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A cyclotron is a charged particle accelerator in an axial magnetic field, which utilizes high frequency (Radio Frequency, RF) electric field to accelerate the charged particle beam which is spiraling on a plane perpendicular to the direction of the magnetic field. There are a variety of cyclotrons, ranging from low-energy types for irradiating semiconductors or for the production of labeled compounds with short half-lives for PET, to high-energy types for nuclear physics or for the production of radioisotopes for SPECT.
AVF is the acronym for “Azimuthally Varying Field.” It means that the magnetic field varies in strength in the azimuthal (beam path) direction in order to converge the beam in radial and axial directions.
To maintain the isochronism in the entire accelerating field, the axial magnetic field in the cyclotron should be strengthen as the acceleration energy and the radius rise. However, the increasing magnetic field with orbit radius diverges the beam axially. Nevertheless, the cyclotron can synchronize with the beam while preventing beam divergence during acceleration by varying the field strength (AVF, Azimuthally Varying Field) along the path of the accelerating particles. The advent of such AVF cyclotrons enables not only accelerating charged particles with high energy, but also acquiring larger beam current values, while increasing RI manufacture efficiency. (The photo is 930 cyclotron in Cyclotron and Radioisotope Center, Tohoku University)
This is a kind of AVF cyclotron, in which the magnet is divided azimuthally into multiple sectors.
Cyclotrons with energy exceeding 100 MeV take the form of a ring cyclotron (or separate sector cyclotron). The features of the ring cyclotron and their comparison with the aforementioned AVF cyclotron are as follows.
|1||This structure substantially reduces the magnetic field in valley parts, which greatly enhances axial beam convergence.|
|2||The RF resonator can be placed outside the magnet, which improves design freedom and allows higher acceleration voltage.|
|3||As it is composed of multiple sector magnets, it can be larger than an AVF cyclotron.|
|4||Sumitomo Heavy Industries have delivered our first four-sector ring cyclotron RRC (Model 6-4A) to RIKEN, which is shown in the above photo. By combining the ring cyclotron with the heavy ion linac and the AVF cyclotron, it can accelerate multiple types of particles, from proton to uranium. We delivered our second ring cyclotron to Research Center for Nuclear Physics, Osaka University. It (Model S-6A) has six sector magnets.|
|Magnet||Number of sector magnets||6||4||4|
|Acceleration||Number of RF cavities||3||2||2|