Particle accelerators


Particle accelerators use electromagnetic fields to bring charged particles to high speeds and contain them in well-defined beams. The most familiar example of a modern particle accelerator is the massive Hadrian collider at CERN, which is used to study the properties of the hypothetical particle known as the Higgs boson. But according to the National Accelerator Laboratory, physicists use a range a range of accelerators today to study everything from environmental science to astrophysics to medicine.

Historians have identified three lines of scientific research that led to the current technology behind particle accelerators. The first was pioneered by Edward Rutherford’s research in the early 1920s into the properties of atomic particles, sparking interest into the possibility of splitting the atom. In 1928, George Gamov predicted that such a device could work if it could generate sufficient power, perhaps 500 keV. Rutherford encouraged John Cockcroft and Ernest Walton to design an electrostatic machine—a 500 kV particle accelerator—and after four years of development, in 1932, they conducted the first fully man-controlled splitting of the atom by splitting the lithium atom with 400 keV protons. This device, which earned Cockcroft and Walton the 1951 Nobel Prize in physics, is housed at the Science Museum, London.

At around the same time, Robert Van de Graaff, an American studying physics at Oxford University on a Rhodes scholarship, invented a more powerful electrostatic generator, but it was not deployed for nuclear physics research until well after the Cockcroft Walton generator’s first successful test. The Van de Graaff generator would later be improved through the inclusion of a high-pressure tank of dry nitrogen (Freon), which enabled up to 10 MV of power. Its voltage would then be accelerated twice over through the use of a tandem accelerator. The most powerful tandem accelerator now operates at the Oak Ridge National Laboratory, which ordinarily runs at 24.5 MV.

The second line of research, which has become dominant in the development of high-energy accelerators like the Hadrian collider, was the invention of resonant acceleration. Unlike the Cockcroft Walton and Van de Graff generators, which relied on direct voltage, resonant acceleration generators employ time-varying fields across drift tubes. Ernest Ising proposed such a system in 1924, theorizing that it could build energies that exceed the highest direct voltage available in the system. To avoid energy loss, the drift tubes would be combined in a linear machine shaped in a circle.

Building on this early work, Ernest Lawrence designed a fixed-frequency cyclotron in 1929. Less than a foot in diameter, it could accelerate protons to 1.25 MeV. In 1932, Lawrence split the atom just weeks after Cockcroft and Walton, and received the Nobel Prize for this achievement in 1939.

A third line of development came with the invention, in 1923 by Rolf Wideroe, and re-invention, in 1940 by Donald Kerst, of the betatron, which could send electrons moving at 2.2 MeV. Kerst would develop the largest betatron in 1950 (300 MeV) in 1950. Betatron’s career as a device for high-energy physics ended soon after, but it continued to be employed as a tool in hospitals and small laboratories.

These three technologies for accelerating particles—direct current acceleration, resonant acceleration, and the betatron, became vital technologies for the creation of the modern accelerator. These devices have the shape of Lawrence’s cyclotron, but avoid its relativistic effects through the principle of phase stability. This concept was used to create the synchrotron, which allows for longitudinal focusing of the particles. The modern synchrotron employs a storage ring collider, which allow a continuous particle beam to circulate for many hours.