an energy of 730 Mev a proton, for example, makes 75,000 revolutions in just 6 milliseconds (msec). It travels a distance of 450 miles and attains a velocity of 152,000 miles per second, or 82% of the speed of light! During this brief journey its mass increases 75%, giving very convincing evidence for the validity of Einstein's theory. Similar data for other ions may be found in the appendix.

DESIGN AND CONSTRUCTION OF THE 184-INCH SYNCHROCYCLOTRON

Magnet

During the rebuilding of the cyclotron, the diameter of the magnet pole pieces was increased from 184 to 188-3/4 inches. Also, the pole gap at the center was reduced from 21 to 14 inches. These changes increased the weight of steel in the magnet from 3700 to 4000 tons.

The main exciting coils, which contain 1300 turns of copper-bar conductor each, were not altered. Two auxiliary coils containing 425 turns each were added. This brought the total weight of copper from 300 to 340 tons. The coils are layer-wound around the pole pieces close to the pole gap. Other data about the coils are given in the appendix.

The effect of these modifications was to increase the field strength at the center of the pole gap from 15,000 to 23,400 gauss. This increase made it possible to obtain the higher-energy ions.

Power is supplied to the coils by two motor generator sets, which produce the direct current required for a steady magnetic field. The direct current from the motor generators is regulated so that the magnetic-field fluctuation is less than one part in 10,000. This is necessary if one wants an external beam of nearly uniform energy.

In order to prevent the beam from becoming unstable and striking the dee, the magnetic field must be strongest at the center and decrease radially (Fig. 4a). With flat pole faces the field does not decrease uniformly. To give the desired rate of decrease, the pole faces are shimmed with concentric steel 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.

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 × 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.

Fig. 5. Radiofrequency cycle for accelerating protons. Sixty-four such cycles are repeated each second.

Radiofrequency power is supplied to the dee by a vacuum-tube oscillator. The frequency of oscillation must decrease during the acceleration cycle, as indicated above. For protons, the frequency at the start of acceleration is 36 megacycles (Mc). At the end of acceleration the frequency is only 18 Mc (see Fig. 5). This change in frequency is achieved by varying the electrical capacitance in the tuned circuit of the oscillator. (This is what you do when you dial a different station on a radio.) This tuned circuit, which is called the cyclotron resonator, is shown in Fig. 6.

Fig. 6. Cyclotron resonator.

Because the frequency must change over such a wide range (from 36 to 18 Mc), the electrical capacitance must be varied by a factor of 20 to 1. This is done by a variable capacitor of unique design. It resembles two giant tuning forks. As the blades of the tuning forks vibrate, the capacitance is alternately increased and decreased by the required amount.

These two tuning forks must be kept in step with great precision. This is to prevent the oscillator from exciting lateral rf resonances. With a cyclotron of this size, this is a problem. These resonances, if excited, would cause loss of beam. The method for keeping the blades moving together is as follows: The blades are made to vibrate at their resonant frequency, which is approximately 64 cycles per second. One set of blades operates at its natural frequency as a tuning-fork oscillator. The second set of blades is driven from an amplified sample of the signal from the first; its natural period is adjusted automatically to equal that of the first. The amplitude of each set is regulated to within 0.003 in.; the phase angle between the blades is regulated to within 1 deg.

Ions are accelerated only when the radiofrequency is decreasing (Fig. 5). The remaining portion of the cycle is "dead time." Thus, 64 pulses, each of about 500 microseconds' duration, are obtained every second. The average ion current of a pulsed beam is much less than for a