Technical design study. BESSY VSR. Variable pulse length Storage Ring. Upgrade of BESSY II

BESSY-VSR is a novel approach to create in the Storage Ring BESSY II long and short photon pulses simultaneously for all beam lines through a pair of superconducting bunch compression cavities. Pulse-picking schemes will allow each individual user to freely switch between high average flux for X-ray spectroscopy, microscopy and scattering and picosecond pulses up to 500 MHz repetition rate for dynamic studies. Thus BESSY-VSR preserves the present average brilliance of BESSY II and adds the new capability of user accessible picosecond pulses at high repetition rate. In addition, high intensities for THz radiation with intrinsic synchronization of THz and X-ray pulses can be extracted from BESSY-VSR. For the scientific challenges of quantum materials for energy, future information technologies and basic energy science BESSY-VSR is the multi-user Synchrotron Radiation facility that allows with the flexible switching between high repetition rate for picosecond dynamics and high average brightness to move classical 3rd generation Synchrotron Radiation science from the observation of static properties and their quantum mechanical description towards the function and the control of materials properties, technologically relevant switching processes and chemical dynamics and kinetics on the picosecond time scale. Strategic relevance of BESSY-VSR for science with photons BESSY-VSR creates for the highly productive Synchrotron Radiation community a uniquely attractive multi user storage ring adding the soft X-ray picosecond dynamics at MHz repetition rate. In particular investigations on reversible dynamics and switching in molecular systems and materials are accessible in a non destructive way. The investigations with X-rays from BESSY-VSR are highly complementary and compatible to dynamic studies conducted by users with optical lasers at their home universities and laboratories. Technologically, the employed superconducting bunch compression cavities in BESSY-VSR are a direct synergy to the CW superconducting cavities developed for bERLinPro. Unique science opportunities by BESSY-VSR for quantum materials for energy, future information technologies and basic energy science BESSY-VSR users will excel to correlate chemical function with molecular dynamics on multidimensional potential energy surfaces and to determine principles underlying functional materials for reversible processes such as switching. The ability to determine through high quality Synchrotron Radiation Spectroscopy, Scattering and Microscopy the geometric and electronic structure as well as the nanoscale composition is the first step to then create and investigate transient states and their picosecond dynamics that govern materials' function. Here both X-ray probes and IR/ THz pulses from BESSY-VSR uniquely enable THz-EPR experiments with picosecond time resolution to reveal the structure-dynamic-function relationship of functional molecular systems.