Key competencies for BESSY III: Undulators

HZB undulators are not only used in the BESSY II synchrotron lightsource. They also enjoy great popularity at other large-scale research facilities. HZB has already supplied undulators to renowned centres such as the Paul Scherrer Institute in Switzerland, DESY in Hamburg, and MAX IV in Sweden. Undulators are key components in the operation of synchrotron lightsources.

The electrons pass through complex magnetic structures that force them into an undulating path. The undulating movements of these high-energy electrons repeatedly generate high-energy photons of great brilliance - synchrotron radiation.

But why are the HZB undulators so popular? “The HZB is one of the few facilities that has developed, built, and also operated undulators from beginning to end. We were there during the planning of BESSY II”, says Johannes Bahrdt, head of the Undulator Department. ”25 years of experience have put us at the forefront of theoretical calculations and the design of undulators. An additional point: We maintain direct contact with users, know what they want, and develop new design concepts for undulators that exactly meet their needs.”

This was also the case with the CPMU-17 undulator installed at the BESSY II storage ring in September 2018. The EMIL laboratory project team approached Bahrdt's department during the EMIL planning phase. "The unique feature of the EMIL laboratory is that researchers are able to examine their material samples over a very wide energy range”, says Marcus Bär, head of the Interface Design Department and supervisor of the SISSY I and SISSY II measuring stations in the EMIL laboratory. ”But to accomplish this, we needed two different beam tubes: one for the soft X-ray light and one for the somewhat higher-energy photon range.“

For Bahrdt's team, this meant developing two undulators with completely different magnetic designs and construction. In the CPMU-17 undulator, the magnetic devices are located in a vacuum chamber and are cooled with liquid nitrogen (cryogenic in-vacuum undulator). This allows significantly stronger magnetic fields to be generated for deflecting the electrons. ”Our newly developed undulator provides BESSY II users in the tender-X-ray range with high-brilliance synchrotron radiation for the first time and gets the maximum photon energy out of our low-energy-range storage ring”, says Bahrdt.

BESSY II is operated at an electron energy of 1.7 GeV, which is optimal for investigations with soft X-ray light. Because the electron energy at BESSY II is comparatively low, much stronger undesirable interactions with this machine can occur than with others elsewhere. “We go to great effort to calculate these effects for every undulator and compensate for them”, says Bahrdt. “We understand theory and the construction techniques better than almost anyone else.” This knowledge is also indispensable for the future BESSY III that will likewise specialise in the soft X-ray range.

In the meantime, Johannes Bahrdt is looking forward to pushing the limits of what is physically possible. In the latest project, he and his team want to combine the advantages of two different types of undulators by merging the APPLE undulators with in-vacuum technology. This newly developed undulator will supply the RIXS measuring station and X-ray microscopy at BESSY II with variable-polarity light. The project is being funded ATHENA Accelerator Project of the Helmholtz Society and will lend a lot of visibility to undulator development at the HZB.

The next step is designing a cryogenic in-vacuum APPLE undulator that will be used in a laser-plasma accelerator at DESY. “We are working on magnetic devices suitable for round electron beams such as those generated in free-electron lasers (FELs) and diffraction-limited storage rings. At the same time, we are developing new technologies, including a soldering process to connect magnets for use in vacuum. We learn a great deal in these projects that is applicable for BESSY III”, Johannes Bahrdt is certain.