Accelerator research
Inside the electron storage ring BESSY II, where electron bunches fly at nearly the speed of light. Many components are needed to generate the brilliant synchrotron radiation. Foto: HZB/M. Janda
Synchrotron radiation sources are used avidly by researchers in just about all fields. In biology, the environmental sciences, chemistry or physics – researchers everywhere are interested in analysing the smallest of atomic structures. This special synchrotron light is generated using beams of fast-moving particles that are guided through special magnetic systems. At BESSY II, the particles accelerated to these high speeds are electrons.
Several groups are working on sophisticated accelerator concepts that are receiving a lot of attention in specialist circles. This has made Berlin into an important hotspot of internationally networked accelerator research.
Researchers are breaking new technological ground in the development of novel accelerators. The limits of feasibility are being continually redefined: researchers are now implementing accelerator concepts that were barely imaginable just a few years ago.
BESSY VSR (variable-pulse-length storage ring)
One of these innovative projects is VSR Demo. In addition to the approximately 15-picosecond-duration light pulses that BESSY II currently generates, BESSY VSR will also produce pulses of a mere 1.5-picosecond duration – at full brightness. This opens up the possibility of observing extremely fast chemical and physical processes. But it also makes it an enormous technological challenge, since a portion of the electron bunches will have to be severely compressed using superconducting cavities.
Superconducting niobium cavities can produce a beam of extremely high stability and quality. Illustration: HZB/E. Strickert
Superconducting cavities
For the future projects bERLinPro and BESSY VSR, teams are developing cavities made of superconducting niobium. Cavities are hollow-chamber resonators. As the accelerated electrons fly through them, the electron bunches can recover some of the energy they have given off as X-ray light. Superconducting niobium cavities can thus help to generate a beam of extremely high quality and stability. This is where HZB is doing pioneering work. Physicists are developing the cavities with high currents in mind and are working out how to suppress unwanted oscillations efficiently.
The HZB is not only researching new types of undulators, they are also being built here.
Undulators
HZB is one of the few establishments that have developed, built and operated undulators “from scratch”. Inside undulators, electrons fly through complex magnetic structures and generate synchrotron radiation at high brightness. New undulators will provide users with even better X-ray light that has the exact properties they need. In the scope of the Helmholtz project “ATHENA”, HZB is developing APPLE undulators (Advanced Planar Polarised Light Emitters) in combination with in-vacuum technology that will provide variably polarised light for RIXS experiments and X-ray microscopy at BESSY II. Planned after that is a cryogenic in-vacuum APPLE undulator that will be used in a laser-plasma accelerator at DESY.
A synchrotron source point image of a bending magnet of the Twin Orbit modus. The first “Twin Orbit User Test week” at BESSY II in February 2018 was a big success. Copyright: HZB
New operating mode
The accelerator physicists at HZB are currently developing a new operating mode for synchrotrons, in which the electron bunches will circle the storage ring in two different orbits. This mode uses a concept known as “Transverse Resonance Island Buckets” or TRIBs. The HZB physicists already have succeeded in creating a stable mode with two different orbits. The first experiments at the Metrology Light Source, at BESSY II and at MAX IV in Lund, Sweden, have all been successful. The new operating mode effectively provides two synchrotron radiation sources in a single ring, by which the needs of different user groups can be served at the same time. The experiments are also helping to determine whether it would make sense to equip a future light source BESSY III, specifically, with the option to use such TRIBs modes.
These magnet devices are used for testing bERLinPro. Photo: Michael Setztpfandt
Linear accelerator with energy recovery and the way to BESSY III
With the future project bERLinPro, HZB experts are building a prototype for an accelerator with energy recovery. They are testing a new accelerator concept for photon sources. For example, they are building novel superconducting high-frequency cavities to perfectly control the energy and shape of the electron bunches. In addition, they are developing another key component: an electron source (gun) to generate a highly brilliant beam. Electrons have already been generated and accelerated in a test system from the interaction of cathode, laser pulse and electric field in the cavity. This research work is carried out together with many partners from Germany and abroad. With bERLinPro we systematically gain experience for the development of accelerator components. We need this knowledge in order to develop and test new ideas for a successor device - the new BESSY III.