – a classic electromagnet. The problem is what to do with the ends? Because particles can move freely along the tube, they can just as freely drift out the end. Spitzer’s solution was to make his tube endless by bending it around into a doughnut shape, or torus, just as Thonemann and Thomson proposed before him. This created a new problem, however: making the field bend around in a curve meant that it was no longer truly uniform. On the inside edge of the curve the turns of the electromagnet coil end up bunched closely together and on the outside edge they’re spaced further apart. This means that the magnetic field the coils produce ends up stronger on the inside of the curve than on the outside. The effect on the particles of this non-uniform field is a vertical force which pushes the electrons up to the top of the tube and the ions down to the bottom, or vice versa. This separation of the electrons and ions creates an electric field which, in combination with the magnetic field, pushes the particles towards the outside edge of the curve. The overall effect is that the particles end up hitting the walls of the tube, the plasma loses energy and fusion doesn’t happen. Spitzer calculated that particles would hit the wall before they had completed one circuit of the torus.
He was still puzzling over this conundrum when he left Aspen but it only took a few more days to come up with a solution: instead of a torus, build a figure-of-8 shape with two crossing straight sections joined by curved ends. With this arrangement, at one curved end the electrons are pushed up and the ions down and when they get to the other end the electrons are pushed down and ions up, so the drift of particles is essentially cancelled out.
Instead of getting down to work on the H-bomb with Project Matterhorn, Spitzer spent the next month writing up a detailed proposal for a power-producing thermonuclear reactor which he called a stellarator, and then sent it to the Atomic Energy Commission (AEC), the government agency set up in 1946 to manage both the development of nuclear energy and America’s nuclear weapons, including Project Matterhorn.
Spitzer was invited to a meeting at the AEC in Washington on 11th May to discuss the issue of controlled fusion power. Also present was James Tuck who was at that time again working at Los Alamos on the H-bomb. Apart from Spitzer, there were probably few other minds in the US as primed and ready to tackle fusion power as Tuck’s. Born and raised in Manchester, Tuck studied physical chemistry first at the Victoria University of Manchester and then at Oxford. Before finishing his doctoral thesis, he joined Leo Szilard working on particle accelerators at the Clarendon Laboratory. The war soon intervened: Szilard moved to the US and Tuck’s head of department, Frederick Lindemann, was appointed to a senior post in the UK government, taking Tuck with him. Lindemann was already a personal friend of Winston Churchill and when the latter became prime minister the former was made his scientific adviser. Tuck, however, disliked the high politics of being a government science adviser, although he did make a valuable contribution to the war effort by developing shaped charges for use in armour-piercing shells.
Tuck’s work on shaped charges got noticed in Los Alamos where they were having grave problems with the implosion mechanism of the plutonium bomb. Manhattan Project officials appealed, via Churchill, for Tuck to join their effort. He moved to New Mexico early in 1944 and helped to develop the explosive lens that was key to the bomb’s success. He was present for the very first nuclear explosion at the Trinity test site in New Mexico in July 1945 and the first test at Bikini Atoll the following year. With the war over he had more time on his hands and so contacted Manchester University about finishing his doctoral thesis but was told that ‘the statute of limitations on presenting a thesis had run out.’ By this time his old boss Lindemann, now Lord Cherwell, was urging him to return to the UK. He arrived back at the Clarendon in autumn 1946, around the same time that Peter Thonemann arrived. Tuck helped install a new particle accelerator called a Betatron at the lab and, because of his fusion work at Los Alamos, was invited by John Cockcroft to the meeting at Harwell in January 1947 when George Thomson’s pinch device was discussed.
So Tuck had a pretty good knowledge of all the fusion work that was going on in Britain when, in 1949, he got a call from Edward Teller to come to Los Alamos to work on the new H-bomb programme. Back in New Mexico, Tuck was set to work measuring the exact reactivity of the deuterium-deuterium and deuterium-tritium reactions but continued to think about constructing a pinch device along similar lines to Thonemann’s. In spring 1951, a graduate student from Princeton spent some time at Los Alamos and told Tuck about Spitzer’s plan for the stellarator. Tall and wire-thin, Tuck was a blunt northerner with an acerbic sense of humour, and one who wasn’t shy of telling others when he didn’t think their ideas were up to much. And that’s exactly what he thought about Spitzer’s stellarator.
For a start he believed that the stellarator, which aimed to hold its plasma at high temperature in a steady state for long periods, would lose too much heat by simple thermal conduction through the plasma and into the walls. The pinch, in contrast, was a pulsed device that squeezed the plasma very fast to achieve bursts of energy. He also distrusted Spitzer’s optimism. He had seen firsthand some of the problems that Thonemann and Thomson had come up against and knew that achieving fusion would require a lot more than just devising a good confinement scheme. So many things were unknown or untested that it was far too soon to talk about power-producing reactors.
At the 11th May AEC meeting both the pinch and the stellarator were discussed. The evidence in support of both approaches was weighed up; both men interpreted it differently and both remained resolutely in favour of their own device. But the AEC officials liked Spitzer’s plan and in July they awarded Spitzer $50,000 for theoretical studies into the stellarator as part of Project Matterhorn. Tuck was busy for most of the rest of 1951 with his studies of reaction rates but towards the end of the year he approached the head of the Los Alamos lab and proposed a new programme of controlled thermonuclear research there. He was awarded $50,000 of the lab’s own funds to get things started. And so the seeds were sown for years of rivalry between Los Alamos and Princeton.
Meanwhile in Argentina, Perón grew tired of waiting to hear of more success from Huemul Island. He appointed a technical committee of physicists and engineers to investigate Richter’s work. They reported to Perón in September 1952 that the temperatures Richter was achieving in his device were much too low to cause fusion reactions. Perón shut down the laboratory later that year. Whethe
r or not Richter genuinely believed he could achieve fusion energy, he was working alone, cut off from the rest of the scientific community and so was unlikely to succeed. But this was not the last time in the history of fusion that extravagant claims turned out to be false.
Spitzer set to work refining the theory of the stellarator as well as sketching out a complete development plan. Work would start with a table-top device (Model A) which would show that a plasma could be created and confined and would heat the plasma’s electrons to 1 million °C. The larger Model B would also heat the ions up to 1 million °C, while the Model C would be a virtual prototype power reactor able to reach thermonuclear temperatures of more than 100 million °C. The whole process would take about a decade.
In November 1951, the governing body of the AEC – five commissioners appointed by the US president – considered whether controlled fusion should become a formal programme. In the optimistic spirit of the time, they were all generally in favour of pursuing new avenues of research. They were also aware that Britain was already working on fusion and suspected that the Soviet Union was too. After the shock of Russia’s first atomic explosion two years earlier, the United States couldn’t afford to fall behind in any important new field of research. It was a time of great insecurity for America: its national laboratories were racing to build the H-bomb before the Russians and its soldiers were fighting communist forces in the hills of Korea. The commissioners asked Thomas Johnson, director of the AEC’s division of research, how much a fusion programme would cost. Plucking a figure out of the air he said that $1 million over three and a half to four years would prove whether fusion is feasible and if not, why not. So the programme was launched with a pot of $1 million.
Along with their generosity, the commissioners stipulated that the programme had to remain classified because of the potential of fusion neutrons for breeding plutonium, so the controlled fusion part of Project Matterhorn had to find a secluded place to do its work. Princeton University had recently acquired some property outside the city which had previously been owned by the Rockefeller Foundation. There Spitzer found a corrugated iron building that formerly housed laboratory animals. They would find out later that this ‘rabbit hutch’ was as hot as hell in the summer, but it would do. Windows were blackened, alarms fitted, barbed-wire fences erected and guards put on duty. Over the weeks that followed, Spitzer and his astrophysics colleague Martin Schwarzschild spent their weekends sitting on the floor of the hutch winding flat copper wire around 2-inch diameter glass tubes as they constructed Model A, America’s first fusion reactor.
Lyman Spitzer and his Model A stellarator.
(Courtesy of Princeton Plasma Physics Laboratory)
Tuck didn’t have to worry so much about security: he was working in a government lab surrounded by people who were already security screened. Especially useful to him would be all the expertise in measuring devices and instruments, or ‘diagnostics,’ that had been developed to analyse bomb tests. One of the major difficulties for fusion researchers at the time was finding out exactly what was happening inside a device. How dense and hot is the plasma, and is it humming along stably or wriggling out of control? Naturally, Tuck began working on a toroidal pinch. It had a pleasing simplicity to it which he hoped would mean a working reactor wouldn’t be hugely expensive. Not everyone was convinced. A sceptical colleague referred to his device as an ‘impossibilitron.’ Tuck countered, ‘Perhaps it will work and perhaps it won’t,’ and resolved to call it the Perhapsatron. Tuck scrounged some parts from a disused Betatron accelerator and got the lab’s technicians to make a toroidal glass tube for the first Perhapsatron. His goals were straightforward: to see if he could produce a pinched plasma and if it is stable. If that worked he would aim to produce neutrons.
A third team also joined the effort. Edward Teller was agitating to set up a second weapons laboratory to work on the H-bomb in addition to Los Alamos. His chosen site was at Livermore, California, a small town in a cattle-rearing and wine-growing valley near San Francisco. The University of California at Berkeley bought an old military camp there in 1950 and built a lab. Herbert York, a young Berkeley physicist, was given the job of setting up the new weapons lab, so in the early months of 1952 York travelled to other labs in the H-bomb project, including Los Alamos and Princeton, to discuss Teller’s plans. He was intrigued by the work of Spitzer and Tuck and decided that it would be a good idea if the Livermore lab had its own controlled fusion project, to broaden its programme beyond weapons work. York didn’t want to simply duplicate the research elsewhere by building a pinch or a stellarator; he needed a device of his own. Spitzer’s original idea, of a straight tube with an applied magnetic field running along the length of it, appealed to him because of its simplicity. The stellarator only got into problems because its tube was bent into curves to make it endless. What if you kept the simplicity of the straight tube and simply plugged the ends in some way to stop particles escaping? His first idea was to plug the ends with intense radio waves, which were known to exert some pressure on matter.
York gave a series of lectures at Berkeley’s Radiation Laboratory on the problems of fusion. In the audience was Richard Post, a recent recruit to the lab. Post was fascinated and loved the fact that it was an area of research with a socially useful goal. His recent doctoral research had involved both radio waves and plasma so he immediately wrote a long memo to York analysing the radio wave plug idea and other possibilities. York called him in and offered him the job of heading up the new lab’s controlled fusion group. Post and some colleagues set to work on the theory of such a device. He soon concluded that it would take a huge amount of power to produce radio waves intense enough to plug the ends of the tube. They began looking at the magnetic fields themselves and found that if the field was strengthened at the ends of the tube – with extra windings of the electromagnet – this forced the field lines closer together. If the conditions of the plasma were right, this crimping of the field at the ends forced the moving particles to reverse direction, an effect known as a magnetic mirror. York and Post now had a fusion device they could call their own.
During the course of 1952, both the stellarator and the Perhapsatron were built and powered up for the first time. Both teams started off filling their tubes with noble gases rather than deuterium because at first they weren’t trying to cause fusion reactions but just to show that they could create a plasma, confine it and heat it. Spitzer was able to produce a plasma in his Model A and, applying a stronger magnetic field, was able to show that the confinement was improved. But particles were still drifting to the wall faster than theorists had predicted, so Spitzer still had a puzzle on his hands. In their theoretical models, they had assumed that all the particles in the plasma moved independently of each other, influenced predominantly by just the magnetic field. That assumption was something of a necessity because to accept that the particles influenced each other as well would make their calculations impossibly complicated in an age before computers were widely available. If the particles do influence each other then ‘collective effects,’ such as waves and turbulence, would have to be accounted for.
The Perhapsatron was also proving to be more of a conundrum than Tuck had expected. When the device, which was about a metre across, was powered up the researchers could see a glowing plasma through windows in the side and when they induced a current in the plasma they could see for a brief instant the plasma being pinched into a fine filament at the centre of the tube before it suddenly disappeared. Using high-speed cameras that take shots at various intervals after the pinch was applied, they could see that it remained stable for just a few millionths of a second before the filament began to violently thrash about and break up. Tuck had come up against the kink instability, just as Bob Carruthers did at Harwell. Tuck grasped the fact that there was a brief period of stability before the plasma broke up and concluded that he could either focus on making the pinches very fast – getting a burst of energy before the instability kic
ks in – or on finding some way to stabilise the plasma.
With Livermore’s magnetic mirror work also progressing, the fusion programme was gaining some momentum. It had its first conference at the end of June 1952 in Denver, with eighty attendees. It also acquired a name: Johnson at the AEC was plotting to close down another research programme called Project Lincoln at the Hood Laboratory and give the money to Tuck. With that conjunction of names, it just had to be Project Sherwood. But it remained a pretty relaxed affair. The priorities of the programme were set by the researchers themselves. There were only a few dozen people in total working on fusion and many of them, including Spitzer, Tuck and Post, were only working on it part-time; they had other lines of research going on in parallel. Nevertheless, none of them ever doubted, despite the severe lack of knowledge of the intricacies of plasma behaviour, that it would be a simple progression over the course of a decade or so from table-top demonstrator devices to larger experimental reactors and then prototype power plants.
* * *
The appointment of Admiral Lewis L. Strauss (pronounced ‘straws’) as chairman of the AEC in mid 1953 stirred up the cosy club of fusion research. Strauss was a man of his time: a firm believer in technology and US scientific pre-eminence, and intensely suspicious of the Soviet Union. He wanted fusion to be achieved to show the power of the capitalist system, and he wanted it done during his tenure at the AEC.
Piece of the Sun : The Quest for Fusion Energy (9781468310412) Page 7