At first, improvements in cyclotron design were directed at the construction of larger machines that could accelerate particles to greater velocities. Soon, however, a new problem arose. Physical laws state that nothing can travel faster than the speed of light. Thus, adding more and more energy to a particle will not make that particle's speed increase indefinitely. Instead, as the particle's velocity approaches the speed of light, additional energy supplied to it appears in the form of increased mass. A particle whose mass is constantly increasing, however, begins to travel in a path different from that of a particle with constant mass. The practical significance of this fact is that, as the velocity of particles in a cyclotron begins to approach the speed of light, those particles start to fall "out of sync" with the current change that drives them back and forth between dees.
Two different modifications-or a combination of the two-can be made in the basic cyclotron design to deal with this problem. One approach is to gradually change the rate at which the electrical field alternates between the dees. The goal here is to have the sign change occur at exactly the moment that particles have reached a certain point within the dees. As the particles speed up and gain weight, the rate at which electrical current alternates between the two dees slows down to "catch up" with the particles.
In the 1950s, a number of machines containing this design element were built in various countries. Those machines were known as frequency modulated (FM) cyclotrons, synchrocyclotrons, or, in the Soviet Union, phasotrons. The maximum particle energy attained with machines of this design ranged from about 100 MeV to about 1 GeV.
A second solution for the mass increase problem is to alter the magnetic field of the machine in such a way as to maintain precise control over the particles' paths. This principle has been incorporated into the machines that are now the most powerful cyclotrons in the world, the synchrotrons.
A synchrotron consists essentially of a hollow circular tube (the ring) through which particles are accelerated. The particles are actually accelerated to velocities close to the speed of light in smaller machines before they are injected into the main ring. Once they are within the main ring, particles receive additional jolts of energy from accelerating chambers placed at various locations around the ring. At other locations around the ring, very strong magnets control the path followed by the particles. As particles pick up energy and tend to spiral outward, the magnetic fields are increased, pushing particles back into a circular path. The most powerful synchrotrons now in operation can produce particles with energies of at least 400 GeV.
In the 1970s, nuclear physicists proposed the design and construction of the most powerful synchrotron of all, the superconducting super collider (SSC). The SSC was expected to have an accelerating ring 51 mi (82.9 km) in circumference with the ability to produce particles having an energy of 20 TeV. Estimated cost of the SSC was originally set at about $4 billion. Shortly after construction of the machine at Waxahachie, Texas began, however, the United States congress decided to discontinue funding for the project.