Abstract
A major challenge in the design of rf cavities for the acceleration of medium-energy charged ions is the need to rapidly sweep the radio frequency over a large range. From low-power medical synchrotrons to high-power accelerator driven subcritical reactor systems, and from fixed focus alternating gradient accelerators to rapid cycling synchrotrons, there is a strong need for more efficient, and faster, acceleration of protons and light ions in the semirelativistic range of hundreds of MeV/u. A conventional way to achieve a large, rapid frequency sweep (perhaps over a range of a factor of 6) is to use custom-designed ferrite-loaded cavities. Ferrite rings enable the precise tuning of the resonant frequency of a cavity, through the control of the incremental permeability that is possible by introducing a pseudoconstant azimuthal magnetic field. However, rapid changes over large permeability ranges incur anomalous behavior such as the ``Q-loss'' and ``f-dot'' loss phenomena that limit performance while requiring high bias currents. Notwithstanding the incomplete understanding of these phenomena, they can be ameliorated by introducing a ``harmonic ratcheting'' acceleration scheme in which two or more rf cavities take turns accelerating the beam---one turns on when the other turns off, at different harmonics---so that the radio frequency can be constrained to remain in a smaller range. Harmonic ratcheting also has straightforward performance advantages, depending on the particular parameter set at hand. In some typical cases it is possible to halve the length of the cavities, or to double the effective gap voltage, or to double the repetition rate. This paper discusses and quantifies the advantages of harmonic ratcheting in general. Simulation results for the particular case of a rapid cycling medical synchrotron ratcheting from harmonic number 9 to 2 show that stability and performance criteria are met even when realistic engineering details are taken into consideration.
Highlights
Ions in a synchrotron accelerate over a speed range βmin ≤ βðtÞ ≤ βmax: (1)Isochronous accelerators maintain a constant revolution frequency as particles gain energy through careful adjustment of path length
An alternative to ferrite-loaded cavities is made possible by the use of magnetic alloys, which feature high permeability values, resilience under high magnetic fields, and large inductances
The subsequent increase in frequency reduces the magnetic flux requirements in the cavity, allowing higher accelerating voltages to be obtained at fixed ferrite length while reducing cavity losses
Summary
Ions in a synchrotron accelerate over a speed range βmin ≤ βðtÞ ≤ βmax:. Isochronous accelerators maintain a constant revolution frequency as particles gain energy through careful adjustment of path length. The ferrite materials suffer dramatic loss effects when pushed to large biasing fields, rapid biasing changes, and large amplitude ac fluxes [6] [7] These difficulties limit the advancement of fast and efficient accelerators for low energy ion beams. Magnetic alloy (MA) cavities have been designed and used for high power rapid cycling synchrotrons [8] Another solution is to chose the energy gain per turn such that cavities are spaced by an integer multiple of the rf period, allowing accelerating bunches to skip rf buckets. This is the motivation and the basic method of harmonic ratcheting
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Physical Review Special Topics - Accelerators and Beams
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
