synchrocyclotron

A part of a magnet from the Orsay synchrocyclotron, now used by the proton therapy center (to be replaced in 2008 by newer technologies)

A synchrocyclotron is a cyclotron in which the frequency of the driving RF electric field is varied to compensate for the mass gain of the accelerated particles as their velocity begins to approach the speed of light. This is in contrast to the classical cyclotron, where the frequency was held constant.

The two fundamental differences between this machine and the orthodox cyclotron are that

1. in this machine, only one dee is used instead of two
2. the frequency of oscillating electric field is made to decrease continuously instead of keeping it constant so as to maintain the resonance with ion frequency. One terminal of the oscillating electric potential varying periodically is applied to the dee and the other terminal is earthed. The proton or deuterons to be accelerated are made to move in circles of increasing radii. The acceleration of particles takes place as they enter or leave D. at the outer edge, the ion beam can be removed with the aid of electrostatic deflector. It was possible to produce 200MeV deuterons and 400MeV α-particle with the first synchrocyclotron

Synchrocyclotrons have not been built since the isochronous cyclotron was developed.

Isochronous cyclotrons maintain a constant RF driving frequency and compensate for the relativistic mass gain of the accelerated particles by increasing the magnetic field with radius. Isochronous cyclotrons are capable of producing much greater beam current than synchrocyclotrons.

Advantages

The chief advantage of the synchrocyclotron is that there is no need to restrict the number of revolutions executed by the ion before its exit. As such, the potential difference supplied between the dees can be much smaller.

The smaller potential difference needed across the gap has the following uses:

1. There is no need for a narrow gap between the dees as in the case of convention cyclotron, because strong electric fields for producing large acceleration are not required. Thus only one dee can be used instead of two, the other end of the oscillating voltage supply being connected to earth.
2. The magnetic pole pieces can be brought closer, thus making it possible to increase greatly the magnetic flux density.
3. The frequency valve oscillator is able to function with much greater efficiency.

The main drawback of this device is that, as a result of the variation in the frequency of the oscillating voltage supply, only a very small fraction of the ions leaving the source are captured in phase-table orbits of maximum radius and energy so that the output beam current is rendered weak. Thus the machine produces high energy ions, though of small intensity

See also

• Cyclotron — this contains a mathematics section that assumes constancy of mass, and an extensive see also section.
• Betatron

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