Double rf systems are critical for achieving the parameters of fourth-generation synchroton light sources. These systems relax stochastic collective effects but also introduce instabilities, such as Robinson and periodic transient beam loading (PTBL) instabilities, which depend heavily on the cavity parameters. In the framework of the SOLEIL II project, we demonstrate how we can use semianalytical algorithms to predict and analyze these instabilities, in conjunction with multibunch tracking simulations, to optimize rf cavity parameters in high-dimensional parameter spaces, thereby maximizing the Touschek lifetime. Finally, the expected performances of the double rf system, using the optimized normal conducting passive harmonic cavity parameters, are presented.

SOLEIL, the French 2.75 GeV third-generation synchrotron light source, continues to operate as a high-performance research infrastructure supporting both academic and industrial communities. This contribution reviews the 2025 operational achievements, with emphasis on accelerator stability, availability, and the delivery of reliable photon beams to 29 beamlines. Key performance indicators and operational trends are presented, together with an analysis of major events and the mitigation strategies implemented to strengthen machine robustness. Significant progress in addressing component obsolescence is reported, including advances in BPM electronics, RF systems, and the global control-system upgrade. Updates on the LINAC modernization programme are provided, along with the use of the current SOLEIL accelerator as an integrated test platform to validate critical technologies for the forthcoming facility upgrade. Preparatory activities for SOLEIL II are outlined, including a progress report for bunch-per-bunch transverse feedback system, development and preparation to the qualification of new 352 MHz low-HOM RF cavities, the 1.4 GHz fourth-harmonic system, and upgraded control architectures, highlighting the technical readiness for the next-generation machine.