Study of beam coupling impedance for SOLEIL II and validation of instability models on SOLEIL

Situated at the heart of the Paris-Saclay cluster, approximately 20 km from Paris, SOLEIL is the French national synchrotron radiation source. In this large research infrastructure, the scientific experiments exploit the light from synchrotron radiation from the infrared to hard X-rays. Since 2008, SOLEIL has been serving many research fields in physics, biology and chemistry with applications in Materials Science, Structural Biology, Pharmaceuticals, Environmental & Earth Sciences and Cultural Heritage. SOLEIL, overseen by the CNRS and the CEA, provides its personnel with a multidisciplinary and internationally renowned working environment.

Duration: 4 to 6 months (from March/April 2025)

Level: Master 1 or Master 2

Contact: Alexis Gamelin (alexis.gamelin@synchrotron-soleil.fr)

Key words: storage ring, collective effects, impedance, simulation, electromagnetism, beam measurement

Detailed presentation:

The SOLEIL synchrotron is a third-generation light source that produces X-rays from an electron storage ring since 2006. The SOLEIL facility is currently undergoing an upgrade plan called SOLEIL II towards a fourth-generation light source to meet the needs of new research domains of the next decades. The SOLEIL II’s electron beam natural emittance will be drastically reduced to about 83 pm rad, compared to 3.9 nm rad in the present machine, to reach the diffraction limit in the soft/tender X-ray range. However, the performance of SOLEIL II could be limited by the interaction of the beam with itself and with its environment. These phenomena, whose impact increases with the intensity of the beam, are called collective effects. To reach an ultra-low emittance, very strong magnets will be needed, which requires a significant reduction in the dimensions of the vacuum chamber and will thereby enhance the impact of these collective effects. Induced disturbances can cause heating of the vacuum chamber, degradation of the beam properties and beam instabilities that can severely limit the total current. The proposed internship consists of two parts:

• As SOLEIL II design is getting closer to full mechanical feasibility, each vacuum chamber of the new synchrotron needs to be verified to check its possible impact on the beam coupling impedance of the machine. The beam coupling impedance quantifies the interaction between the beam and its environment. The student will take part in the simulation study of the beam coupling impedance using 2D and 3D electromagnetic codes for the SOLEIL II vacuum chambers.

• Efforts have been made in the last years to build improved instability models using tracking code simulating the multi-bunch beam dynamics for the SOLEIL II accelerators. These models have been benchmarked mostly against other tracking codes or theories. Using the impedance model from SOLEIL construction, the student will simulate the present SOLEIL beam dynamics to compute the instability threshold of different instabilities (including different effects such as intra-beam scattering and space charge) to validate the new tracking models. An experimental validation of the obtained results on the current ring of SOLEIL will be conducted by the student on a dedicated machine shift.