The Alternating Gradient Concept

Even as Brookhaven's first accelerator, the Cosmotron, went into operation in the early 1950's, scientists knew that achieving the higher energies needed for future research was going to be a difficult problem. Calculations showed that, using existing technology, building a proton accelerator ten times more powerful than the 3.3-billion electron volt (GeV) Cosmotron would require 100 times as much steel. Such a machine would weigh an astronomical 200,000 tons. Brookhaven physicists Ernst Courant, M. Stanley Livingston, and Hartland Snyder overcame this barrier by co-inventing the alternating gradient or strong-focusing principle of propelling protons.

In ring-shaped accelerators such as the Cosmotron, particles travel through a magnetic field, which keeps them on their circular course by bending their trajectories. As a beam of particles achieves higher energies in a machine like this, the beam remains well focused in the vertical direction, but its trajectory becomes unstable in the horizontal direction, leading to beam loss. This could only be overcome by using more powerful (and far heavier) magnets and drastically increasing the size of the machine.

In the Cosmotron, all the magnets were C-shaped, with the open side and the magnetic field, facing outward. The breakthrough occurred by alternating the orientation of these magnets, so some of their field gradients faced outward and some inward. Brookhaven physicists found that the net effect of alternating the field gradient was that both the vertical and horizontal focusing of protons could be made strong at the same time, allowing tight control of proton paths in the machine (right). This increased beam intensity while reducing the overall construction cost of a more powerful accelerator.

When the magnetic field gradients were made stronger, Courant, Livingston and Snyder found that there was no theoretical limit to the energies to which protons could be accelerated -- provided that alternations are made more frequently as the field gradients increased. Also, using strong focusing, magnet apertures could be as small as 1 or 2 inches in diameter, as opposed to the 8 by 24 inches within the Cosmotron. Without strong focusing, a machine as powerful as the Alternating Gradient Synchrotron (AGS) would have needed apertures (the gaps between the magnet poles) as large as perhaps 20 by 60 inches instead of apertures of only a few inches. The following diagram shows the size comparison between the Cosmotron's weak-focusing magnet (left) and the AGS alternating gradient focusing magnets.

The strong-focusing principle revolutionized accelerator design. The principle's practicality was demonstrated in 1954, when Cornell's 1.3-GeV electron accelerator began operation. Then the new technology was applied to larger machines. In 1959, the 25-GeV Proton Synchrotron went into operation at CERN, the European high energy physics laboratory, and in 1960, the 33 -GeV AGS was commissioned. These alternating gradient synchrotrons were constructed using only twice the amount of steel (4,000 tons) needed to construct the weak-focusing, 3.3-GeV Cosmotron. Since its discovery, strong focusing has been one of the guiding principles behind every new accelerator in the world.

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

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