Lawrence Radiation Laboratory

“ The main parts of a cyclotron are represented in Fig. 2. Charged particles (ions) are accelerated inside an evacuated tank. This is to prevent the beam from colliding with [Pg 4] air molecules and being scattered. The vacuum tank is placed between the poles of an electromagnet, whose field bends the ion beam into a circular orbit.
The operation begins when the ions are introduced into the region between two accelerating electrodes, or "dees." [2] Because the ions carry a positive electric charge, they are attracted toward that dee which is electrically negative at the moment. ”

“ For example, the [Pg 5] resonant frequency for protons accelerated in a 15,000-gauss magnetic field is 23.7 megacycles (Mc) . [3] We call such a rapidly alternating potential a "radiofrequency voltage" and the electronic circuit for producing it a "radiofrequency oscillator."
The energy E of an ion emerging from the cyclotron is given by
where H, e, and m are as defined above, and R is the radius at which the beam is extracted. ”

“ 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. ”

“ 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. ”

“ But since the dee potential continues alternating at a constant frequency, an ion soon begins to arrive "late" at the dee gap. By the time it has made about 70 to 100 turns an ion is so badly out of phase that it is no longer accelerated.
Suppose now that we want to obtain an energy of 10 Mev. Because an ion can make a maximum of about 100 [Pg 6] turns, the accelerating potential would have to be about 100,000 volts. However, Professor Lawrence hoped to reach 100 Mev with the new 184-inch cyclotron. This meant that the accelerating voltage would have to be about 1,000,000 volts. ”

“ 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 continuous beam, such as that obtained from a conventional cyclotron (see Table I) . This is part of the price paid for higher energies.
Internal Targets and Beam Extractor
The simplest target is one placed inside the vacuum tank where the circulating beam will strike it. ”

“ 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. ”

“ This causes them to spiral outward. Finally they strike a target inserted into their path or are extracted from the cyclotron for use as an external beam.
The time required for an ion to complete one loop remains constant as it spirals outward. This is because its velocity increases sufficiently to make up for the increased distance it travels during each turn. This means that the electric potential applied to the dees must alternate at a constant frequency, called the "resonant frequency."
The resonant frequency f is given by the relationship
where H, e, π, c, and m are constants. ”