The BESSY II accelerator complex of pre-injector, booster synchrotron, storage ring and related technical infrastructures is in its 25th year of operation. Modernisation and maintenance measures for the accelerator complex and for the instruments are the backbone for the reliable and stable operation of the facility. Some of these measures are simply intended to replace components which are becoming obsolete and no longer maintainable. Others are not just replacements but aim to bring a new quality to user operation in terms of higher reliability and stability, or improved or not previously accessible beam parameters. Moreover, some of these projects will also pave the way for the development and realization of technologies needed for the new BESSY III facility. Among the planned measures are: - A Cryogenic Permanent Magnet Undulator CPMU20 for the SoTeXS beamline - New superconducting 7T wavelength shifters (WLS) – maintaining hard X-rays for BAM, PTB and the MX community, including measures to improve the cryogenics of CPMU devices - Normal-conducting active higher-harmonic-cavities (HHC) for improved lifetime control and longitudinal phase space shaping (including new digital low-level RF) - Permanent magnets for improved energy efficiency - A BESSY II digital twin for accelerator operation and development - General preservation and modernisation measures
