page_title: ZETA (fusion reactor)
section_title: Thomson scattering and tokamaks
hierarchical_section_title: ZETA (fusion reactor) / Thomson scattering and tokamaks
caption_attribution_description: English: Mike Forest operates a hand-built laser that is part of a Thompson scattering system, demonstrated in Nature in 1964. A similar system was later used to measure temperatures in ZETA reactor. It was later be used to test the tokamak devices in Novosibirsk and prove they were meeting the performance numbers the Soviet scientists were claiming.
context_page_description: ZETA, short for "Zero Energy Thermonuclear Assembly", was a major experiment in the early history of fusion power research. Based on the pinch plasma confinement technique, and built at the Atomic Energy Research Establishment in the United Kingdom, ZETA was larger and more powerful than any fusion machine in the world at that time. Its goal was to produce large numbers of fusion reactions, although it was not large enough to produce net energy.
  ZETA went into operation in August 1957 and by the end of the month it was giving off bursts of about a million neutrons per pulse. Measurements suggested the fuel was reaching between 1 and 5 million kelvins, a temperature that would produce nuclear fusion reactions, explaining the quantities of neutrons being seen. Early results were leaked to the press in September 1957, and the following January an extensive review was released. Front-page articles in newspapers around the world announced it as a breakthrough towards unlimited energy, a scientific advance for Britain greater than the recently launched Sputnik had been for the Soviet Union.
context_section_description: ZETA's failure was due to limited information; using the best available measurements, ZETA was returning several signals that suggested the neutrons were due to fusion. The original temperature measures were made by examining the Doppler shifting of the spectral lines of the atoms in the plasma. The inaccuracy of the measurement and spurious results caused by electron impacts with the container led to misleading measurements based on the impurities, not the plasma itself. Over the next decade, ZETA was used continuously in an effort to develop better diagnostic tools to resolve these problems.
  This work eventually developed a method that is used to this day. The introduction of lasers provided a new solution through a British discovery known as Thomson scattering. Lasers have extremely accurate and stable frequency control, and the light they emit interacts strongly with free electrons. A laser shone into the plasma will be reflected off the electrons, and during this process will be Doppler shifted by the electrons' movement. The speed of the electrons is a function of their temperature, so by comparing the frequency before and after collisions, the temperature of the electrons could be measured with an extremely high degree of accuracy. By "reversing" the system, the temperature of the ions could also be directly measured.
  Through the 1960s ZETA was not the only experiment to suffer from unexpected performance problems. Problems with plasma diffusion across the magnetic fields plagued both the magnetic mirror and stellarator programs, at rates that classical theory could not explain. Adding more fields did not appear to correct the problems in any of the existing designs. Work slowed dramatically as teams around the world tried to better understand the physics of the plasmas in their devices. Pfirsch and Schluter were the first to make a significant advance, suggesting that much larger and more powerful machines would be needed to correct these problems. An attitude of pessimism took root across the entire field.
  In 1968 a meeting of fusion researchers took place in Novosibirsk, where, to everyone's astonishment, the Soviet hosts introduced their work on their tokamak designs which had performance numbers that no other experiment was even close to matching. The latest of their designs, the T-3, was producing electron energies of 1000 eV, compared to about 10 eV in ZETA. This corresponded to a plasma temperature of about 10 million K. Although the Soviet team was highly respected, the results were so good that there was serious concern their indirect temperature measurements might be unreliable and they had fallen prey to a measurement problem like the one that had occurred with ZETA. Spitzer, once again, expressed his scepticism rather strongly, sparking off an acrimonious debate with Artsimovich.
  The Soviets were equally concerned about this, and even though it was the height of the Cold War, Artsimovich invited UKAEA to bring their laser system to the Kurchatov Institute and independently measure the performance. Artsimovich had previously called their system "brilliant." The team became known as "the Culham five", performing a series of measurements in late 1968 and early 1969. The resulting paper was published in November 1969 and convinced the fusion research field that the tokamak was indeed reaching the levels of performance the Soviets claimed. The result was a "veritable stampede" of tokamak construction around the world, and it remains the most studied device in the fusion field.
  Tokamaks are toroidal pinch machines. The key difference is the relative strengths of the fields. In the stabilised pinch machines, most of the magnetic field in the plasma was generated by the current induced in it. The strength of the external stabilisation fields was much lower and only penetrated into the outer layers of the plasma mass. The tokamak reversed this; the external magnets were much more powerful and the plasma current greatly reduced in comparison. Artsimovich put it this way:
  The longitudinal field intensity must be
caption: Mike Forrest operates a hand-built laser that is part of a Thomson scattering system used to measure temperatures in ZETA. This became a major diagnostic technique in the fusion field, used to this day.
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