Cyclotrons and Fixed-Field Alternating-Gradient Accelerators

Abstract

This article describes particle accelerators using magnets whose field strengths are fixed in time to steer and focus ion beams in a spiral orbit so that they pass between (and can be accelerated by) the same electrodes many times. The first example of such a device, Lawrence's cyclotron, revolutionized nuclear physics in the 1930s, but was limited in energy by relativistic effects. To overcome these limits two approaches were taken, enabling energies of many hundreds of MeV/u to be reached: either frequency-modulating the rf accelerating field (the synchrocyclotron) or introducing an azimuthal variation in the magnetic field (the isochronous or sector-focused cyclotron). Both techniques are applied in fixed-field alternating-gradient accelerators (FFAGs), which were intensively studied in the 1950s and '60s with electron models. Technological advances have made possible the recent construction of several proton FFAGs, and a wide variety of designs is being studied for diverse applications with electrons, muons, protons and heavier ions. All fixed-field accelerators offer high beam intensity: classical and isochronous cyclotrons operate in cw mode and in some cases deliver beams of 2 mA; synchrocyclotrons and most FFAGs operate in pulsed mode, but are capable of much higher pulse repetition rates (≤ kHz) than synchrotrons.

Current and future accelerator technologies for charged particle therapy

“…A compromise between cyclotrons and synchrotrons is the fixed-field alternatinggradient accelerator (FFAG) [77,78,79]. The FFAG is a strong-focusing variant of the cyclotron first proposed in the 1950s, and retains the advantages of having a fixed dipole field (so that superconducting magnets are an option) whilst giving a greater energy reach than is possible with a cyclotron.…”

Technologies for delivery of proton and ion beams for radiotherapy

“…dielectric wall accelerators), laser-generated proton beams, 87,88 or novel forms of conventional accelerators such as fixed-field alternating gradient (FFAG) rings. 89,90,91 Those technologies in or close to clinical implementation are described below, whilst some others of interest are discussed in Section 7.…”

Cyclotrons

“…Some excellent reviews on this subject are presented in Friesel and Antaya (2009), Craddock and Symon (2008), and Craddock (2010). Their characteristics are described chronologically in the following subsections according to the history of their development.…”

“…Reproduced fromCraddock MK and Symon KR (2008) Cyclotrons and fixed-field alternating-gradient accelerators. Schematic of a radial-sector-type AVF cyclotron and the noncircular orbit.…”