Posted on: 3rd November 2005

“Measurements have been made of the electron temperature and density of the plasma in the toroidal discharge apparatus Tokamak T3 at the Kurchatov Institute, using Thomson scattering by the plasma electrons of 6943 Å light from a Q-spoiled ruby laser. Important features of recent measurements on Tokamak T3 have been the high total energy of the plasma, the long confinement time and the evidence for thermonuclear reactions in the confined plasma column. In the T3 torus (which has a major diameter of 2m and a minor diameter of 0.4m) the electron energy has previously been obtained only for a short (20ms) current pulse using the diamagnetic technique. In the Thomson scattering experiment on T3 the discharge period is 70ms, with a flat topped current pulse. (…) Temperatures of up to about 1keV have been measured.”

N.J. Peacock, D.C. Robinson, M.J. Forrest, P.D. Wilcock and V.V. Sannikov in “Measurement of the Electron Temperature by Thomson Scattering in Tokamak T3″, Nature Vol. 224, November 1, 1969

In the 1950s, physicists believed that mastering thermonuclear fusion would be straightforward, and there were even a few premature claims of the controlled release of major fusion power. Following significant developments in plasma diagnostics, a quite pessimistic period followed in the 1960s. It was demonstrated that man-made plasmas could not confine energy as well as was theoretically predicted. Consequently, achieved temperatures were quite low in comparison with the requirements for thermonuclear fusion. Besides, due to the bitter experience of blunders in the 1950s, scientists were sceptical of any extraordinary claims.

cartoon 'Evolution of the Tokamak'

Drawing from the talk “Evolution of the Tokamak” given in 1988 by B.B. Kadomtsev at Culham.

cartoon ‘Evolution of the Tokamak’

Drawing from the talk “Evolution of the Tokamak” given in 1988 by B.B. Kadomtsev at Culham.

This was still the case at the 1968 conference in Novosibirsk, where scientists from the Moscow Institute of Nuclear Physics announced that their T-3 facility could produce plasmas with temperatures above 1 keV (more than 10 million degrees). Although the Russian team was highly respected, the result seemed too good to be true and doubts were cast on the reliability of the method used to evaluate plasma temperature. Indeed, temperatures on T-3 had been measured indirectly by using the plasma’s magnetic properties. British scientists at Culham had just mastered a more trustworthy, direct approach to measuring very high temperatures based on the then novel method of laser light scattering on plasma electrons (Thomson scattering). The obvious need to validate the T-3 performance was of such importance that it transcended political difficulties. The Soviets invited the British team to Moscow, and the Brits accepted the invitation. In the winter of 1968/1969, an apparatus of several tons was dispatched to Moscow and four Culham experts were sent on mission there.

The mission was a resounding success. Surprisingly high temperatures of the T-3 plasmas were confirmed, blowing a fresh wind through fusion research worldwide. In particular this was a major breakthrough for the tokamak concept which had, until then, only been developed in the U.S.S.R. (tokamak is the Russian acronym for “toroidal chamber with magnetic coils”).

Following Nature’s publication of the above article in November 1969, the U.S. scientists in Princeton immediately decided to convert their toroidal experiment to a tokamak (giving birth to the ST device) and the French designed the TFR tokamak. Given their imminent success, projects for large tokamaks including JET emerged in the 1970s.

Picture of the month

setup to exploit Thomson Scattering

A British spectrometer (left) coupled to the Soviet tokamak T-3 (right). To exploit the Thomson Scattering phenomenon for plasma temperature measurements, a powerful laser and sensitive spectrometer are required. The laser fires light through the tokamak’s plasma, while the spectrometer measures the wavelengths of light that plasma scatters from the laser’s path. Changes in wavelength are then directly linked to the temperature of plasma electrons. See Focus On : Lidar-Thomson Scattering Diagnostic on JET