rings of varying thickness, as shown in
Fig. 4b. In a radially decreasing magnetic field, the lines of magnetic
flux bow outward, as represented in Fig. 4b. Ions moving in a magnetic
field are deflected at right angles to these flux lines. Ions above the
midplane of the cyclotron are directed downward; those below the
midplane are directed upward. In this way an ion oscillates about the
midplane and vertical focusing is achieved.

[Illustration: Fig. 4.

(a) Plot of magnetic-field strength vs radius. The field strength
decreases gradually out to a radius of about 83-in., after which it
falls off sharply. This point marks the maximum usable radius for
particle orbits. Further out they are unstable.

(b) Magnetic flux lines are represented as broken arrows, and focusing
forces as solid arrows. An ion above the midplane is directed downward,
while an ion below the midplane is directed upward.]

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 x 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
groun