poles 6 meters in diameter and weighing 10 ,000 tons—a weight comparable to that of the Eiffel Tower! [15]

very closely, however, and the tube still needs to be about 20 to 40 inches wide. The first synchrotron, called the Cosmotron (at Brookhaven National Laboratory on Long Island, New York), relied on weak focusing to achieve an unheard-of 3 GeV (3 billon eV) of energy.[1]

A large improvement in beam confinement was achieved with the development of “strong focusing” in 1952. A combination of different magnets alternately focus and de-focus the beam in both the horizontal and vertical directions. The combination of magnets can be arranged to achieve a net focusing of the beam. This idea is best understood by considering the optical analog of alternating convex and concave lenses, shown in Fig. 5.[ 11 ] Strong focusing squeezes the beam considerably so that the vacuum tube can be reduced to 3 to 5 inches in diameter ([16], p. 230). This reduction in size is not only useful for focusing the beam onto the target, but also brings immense because far smaller magnets can be used. The strong focusing mechanism was first applied in a 1.5 billion eV electron synchrotron, constructed in 1953-54 at Cornell University. In 1959, the proton synchrotron at CERN (the European Laboratory for Particle Physics) relied on the strong focusing technique to reach a whopping 28 GeV (=2810⁹ eV).

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Figure 5: Optical analogy of strong focusing. In the synchrotron, magnetic lenses take the places of these optical lenses.

The phase focusing process relies on the fact that the speed of the particles in the synchrotron is nearly constant, approaching—but never reaching—the speed of light. Suppose for the moment that one of the particles in the group is lagging behind the optimal position. It crosses the voltage gap a little late when the voltage has already decreased to below the value that keeps the ideal particle in orbit. At the same time, the magnetic field of the bending magnets is increased (because the synchrotron considers only the particles near the ideal position, i.e., those that were accelerated.) The higher magnetic field will cause the straggler to bend into a smaller radius than the “ideal particle.” Since both the straggler and the ideal particle are moving at about the same velocity, the straggler will then arrive a bit earlier at the next gap. Through a similar process, a particle that arrives at a gap too early receives more energy than the ideal particle, which causes it to follow a slightly larger circle, arrive slightly too late at the next gap, and receive less energy. Like the marble in the salad bowl, the particles in the group oscillate about the ideal position, and a stable equilibrium is achieved. ([16], p. 219)