Accelerator Upgrade

Emittance describes how “spread out” the electrons are as they circulate in the storage ring. A lower emittance means the X-ray beams produced are sharper, more parallel, and more laser-like. Reducing emittance is analogous to replacing a flashlight with a precision spotlight: the same total brightness, but concentrated into a much smaller, cleaner beam. For experiments, this translates into more photons hitting the sample per unit area, higher coherent flux, and beams that can be focused to smaller spots — all of which directly improve measurement speed, spatial resolution, and detection sensitivity.

MAX 4^U targets a natural bare lattice emittance (i.e., for the lattice without Insertion Devices and at zero current) below 75 pm·rad, with the aspirational goal of reaching even lower emittance.
Exploration of the complex phase space of accelerator parameters has been underway since 2024. A lattice candidate at around 65 pm·rad was identified at the conclusion of the Conceptual Design Phase in 2025 and will be further developed during the technical design phase. The MAX IV 3 GeV ring current natural emittance is 328 pm·rad.

In 2016, MAX IV achieved 328 pm·rad — three to twenty times better than any other lightsource in the world at the time. By the early 2030s, competing facilities will have emittances ranging from 80 to a few hundred pm·rad. With MAX 4^U targeting below 75 pm·rad, MAX IV will be the very best medium-energy (i.e., below 4 GeV) storage ring in Europe and the world.

No.
Unlike traditional third-to-fourth-generation upgrades, MAX 4^U is a “surgical” upgrade of an already fourth-generation lightsource. It is, in fact, the first upgrade of a fourth-generation source.
It does not involve replacing the entire storage ring, relocating beamlines, or major infrastructure reconstruction. The goal is to achieve major performance gains while minimizing disruption to users.

MAX 4^U leverages the strong foundation of the existing fourth-generation design. A targeted upgrade allows major performance gains with significantly less disruption, cost, and infrastructure replacement than a full rebuild.

The Conceptual Design Report (CDR) was concluded in 2025 and is available on the MAX 4^U webpage. Since January 2026, the detailed technical lattice design has been under way.
What is confirmed is that the upgrade achieves its performance gain primarily by modifying the magnetic lattice of the R3 ring to reduce horizontal emittance, rather than by wholesale replacement of the machine.

We are currently considering the implementation of an advanced injection scheme that would increase injection transparency in top-up mode. However, we do not plan to adjust the filling pattern.