Abstract
A scaling fixed field alternating gradient (FFAG) accelerator provides large momentum acceptance despite the fact that magnetic guiding fields are constant in time. Optical functions are identical over the large momentum range as well. We have designed a straight beam transport line (BTL) using a scaling FFAG type magnet which has a field profile of ${y}^{k}$, where $y$ is the horizontal coordinate and $k$ is the magnetic field index. This FFAG-BTL has very large momentum acceptance and optical functions that, practically speaking, do not depend on momentum. We also designed a dispersion suppressor at the end of the line by combining unit cells with a different field index $k$ so that the momentum dependence of orbit location should be eliminated at the exit. An obvious application of this design is the BTL after an FFAG accelerator to a patient in a hadron therapy facility or to a target in general. This could be an alternative to the conventional BTL with solenoids or quadrupoles because of the strong focusing nature of a quadrupole and the large momentum acceptance like a solenoid.
Highlights
An ordinary synchrotron with an alternating gradient focusing structure cannot transport a beam with large momentum spread because the sizable dispersion function causes a significant shift in the closed orbit of the off momentum particle and it hits a vacuum chamber
An fixed field alternating gradient (FFAG) has the potential to deliver a beam with different momentum in a pulse by pulse mode. Since such an accelerator has the capability of fast switching beam momentum, the following beam transport line (BTL) should have a large momentum acceptance as well
A BTL with a large momentum acceptance is desirable as a dump line of a high intensity synchrotron where a beam that has tripped during acceleration goes through
Summary
An ordinary synchrotron with an alternating gradient focusing structure cannot transport a beam with large momentum spread because the sizable dispersion function causes a significant shift in the closed orbit of the off momentum particle and it hits a vacuum chamber. The scaling fixed field alternating gradient (FFAG) accelerator keeps the closed-orbit shift depending on particle momentum very small. The focal length is scaled with only the average orbit radius so that the tune is independent of momentum This can be done with a magnetic field profile which has steeper gradient toward a large orbit radius. The proof of principle proton FFAG demonstrated a 1 kHz operation, which is impossible with an ordinary synchro- Since such an accelerator has the capability of fast switching beam momentum, the following beam transport line (BTL) should have a large momentum acceptance as well. The idea of using the scaling FFAG optics, where the closed-orbit shift depending on particle momentum is very small and the chromaticity is zero for a large momentum range, for a BTL has been around for years, there has been no design up to now. III, we will describe a way of obtaining the dispersion suppression
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