ded 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.

[Illustration: 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.

[Illustration: 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 le