Multi-bend achromat
A multi-bend achromat (MBA) is a type of magnetic lattice used in synchrotron storage rings to achieve extremely low electron beam emittance. By increasing the number of bending magnets (dipoles) within a single achromatic cell beyond the traditional two (DBA) or three (TBA), MBA lattices allow fourth-generation light sources to reach the "diffraction limit," producing X-ray beams with unprecedented brilliance and coherence.[1]
Background and theory
The horizontal emittance ($\epsilon_x$) of an electron beam in a storage ring scales with the third power of the bending angle ($\theta$) of the dipole magnets: $$\epsilon_x \propto \theta^3 \propto \frac{1}{N^3}$$ where $N$ is the number of dipoles in the ring. By increasing $N$—effectively breaking a few large bends into many smaller ones—the emittance can be reduced by orders of magnitude.[2]
Standard 3rd-generation sources typically use DBA lattices. In contrast, 4th-generation sources utilize MBA designs, such as 7-bend (7BA) or 6-bend (H6BA) achromats, to reach emittance levels below 100 pm·rad.
History
The concept of the multi-bend achromat was proposed in the early 1990s by Dieter Einfeld and Mark Plesko as a means to reach "diffraction-limited" storage rings (DLSR).[1] However, technical challenges—including the need for extremely strong quadrupole and sextupole magnets to manage the resulting chromaticity—delayed its practical implementation for two decades.
The first facility to successfully implement a full MBA lattice was MAX IV in Sweden, which began operation in 2016. This was followed by the ESRF-EBS (Extremely Brilliant Source) upgrade in France in 2020, which introduced the Hybrid Multi-Bend Achromat (HMBA) lattice, further optimizing the design by adding "dispersion bumps" to improve beam stability.[3]
Design variations
| Type | Description | Example Facilities |
|---|---|---|
| Standard MBA | Simple repetitive cells of dipoles and quadrupoles. | MAX IV (7BA) |
| Hybrid MBA (HMBA) | Features longitudinal gradient bends and dispersion bumps for easier chromaticity correction. | ESRF-EBS |
| Modified Hybrid MBA | Incorporates mid-cell straight sections for additional insertion devices. | Diamond-II (Planned) |
Facilities using MBA lattices
- MAX IV (Lund, Sweden) – First 4th-generation source (7BA).
- ESRF-EBS (Grenoble, France) – First high-energy HMBA upgrade.
- Sirius (Campinas, Brazil) – 5BA lattice.
- APS-Upgrade (Argonne, US) – 7BA lattice (operational 2024).
- SPring-8-II (Hyogo, Japan) – Planned upgrade.
References
- ↑ 1.0 1.1 Einfeld, Dieter; Plesko, Mark (2014). "First multi-bend achromat lattice consideration". Journal of Synchrotron Radiation. 21 (5): 856–861. Bibcode:2014JSynR..21..856E. doi:10.1107/S160057751401193X. PMC 4181637. PMID 25177977.
- ↑ Martensson, N.; Eriksson, M. (2018). "The saga of MAX IV, the first multi-bend achromat synchrotron light source". Nuclear Instruments and Methods in Physics Research Section A. 907: 97–104. doi:10.1016/j.nima.2018.03.010.
- ↑ Raimondi, P. (2016). "The ESRF Extremely Brilliant Source". Synchrotron Radiation News. 29 (6): 8–15. doi:10.1080/08940886.2016.1244460.
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