Synchrotrons cont'd

DOE-HDBK-1108-97

Radiological Training for Accelerator Facilities

Student's Guide

power supply in which the frequencies would decrease with time to match exactly the slower

revolutions of the particles.

To do this, a machine was designed that operated in a pulsating manner (protons being

accelerated in bursts). Unlike the fixed-frequency cyclotron, which can continuously accelerate a

stream of particles, the new machine, the synchrocyclotron, has to push one group of protons

through an entire cycle, from the initial highest frequency to the final lowest one, before it can

begin accelerating a new group.

The first synchrocyclotron was the one at Berkeley, and it began operation in 1946. Exciting

experiments, not possible with lower energy machines, were quickly begun.

There is not a theoretical limit on the energy of a synchrocyclotron. There is, however a

practical limit from the point of view of economics. As the desired energy increases, so must

the radius of the magnet. As this radius increases, the magnet's area increases as the square of

the radius and its cost may increase by a factor almost equal to the cube of the radius. Thus, the

cost rises much too steeply for the synchrocyclotron technique to be used to reach energies in

the billion electron (GeV) region.

Accordingly, in seeking such energies, scientists tried another technique. This was to develop

machines in which the magnetic field rises in step with the momentum of the particles being

accelerated. This keeps the particles moving in a circle of virtually constant radius rather than

in the widening spirals of cyclotron and synchrocyclotrons. The advantage is that it eliminates

the entire center section of the magnet, with resultant cost savings.

The size of the magnet in a particle synchrotron is determined by two factors. One is the energy

desired, which determines how wide a circle the particles must traverse. The other is the degree

to which the particles are concentrated or "focused" in the magnetic field. This determines the

size of the vacuum chamber.

The first particle synchrotron completed was the Cosmotron at Brookhaven National

Laboratory. It accelerated protons to 2.3 GeV in 1952 and later to 3 GeV.