Early Accelerator Builders at the University of Illinois

Figure 1. Cyclotron built at the University of Illinois in 1936.
Illinois’ pioneering leadership in building particle accelerators began in 1936, four years after the cyclotron was invented by E.O. Lawrence, when Professor P. Gerald Kruger and his students built the world’s third cyclotron, the first to be built outside of Berkeley and the first to have an external beam (Fig. 1)

A cyclotron is a cyclical particle accelerator in which charged particles (protons) are held in a spiral trajectory by a static magnetic field while being accelerated by a rapidly varying rf field. Lawrence received the 1939 Nobel Prize in Physics for the invention of the cyclotron, which remained the most powerful particle-accelerator technology until the 1950s, when the synchrotron was developed. The physicists at Illinois used their cyclotron to perform some of the earliest studies on nuclear disintegration.

Figure 2. From left, John Manley, Maurice Goldhaber, and Leland Haworth with their linear accelerator, ca. 1938.
A year later, new assistant professors Maurice Goldhaber, Leland J. Haworth, and John H. Manley built a linear accelerator in the basement of the old Physics Building (now MSEB) that used a Cockcroft–Walton (CW) generator to produce a modulated source of slow neutrons (Fig. 2). The CW generator, an electric circuit that generates high dc voltage, was also invented in 1932 by British physicist John Douglas Cockcroft and Irish physicist Ernest Thomas Sinton Walton. Particle accelerators today still use CW generators.

The Illinois physicists used their device to determine the velocity distributions and energy dependence of various neutron reactions, including the mean life of neutrons in water, the hydrogen capture cross section, and the velocity dependence of the absorption of boron for slow neutrons.

In the early 1940s, the accelerator was dismantled and shipped to “Site Y” (now Los Alamos National Laboratory) for use in the Manhattan Project. John Manley, who accompanied the accelerator to New Mexico, later became the deputy director of the national laboratory. Goldhaber would go on to become the director at Brookhaven National Laboratory (the current director is also a Physics Illinois alumnus, Dr. Doon Gibbs), and Haworth served as a director of the Atomic Energy Commission and later was appointed by President Kennedy to head the National Science Foundation.

Figure 3.Donald Kerst and the first betatron, which achieved acceleration of electrons on July 15, 1940, at the University of Illinois.
The next chapter in Illinois accelerator building came in 1940 with Donald Kerst and the invention of the betatron—the world’s first magnetic induction electron accelerator. Kerst’s apparatus, which accelerated electrons to an energy of 2.35 million electron volts (MeV), was the first successful device to exploit the electromotive force associated with a changing magnetic flux to accelerate charged particles (Fig. 3). It was also the first device in which the magnetic field at the orbit was designed to contain the circulating particles indefinitely in a fixed orbit. The stable oscillations about the fixed orbit were described in a seminal paper by Kerst and theorist Robert Serber in 1941. The betatron became the prototype for focusing beams of charged particles employed in subsequent circular accelerators, and the stable oscillations of particles in these accelerators are called “betatron oscillations” in recognition of the pioneering Illinois work.

After the new machine was referred to variously as a “rheotron,” an “inductron,” a “Super-X-Ray Machine,” and a “cosmic ray machine,” in early press releases, a departmental contest was held to name it. “Außerordentlichhocgeschwindigkeitelektronenentwickelndenschwerarbeitsbeigollitron” (loosely translated from the German as “hard-working machine for generating extraordinarily high-velocity electrons, by golly”) was one of the more ingenious entries. Kerst settled on “betatron.” The original 2.4-MeV betatron is now housed at the Smithsonian Institution in Washington DC, but you can see the glass vacuum chamber “doughnut” from the tabletop machine on display in the hallway to the north of Room 151 Loomis, as well as a second-generation 24-MeV machine that was used for x-ray radiography detection of flaws in metals and armaments during World War II. If you peek in the side of the machine, you’ll see the ceramic doughnut that replaced the blown glass containment vessel of the original tabletop machine.

In 1941, Kerst completed his second betatron, a 24-MeV machine which was commercially produced by the Allis-Chalmers company and adopted for a variety of industrial uses. The high-energy x-rays produced by the betatron allowed engineers to “take pictures” through thick metal castings and forgings to detect material flaws or manufacturing defects. Kerst was drafted to work at Los Alamos in 1943, where he requisitioned and diverted the first of the commercial 24-MeV betatrons to take stop-action x-ray images of explosions. Betatrons were also used at the Rock Island, Illinois, and Picatinny, New Jersey, U.S. military arsenals during World War II for nondestructive testing of internal mechanisms and material integrity of military equipment and armaments before materiel was shipped to combat zones.

Figure 4. Donald Kerst (L) and his student H. William Koch measuring the depth-dose of ionization distributions produced by 24-MeV x-rays, ca. 1942. They carried out the first measurements of dose distributions in human-tissue-like materials.
The 24-MeV betatron was also used for medical applications, as a source of high-energy x-rays and of free electrons for cancer treatment (Fig. 4). The first betatron used solely for medical purposes was installed at the University of Illinois College of Medicine in Chicago in 1949.

Despite its commercial and medical applications, the betatron was first and foremost an instrument for nuclear physics research. The 24-MeV machine was used to carry out the first photofission experiments on the isotopes of uranium and plutonium during World War II under highly secret conditions. Later the betatron was used in groundbreaking experiments on photo-induced nuclear processes and the first experiments on electron scattering.

In his quest for ever-higher electron energies, Kerst scaled up plans again in 1948, first for an 80-MeV prototype, and then for a whopping 340-MeV third-generation behemoth, which had its own purpose-built building on the corner of Stadium Drive and Oak Street in Champaign. (The machine’s power consumption was so large [170 kw], it had to be sited as close as possible to the University’s Abbott Power Plant).

Figure 5. Assembling the 300-ton laminated iron magnet for the 340-MeV betatron, ca. 1949. Note the person standing in the center of the magnet.
The “big” betatron—with more than 300 tons of laminated iron in its magnet (Fig. 5)—was used for ground-breaking studies in nuclear physics from the time it went online in 1950 until its decommissioning in 1969. Important Illinois contributions to nuclear physics made using the 340-MeV betatron include photodisintegration of nuclei, pion photoproduction, and the interactions of photons with isotopes of hydrogen and helium.

The last accelerators built at the University of Illinois were racetrack microtrons, which used superconducting rf cavities as their accelerating structure. Instead of accelerating electrons held in a circular orbit, the microtrons accelerated electrons in bunches by subjecting them to a series of oscillating electric potentials along a linear beamline, a “linear accelerator,” or linac. The electron beam was passed through the superconducing rf cavities to achieve higher energies. In addition, these accelerators, unlike all other electron accelerators at that time, produced a continuous beam having exceptional beam quality.

Figure 6. Professors Peter Axel (L) and Alfred Hanson (R) with the completed MUSL-1 superconducting electron linac, March 1972.
The first of these accelerators, called the “Microtron Using a Superconducting Linac” (MUSL-1), was designed and built by Illinois Physics faculty and students beginning in 1969, when the big betatron was decommissioned and sold for scrap. Although superconducting rf cavities are now the accelerating structure used in all high-energy accelerators, the Illinois group was among the very first to exploit this technology. MUSL-1, basically a prototype, was followed by MUSL-2, which achieved a maximum energy of 100 MeV. MUSL-2 was operated for nuclear physics research at Illinois from 1972 to 1992.

Five major experimental instruments were built to exploit the continuous beam of MUSL-2, including an electron-scattering coincidence facility, low- and high-energy bremsstrahlung detectors, and two photon-tagging systems. The accelerator served both University of Illinois faculty and students and researchers from Canada, Germany, Israel, Brazil, and Japan.

Today, the department’s physicists carry out research at a variety of national and international accelerator facilities, including Fermilab and Brookhaven National Laboratory in the United States and the Large Hadron Collider in Geneva, Switzerland. You can learn more about the current accelerator builders in our department at:
https://physics.illinois.edu/research/groups-and-centers/nuclear-physics.html and:
https://physics.illinois.edu/research/groups-and-centers/high-energy-physics.html.
Be sure to go look at the betatron displays in the Loomis Laboratory Lobby.

Celia Mathews Elliott
Urbana, Illinois
August 2018


Bibliography
Gerald Almy, A Century of Physics at the University of Illinois (University of Illinois Department of Physics, Urbana, IL, 1967).

P. Axel, L.S. Cardman, R.A. Daniel, A.O. Hanson, R.A. Hoffswell, R.M. Laszewski, W.C. Selley, N. Towne, and A.M. Vetter, “The University of Illinois Nuclear Physics Laboratory, 1982,” IEEE T. Nucl. Sci. 30, 1112–1114 (1983).

Paul T. Debevec, private communication.

Celia M. Elliott, “History of Excellence,” Department of Physics, University of Illinois at Urbana-Champaign, https://physics.illinois.edu/history. Accessed 08/13/2018.

R.A. Kingery, R.D. Bert, E.H. Schillinger, Men and Ideas in Engineering: Twelve Histories from Illinois (University of Illinois Press, Urbana, IL, 1967), pp. 66–77.

David Lazarus, The Loomis Legacy (University of Illinois Department of Physics, Urbana, IL, 1987).

Lisa Warne, Vignettes from a Century of Service, 1891–1990 (University of Illinois Department of Physics, Urbana, IL, 1991).