Author:
Ren Zhiliang,Wei Bingfeng,Yang Penghui,Huang Yuejing,Xu Hongliang,Bai Zhenghe
Abstract
Abstract
Diffraction-limited storage rings (DLSRs) based on
multi-bend achromat (MBA) lattices are being developed towards much
lower emittances. For instance, for the future development, MAX IV
designed a 19BA lattice to replace the present 7BA lattice,
achieving an emittance of 16 pm·rad. While this 19BA lattice,
with a significantly increased number of unit cells, requires
extremely strong quadrupoles and sextupoles, which will result in
very small vacuum chambers and can thus significantly increase beam
coupling impedance and potentially enhance beam instabilities. This
issue raises a question of how we can reduce magnet strengths while
achieving the emittance goal. In this paper, based on a simplified
model where an MBA lattice is considered to consist of a number of
identical unit cells, emittance optimization is numerically studied
under magnet strength limitation. The studied results show that for
a given arc length and emittance goal, choosing an appropriate
number of unit cells with relatively large horizontal cell tunes can
significantly reduce magnet strengths. As a practical example, an
11BA lattice with relatively large unit cell tunes is designed and
compared with a 17BA lattice with smaller unit cell tunes. The two
lattices have roughly the same emittance with the same energy, ring
circumference and number of periods, while the magnets of the 11BA
lattice are significantly weaker.
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