It’s all about stability: research in the accelerator division at MAX IV

It’s all about stability: research in the accelerator division at MAX IV

The Accelerator Development group at MAX IV is constantly researching and testing solutions to improve the quality and stability of the electron beam. A new article published in Physical Review Accelerators and Beams builds upon years of research and previous publications and provides new insights on the impact of an effect called ‘transient beam loading’ on beam stability.

In synchrotron facilities, electrons are stored in dedicated storage rings. MAX IV has two such storage rings, one at 3.0 GeV energy, and one at 1.5 GeV. X-ray light is produced by “shaking” the electrons as they circulate in the storage ring, which causes the electron beam to lose some of its energy in the form of photons.

In order to maintain a constant energy level in the storage ring, the energy lost by electrons in producing synchrotron light is returned using electromagnetic standing waves produced in the radiofrequency (RF) cavities. There are several such RF cavities along MAX IV’s storage rings, and they allow to maintain a constant, high-quality electron beam while extracting photons to produce X-ray light.

Further to restoring the energy of the circulating electrons, the electromagnetic standing waves produced in the RF cavities “pack” the electrons together in so-called bunches. The number of bunches that can be stored is determined by the frequency of the electromagnetic standing waves and the size of the ring. In the 3.0 GeV ring, up to 176 electron bunches can be stored simultaneously.

Measured electron-bunch shapes in the large ring at MAX IV Laboratory compared with predictions made using a computationally-efficient method developed at MAX IV.

Hence, the RF cavities are crucial for maintaining a constant energy in the storage ring, but they come with some side effects that can cause instability in the electron beam. The RF cavities present some unwanted electromagnetic resonances called higher order modes (HOMs), which interact with the electron bunches. The electron bunches can excite the HOMs, and through the HOMs, the electron bunches interact with each other. Finally, some HOMs at specific frequencies can cause the electron bunches to oscillate in energy, ultimately causing the electron beam to become unstable.

To prevent these instabilities in the electron beam of the 3.0 GeV ring, one of the strategies of the Accelerator Development team is to avoid having all electron bunches at the same charge level. More specifically, about ten out of the 176 electron bunches are filled with a lower amount of charge. Thanks to a process called ‘transient beam loading’, this difference in charge causes the different electron bunches to oscillate at different frequencies, which in turns, helps preventing the bunches from becoming unstable. At MAX IV, this stabilising effect is further enhanced thanks to the use harmonic cavities, which are used to lengthen the electron bunches.

Oscilloscope trace of the bunches in the MAX IV large ring with ten lower-charge bunches towards the left.

The Accelerator Development group at MAX IV is intensively involved in understanding the transient beam loading and in creating solutions that can predict its effects accurately and efficiently, and without the need for intense computing power. These activities led to a publication in 2018 which is now followed up by this new study exploring the impact of the transient beam loading on beam stability. All this knowledge is built upon even earlier research at MAX IV on the effects of harmonic cavities on the electron bunch shape.

Read the articles here

F. J. Cullinan, Å. Andersson, and P. F. Tavares
Harmonic-cavity stabilization of longitudinal coupled-bunch instabilities with a nonuniform fill
Phys. Rev. Accel. Beams 23, 074402

T. Olsson, F. J. Cullinan, and Å. Andersson
Self-consistent calculation of transient beam loading in electron storage rings with passive harmonic cavities
Phys. Rev. Accel. Beams 21, 120701

Pedro F. Tavares, Åke Andersson, Anders Hansson, and Jonas Breunlin
Equilibrium bunch density distribution with passive harmonic cavities in a storage ring
Phys. Rev. ST Accel. Beams 17, 064401