Accelerator Physics: First electron beam in SEALab
The small spot on the screen is the electron beam. The large cloud to its left is caused by the reflection of the laser beam from the photocathode. © SEALab/HZB
The SEALab team has achieved this breakthrough after years of work. Here you can see the commissioning team. © SEALab/HZB
The SEALab team at HZB has achieved a world first by generating an electron beam from a multi-alkali (Na-K-Sb) photocathode and accelerating it to relativistic energies in a superconducting radiofrequency accelerator (SRF photoinjector). This is a real breakthrough and opens up new options for accelerator physics.
This success paves the way for the further development of superconducting radio-frequency accelerators (SRF photoinjectors) for high-brilliancy electron sources. The achievement holds significant potential for applications in free-electron lasers, energy recovery linac (ERL) class accelerators, detector development and ultrafast electron scattering experiments (UED).
Years of dedicated work, first in the bERLinPro project and then in the SEALAB team, have been invested to achieve this success. There were numerous challenges along the way, including delays due to the COVID-19 pandemic and the cyber attack. Despite this, the team has made great progress. The successful test generated an average current in the microampere range at a repetition rate of 1 MHz, demonstrating the viability of the sodium-based photocathode in combination with SRF acceleration.
Axel Neumann, SEALab project manager, emphasises: ‘This great success is the result of many dedicated individuals who contributed to bERLinPro and SEALab over the past years, often under high levels of stress. We also thank all former team members who were involved in the original project.’
Thorsten Kamps, deputy project manager, now sees the fruits of the intensive work for the photoinjector: ‘We have completely revamped the preparation and characterisation of photocathodes in recent years and are now seeing the success. This will have a significant impact on similar projects.’
With this successful test, the SEALab team has demonstrated that it is possible to use a robust multi-alkali photoemissive source to accelerate an electron beam in an SRF photoinjector to relativistic energies and at a high repetition rate. These results could help to further improve the performance of the next generation of electron injectors. The SEALab team will now also investigate the different beam parameters to expand the possibilities of SRF photoinjectors.
- Perovskite triple-junction solar cells: Even more efficient with GO/SAM bilayersPerovskite semiconductors efficiently convert sunlight into electrical energy; they are also inexpensive and extremely lightweight. A team at HZB has developed a triple-junction solar cell comprising different perovskite semiconductors, with a novel bilayer of graphene oxide (GO) and a self-assembled monolayer (SAM) as the hole conductor. This bilayer significantly increases both efficiency and long-term stability. The efficiency of the novel perovskite triple-junction solar cell is 27.3% and shows hardly any decline even after more than 770 hours of operation. The study has been published in the renowned journal Joule.
- 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.
- CIGS-perovskite tandem cell achieves record efficiency of 25.5 %A Berlin-based team from HZB and Center for the Science of Materials Berlin (CSMB) at the Humboldt-Universität zu Berlin has set a new record for a tandem solar cell. Using a combination of a CIGS semiconductor layer and perovskite, along with several optimised intermediate layers, they were able to convert 25.5% of sunlight into electrical energy. The previous record for this combination of materials and this size of cell stood at 24.6%. The new record has been certified and is visible in the prestigious Solar Cell Efficiency Tables (the "Green Tables"), which serve as the definitive ledger for the global photovoltaic community.