Radial focusing is accomplished in a somewhat analogous manner. If the magnetic field decreases with radius, radial restoring forces are established. An ion at too large a radius is directed inward, and an ion at too small a radius is directed outward. In this fashion, the ion oscillates about the synchronous orbit. Thus, radial focusing is achieved.
Vacuum System
The vacuum tank (acceleration chamber) is a steel box 20 × 25 ft and 4 ft high. It is evacuated to a pressure of 10-5 millimeter of mercury (about one 100-millionth of atmospheric pressure). The pumping equipment consists of six oil-diffusion pumps and four mechanical vacuum pumps. The pumping speed of the six 20-in. oil-diffusion pumps is a total of 20,000 liters/sec.
Ion Source
The ion source is a simple arc-type. Hydrogen gas is allowed to leak into the ion-source enclosure near a tungsten filament, which is heated to incandescence. Electrons emitted by the filament knock off electrons from hydrogen atoms, leaving free protons. The protons then escape into the acceleration chamber through a hole in the ion-source housing. Once inside, the protons are accelerated by the dee potential.
Deuterons or alpha particles are obtained in a similar fashion using deuterium or helium gas in place of hydrogen.
Radiofrequency System
The 184-inch synchrocyclotron has a single dee instead of the double-dee arrangement described above for illustrative purposes. The accelerating electric field is developed between the dee and a dummy dee which is grounded to the vacuum tank. Using a single dee does not change the principle of operation, yet it offers the advantage of allowing more space for auxiliary equipment inside the vacuum tank. Also, the construction is much simpler. The dummy dee is not essential for operation, but it does improve performance.
