New cavities successfully installed
The new cavities during their installation. © C. Jung /HZB
A glimps at the water circuits. © C. Jung / HZB
At the heart of BESSY II are four cavities, hollow resonators, providing the energy that electrons in the storage ring re-absorb after they have released it as light packets. The old cavities were still from the 1970s and were employed at DESY in Hamburg, then at BESSY I, and finally at BESSY II beginning in 1998. “However, the limit of their operating life has been reached”, says Dr. Wolfgang Anders from the HZB Institute of SRF – Science and Technology. Anders, an expert in the field, was responsible for replacing two of the four old cavities with new units during the summer shutdown.
The new cavities have been developed as an EU Project under the supervision of HZB. They are constructed of copper, as before, but have a somewhat more complex shape that is optimised to effectively avoid exciting undesirable oscillations, known as “higher-order modes”. These higher modes are excited by high beam energies and can lead to instabilities.
“The new cavities are significantly more robust and can store up to 80 kilowatts – twice as much as the old cavities. As a result, the requirements for cooling have increased as well”, explains Anders, pointing out the multitude of coolant pipes around the new cavities.
The beam energy at BESSY II is presently still restricted because the vacuum in the new cavities is still improving slowly. “We will be normalising the vacuum conditions over Christmas so that we can operate BESSY II at full beam energy afterwards”, says Anders.
Both of the other old cavities will be replaced during the 2015 summer shutdown so that all the advantages of the new cavities can come into play.
- BESSY II: New sample environment allows glimpse into thermocatalytic processesA novel measurement cell allows, for the first time, soft and hard X-ray investigations under high pressures of up to 20 bar and temperatures of up to 400°C. This provides new insights into thermocatalytic processes, such as the Fischer-Tropsch synthesis for producing synthetic fuels. The development of the measurement cell is considered a significant achievement within the Care-O-Sene project.
- Magnetic imaging: Micro-flowers increase the local magnetic fieldMaterials with magnetic nanostructures have many potential applications such as in spintronics. To explore such materials, nanoscale magnetic-sensitive imaging techniques are very useful, but up to now only weak magnetic fields could be applied during the imaging process. Now an international collaboration led by Dr. Sergio Valencia, HZB, has developed an approach that overcomes this limitation. The team designed tiny magnetic flux concentrators (MFCs), into which the sample is placed. The geometry of the MFCs resembles a flower with a number of petals which focus the applied magnetic field into its center. This greatly expands the magnetic field range available during imaging, and so the range of magnetic systems that can be investigated. The micro-flowers, enhancing magnetic fields locally, can find application in different nanometric magnetic microscopy techniques.
- Disorder creates new properties in compound semiconductorsAn international research team has demonstrated that the intrinsic disorder of the compound semiconductor CuInSnS₄ can be exploited to influence its optical properties. While the atomic vibrations also sense the local disorder, their response is averaged over many different local environments and therefore appear isotropic, as expected for a cubic crystal. In contrast, the optical excitations, known as excitons, are much more sensitive to the local arrangement of atoms. Surprisingly, they show a direction-dependent optical response even though the average crystal structure is cubic. These findings shed new light on the relationship between disorder and material properties, opening up new options for targeted 'disorder engineering' in optoelectronic and photocatalytic devices.