The “Pre-Conceptual Design Report” (preCDR) of the BESSY III facility (https://doi.org/10.5442/r0004) has been finalized at the end of August 2022 and reviewed by a Project Advisory Committee beginning of September 2022. In this paper, we give a status report of the BESSY III facility project and will discuss aspects of lattice design, technical specifications, initial developments and a first estimate of power consumption compared to BESSY II.

The superconducting radio-frequency photoelectron injector (SRF photoinjector), now under commissioning at the SEALab accelerator test facility, has the potential to cover a fast area of beam parameters. Electron bunches from fs to ps length, with fC to nC charge can be accelerated to a couple of MeV beam energy. The legacy from the energy-recovery linac (ERL) test facility bERLinPro, the foundation of SEALab, allows us to operate the SRF photoinjector at very high repetition rate, with energy recovery (ERL), in a sustainable way for fundamental accelerator research into novel, energy-efficient electron accelerators. In this paper preparatory work for two applications is detailed. One is the use of the SRF photoinjector as a direct beam source for ultrafast scattering experiments with high 6D coherence, the other are experiments towards an ERL application for high-energy physics at high average current.

The lattice design process for BESSY III is based on a systematic & deterministic approach where sub-structures of the MBA lattice are analyzed and optimized before the lattice is composed. During this process, 5 standardized Higher-Order-Multi-Bend-Achromat (HO-6MBA) lattices were developed utilizing different combinations of homogeneous and gradient bends in the unit and the dispersion suppression cell. All lattices yield basically the same emittance, momentum compaction factor, working point, maximal field strengths, and drift lengths. Therefore, they are equivalent to the linear optics. This enables us to attribute any differences in their nonlinear behavior to the specific lattice structure. A detailed description and analysis of the trade-offs of these standardized lattice structures are given. Based on this analysis, the choice of the BESSY III lattice type is motivated.

Permanent Magnet (PM) based bending magnets are state-of-the-art concepts to gain stable beam operation and to reduce the power consumption of the magnetic system in an accelerator. This is even more true in injector and beam transport beamlines with fixed beam parameters and low repetition rates. An example is the B2PT magnet in the BESSYII transfer beamline between booster and storage ring. It is the last dipole magnet for the final 7.8 deg bending into septum. This one meter long, compact, high current dipole will be replaced by three 300mm long Variable Permanent Hybrid Magnets. They combine a PM driven strong and stable magnetic field with a small field variability via compact corrector coils. With this new magnet we can reduce fringe fields and vibrations next to stored beam, as well as the total power consumption of the injector by almost 30 kW. In this paper, the design and construction process of the new B2PT magnet will be presented.

Superconducting radio-frequency (SRF) photoinjectors offer a broad range of electron beam parameters and are therefore suitable for many applications such as energy recovery linac (ERL) driven lightsources, particle colliders, or for ultrafast electron scattering experiments. We are now nearing completion of the setup a SRF photoinjector with a SRF gun and SRF booster linac at the SEALAb accelerator test facility at HZB. The goal here is to realize an electron source with high brightness and high average current. In this work, the general planning for the commissioning phase, the operation modes and investigations into the diagnostic tools for achieving the expected beam parameters will be presented. The focus of the instrumentation is to provide information on the beam parameters at large dynamic range and on mechanisms for beam loss generation.