Primer: Lawrence’s Cyclotron December 3, 2008Posted by Will Thomas in EWP Primer.
Tags: Bob Seidel, Charles Lauritsen, Ernest Lawrence, Ernest Rutherford, John Cockroft, John Heilbron, M. Stanley Livingston, Merle Tuve, Robert Van de Graaff, Rolf Wideröe
In the early 1930s, the acceleration of electrons and protons was a popular project. While the spectacular theoretical developments in quantum mechanics had stolen the show in physics in the 1920s, the problem of understanding the atomic nucleus had also become a subject of renewed interest following on experiments performed by Ernest Rutherford and his coterie at the Cavendish Laboratory at Cambridge University. They had shown that bombarding nuclei with the natural radiation of radioactive materials could transmute the subject nuclei into different elements. However, natural radioactive materials were expensive, and their ability to provide incident particles was uncontrolled and inefficient. It was understood that providing some artificial source of high energy (high velocity) particles would make bombardment easier, and the exploration of atomic nuclei more systematic and reliable.
The obvious means of creating a source was to send particles streaming across a high electrical potential difference (high voltage). Lightning accelerated electrons in an uncontrolled way between the sky and the ground—and had, in fact, been marshaled as a source of ephemeral high voltages. The electrical industry had been vigorously seeking ways of creating high voltages so as to transmit electricity over long distances more effectively. In the laboratory, since the late 19th century, cathode ray tubes had been sending electrons through a vacuum between oppositely charged plates. Cathode rays also created x-rays, which were of interest as a therapy for cancer; and it was expected the higher energy x-rays created in higher voltages would lead to more effective therapies than those already tried.
So, the production of reliable, high energy accelerators became a scientific premium during the 1920s. Andthere were a lot of takers. Some physicists, such as John Cockroft at Cavendish, Charles Lauritsen at Caltech, and Merle Tuve at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington accelerated particles using variants on the original concept of creating large potential differences. In the early 1930s, Robert Van de Graaff invented his spectacular generator, which created enormous potentials by transporting charges to a large sphere. The German electrical engineer Rolf Wideröe (who was interested in constructing electrical devices rather than investigating the nucleus) successfully constructed a method of accelerating particles in steps, and published his method in the Arhiv für Electrotechnik in 1928. Unlike single potential differences, particles could be accelerated in steps over and over again by adding more accelerating electrodes, leading to far larger particle energies than was otherwise considered practically possible (or safe).
The physicist Ernest Lawrence (1901-1958) of the University of California at Berkeley (and a boyhood friend of Tuve’s from South Dakota) was a key player in the accelerator sweepstakes. He happened upon Wideröe’s article, and, one among others, elected, with his graduate students David Sloan and M. Stanley Livingston, to construct accelerators according to this series design—including a design that arranged the electrodes so as to accelerate the particles in a circle. While certainly not the first to imagine what became known as the “cyclotron”, in 1931 Lawrence and Livingston were the first to construct a cyclotron with a good enough design so as to keep the particle on its proper track.
Most accelerator development in the late-1920s and early-1930s had been done with the idea of creating particles with a sufficiently high energy to facilitate nuclear research, around one million electron-volts (MeV, the energy gained by an electron across a potential difference of one million volts). However, the cyclotron had a strong potential for scaling well beyond that level, and Lawrence soon dedicated his work to the construction of cyclotrons boasting increasingly high energies as well as to nuclear research.
To provide a place for accelerator construction, Lawrence established an independent laboratory at Berkeley, which became known as the Radiation Laboratory. However, scaling the cyclotron up was not trivial. Although the cyclotron design solved scale problems inherent to the linear accelerator, it, too, required larger and more sophisticated equipment, and, of course, more money. To solve the second problem and thereby the first, Lawrence appealed to the non-profit Research Corporation (which took academic patents and channeled the proceeds into research grants, and which also funded Van de Graaff’s generators) and Chemical Foundation. He also salvaged used magnets and radio equipment. Initial success led to more money from other sources, competition from other experiments including in industry, and thus to demands from his sponsors that he protect his intellectual property. This process led onward, to larger cyclotrons, which required staffs for their design, construction and use. Ultimately, this scaled up, big money, big staff, big equipment research became a new kind of fundamental physical research.
Lawrence would win the 1939 Nobel Prize in physics, and his cyclotron and the foundation of the Radiation Laboratory are often marked as a turning point between the physics of the small university laboratory and expensive “big” physics. It is no coincidence that accelerators, such as the new Large Hadron Collider at CERN (7 TeV, or 7 million MeV), are the poster apparatus for this scaled up brand of physics. For much of the 20th century, accelerators were a core experimental technology for particle physics well beyond the problem of the behavior of atomic nuclei. Lawrence’s laboratory itself (now called the Lawrence Berkeley National Laboratory) is a part of the Department of Energy’s system of national labs. Beyond its symbolic historical value, though, it is also worth keeping track of the specific scientific and technological contexts surrounding Lawrence’s original work. Research on the nucleus, for example, persisted using accelerators still on the scale of some of Lawrence’s earlier models.
The go-to book on this subject is John Heilbron and Bob Seidel’s Lawrence and His Laboratory (1989), which based on work done with Bruce Wheaton for the lab’s 50th anniversary celebration in 1981. A website based upon this original work, plus visual archives, are available at the lab’s web site. Also see the American Institute of Physics’ web exhibit. Note that’s two Heilbron books featured in two consecutive weeks, on quite different technical topics; that kind of chronological adventurousness is rare in today’s history of science profession.