ALBA is working on the upgrade project that shall transform the actual storage ring, in operation since 2012, into a 4th generation light source, in which the soft X-rays part of the spectrum shall be diffraction limited. The project was launched in 2021 with an R&D budget to build prototypes of the more critical components. The storage ring upgrade is based on a MBA lattice which has to comply with several constraints imposed by the decision of maintaining the same circumference (269 m), the same number of cells (16), the same beam energy (3 GeV), and as many of the source points as possible unperturbed. At present, the lattice optimization, iterating with the technical constraints of space and performance, is ongoing. This paper presents the status of the project, with the present proposed lattice, the proposed design for magnets, vacuum chambers and girders, the proposed RF system with fundamental and harmonics cavities, and the general context of the upgrade.

A collaboration agreement between the ALBA, DESY and HZB institutions was signed on 2021 in order to commission the 3rd harmonic normal conducting, HOM damped, active cavity designed and prototyped by ALBA. The cavity prototype arrived to ALBA in December 2021, and successfully passed the low power RF and vacuum tests. Afterwards, in January 2022 it was sent to HZB and mounted in the SUPRALAB@HZB, in the HoBiCat bunker for bead-pull measurements and high-power conditioning. Finally, in May 2022 the cavity was installed in the BESSY-II ring for test with beam. In this contribution we summarize and highlight the major results after two years of commissioning with beam including the lengthening capability of the cavity for single bunch and homogeneous filling pattern, lifetime increase measurements, HOM damping capability and transient beam loading effects due to the presence of a gap in the filling pattern.

A digital direct RF feedback (DDRF) has been implemented in the DLLRF of the ALBA synchrotron light source in order to mitigate the beam loading effects of the beam. Specifically, with the DDRF implemented in the main cavities, the tune shift due to beam loading can be minimized and therefore the DC-Robinson instability can be mitigated. It can also be used to minimize voltage drops in case of transients after a cavity loss during the operation. Also, for the next generation machine ALBA-II and applied to the active 3rd harmonic system, it can be used to fight against AC-Robinson instability induced by the harmonic cavities, increasing the damping rate of the fundamental CBI mode, and reducing the transient beam loading effects. In this contribution we present measurements with beam of the DDRF applied to ALBA main cavities, demonstrating a reduction of a factor two in the effective impedance of the cavity as seen by the beam.