Hard on the heels of the 50th anniversary of the synchrotron (CERN Courier, September 1996, page 10) comes the 50th anniversary of synchrotron radiation. On 24 April 1947, Herb Pollock, Robert Langmuir, Frank Elder and Anatole Gurewitsch saw a gleam of bluish-white light emerging from the transparent vacuum tube of their new 70 MeV electron synchrotron at General Electric's Research Laboratory, Schenectady, New York. Synchrotron radiation had been seen.
However the synchrotron radiation story goes back a long way. The General Electric machine was the second synchrotron to operate, the first having been a small 8 MeV machine at the UK Telecommunications Establishment, Malvern. Unfortunately the UK machine did not have a transparent vacuum tube.
Nor is synchrotron radiation restricted to synchrotrons. Electromagnetic radiation is produced whenever a beam of charged particles loses energy when it is bent in a field, a sort of electromagnetic centrifugal effect. Thus it could equally well be called 'betatron radiation', in honour of the first circular accelerators built to handle electrons. The betatron was first demonstrated by Donald Kerst and Robert Serber at Illinois in 1941, and development work was taken up by General Electric. However these early betatrons, like the UK synchrotron (which in fact was an imported GE betatron), did not have transparent vacuum tubes.
Although it was invisible, 'synchrotron' radiation was nevertheless there and was sapping the energy of the betatron beam. In 1945 John Blewett, then at General Electric, calculated that synchrotron radiation resulted in an energy loss of about 10 electronvolts per revolution, and predicted a resultant tiny orbit shrinkage. The measurement of this shrinkage was the first indirect observation of synchrotron radiation effects.
But the idea of synchrotron radiation goes back much further. Maxwell's grand formulation of electromagnetism in 1864 was a watershed in understanding and new insights proliferated in the ensuing years. Widely credited as the 'inventor' of synchrotron radiation is the French physicist Alfred Lienard of the Ecole des Mines in Paris, who in a 1898 paper in the journal "L' Eclairage Electrique" (Electric Lighting) described the concept of retarded potentials in the calculation of the effects due to the motion of charged particles, and worked out a basic theory of what is now known as synchrotron radiation and which is still followed in modern textbooks. This work was supplemented by Emil Wiechert at Göttingen, so that the formalism is generally known as the Lienard-Wiechert potentials.
Lienard's paper appeared just after the discovery of the electron by J.J. Thomson exactly one hundred years ago at Cambridge. (The electron concept and the elucidation of cathode rays had been in the air for some time, but Thomson's measurement of the ratio of the electron's charge/mass ratio is generally accepted as the official discovery.) However, according to synchrotron radiation pioneer John Blewett, an embryonic idea of synchrotron radiation can be traced as far back as 1867, to Ludwig Lorenz.
The next major development in the formulation of synchrotron radiation theory came in 1908 from G.A. Schott, first as a student at Cambridge, then at Aberystwyth, Wales, in a prizewinning paper on the mechanical reactions of electromagnetic radiation.
The idea lay dormant for several decades before being picked up by Isaak Pomeranchuk in Russia in 1940, at about the same time that Kerst and Serber were getting their betatron to work. Betatrons were also being developed in Russia, where Pomeranchuk and D. Iwanenko in 1944 looked at the problem of radiation losses in higher energy machines.
Perhaps the crowning achievement in this theoretical understanding came from an unexpected source - Julian Schwinger. Unexpected, in the sense that Schwinger's name is eternally linked with the development of relativistic quantum electrodynamics in the immediate post-war period, rather than with classical problems.
However Schwinger had mastered the trade of classical electrodynamics in his wartime efforts on microwave propagation at the MIT Radiation Laboratory, work which had a direct bearing on highly effective new radar techniques.
Schwinger continued to follow this up, and showed that synchrotron radiation contains many higher harmonics, extending into the visible range. In 1949, shortly after his monumental papers on relativistic electrodynamics for which he went on to receive the 1965 Nobel Prize, he published another elegant masterpiece - 'On The Classical Radiation of Accelerated Electrons', which definitively updated earlier work.
Synchrotron radiation was initially a nuisance, sapping energy from circulating electron beams and upsetting calculations. Gradually physicists learned that the tangential fan of synchrotron radiation 'waste' from high energy electron rings could be used to probe the structure of a wide range of samples, and parasitic synchrotron radiation studies began to be carried out at machines built to supply electrons for particle physics studies.
In the 70s, as more applied researchers clamoured for time at these synchrotron radiation sources, a new generation of purpose-built machines appeared. As well as basic research, synchrotron radiation studies went on to be used in the chemical, materials, biotechnology and pharmaceutical industries. A new research community had come of age. Synchrotron radiation is now a flourishing branch of science, with dedicated major facilities such as the European Synchrotron Radiation Facility at Grenoble.
Reproduced by kind permission from the CERN Courier (May 1997), 37(4), 11-12.