John D Lawson (FRS) 1923–2008
A pioneer of accelerator and fusion physics, John D Lawson, passed away on 15 January at his home in Abingdon, Oxfordshire, England.
John Lawson was educated in classics at an English grammar school. He was nevertheless fascinated by science and studied enough physics to try for a place at Cambridge University. However, because a qualification in chemistry was also required he joined the engineering faculty to do mechanical sciences. He used to remark that his engineering training made him think in a slightly different way from those who had done straight physics.
The Second World War was still raging when he finished his degree, and his first scientific job was with the Telecommunications Research Establishment (TRE) Malvern, where the war effort on radar was underway. He decided to work on antenna design, thinking that it would lead to an outdoor life, trekking over the countryside. Although disappointed to find he had to spend most of his time in the laboratory, he became fascinated with ways of narrowing down the polar diagram of aerials. During this time he hit upon an idea now commonly seen on rooftops around the world – the parabolic dish. While at TRE Malvern he was also involved with the reverse engineering of equipment salvaged from downed German aircraft.
When the war came to an end Lawson's expertise in periodic arrays of antennas led him to work at Harwell, where a small team led by his wartime group leader Don Fry was assembled to build two entirely new types of accelerator: a microwave travelling-wave linac and a synchrotron for measuring nuclear cross sections. Lawson worked on the 30 MeV synchrotron, which first produced beam in 1948 – only two years after the first ever synchrotron accelerator built by Frank Goward at the Woolwich Arsenal. This was the beginning of a long career in accelerator physics, punctuated from time to time by research into nuclear fusion.
With the outbreak of the Korean War in 1950, researchers who had worked on defence projects were encouraged to return to defence work. Lawson, because of his experience with microwaves, joined a group led by Peter Thonemann to build a klystron, a device for producing high-power microwaves. Thonemann was in charge of the development of the ZETA fusion experiment, which first operated in 1952.
It was while working at Harwell in the mid-1950s that he came up with his famous criterion – the Lawson criterion – for net energy gain in a fusion device. Lawson showed that to achieve net power, the fusion plasma must be sustained at a temperature such that the product of the plasma density and confinement time exceed a certain value. The goal of all fusion research is to exceed this product of density and confinement time. The then Harwell director, John Cockcroft, suggested that Lawson present a paper on fusion power in Dublin in September 1957 at the meeting of the British Association for the Advancement of Science. It was the first time that anything was said about fusion in public. ZETA and other experiments were classified because of the fact that they could produce neutrons, but the Lawson criterion was unclassified and so was allowed to be presented. The Lawson criterion remains the holy grail of fusion research.
In the late 1950s Lawson became interested in the new alternating gradient proposal for synchrotrons and found that in the extreme form first proposed, particle resonances from magnetic errors would destroy the beam. Designs of both the AGS at Brookhaven and the PS at CERN were modified to make them work, and Lawson considered this among his main achievements. At the same time, the UK had to decide between a high-intensity, weak-focusing machine and the less predictable intensity of a smaller version of the alternating-gradient proton–synchrotron proposed for CERN. After a "shoot out" in Cockcroft Hall – the Harwell auditorium – Cockcroft made the decision for the high-intensity, weak-focusing machine NIMROD; Lawson claims that his warning that particle resonances might destroy the beam was not the principal reason.
In 1961 Lawson transferred to the newly established National Institute for Research in Nuclear Science next door to Harwell – later to become the Rutherford Laboratory and finally the Rutherford Appleton Laboratory – to continue his work on accelerators and help build NIMROD. He continued to specialize in accelerator design and microwaves, but also played a leading international role in promoting and critically examining ideas for future accelerators. In the early 1980s he recognized the potential that high-power lasers could have for particle acceleration, and set up a small research group in laser plasma accelerators. The remarkable success achieved by plasma accelerators over the past few years is a testament to his vision that one day they may take over from the large conventional accelerators that he called "dinosaurs".
Lawson was particular about scientific accuracy and would point out mistakes in his colleagues' reasoning (and sometimes his own) in a gentlemanly manner. He was one of the few physicists to encourage younger colleagues working in big laboratories to concentrate on new ideas, consequently gaining the respect of all generations for his vision and imagination. Those of us who worked with him will remember not only his penetrating questioning but also his warm and friendly behaviour. He is survived by his wife Kay, son Chris and daughter Catriona.
Bob Bingham, Centre for Fundamental Physics, Rutherford Appleton Laboratory, and University of Strathclyde, and Ted Wilson, CERN and the John Adams Institute For Accelerator Science, Oxford.