Milestone for bERLinPro: photocathodes with high quantum efficiency
Photocathode in superconducting photoinjector system. © J. Kühn/HZB
The superconducting photoinjector system (1): The photocathode (3) is excited by a green laser (2) and emits electrons (4) which are accelerated in the superconducting RF cavity. © Britta Mießen
Photocathode after its production in the preparatory system. © J. Kühn/HZB
A team at the HZB has improved the manufacturing process of photocathodes and can now provide photocathodes with high quantum efficiency for bERLinPro.
Teams from the accelerator physics and the SRF groups at HZB are developing a superconducting linear accelerator featuring energy recovery (Energy Recovery Linac) as part of the bERLinPro project. It accelerates an intense electron beam that can then be used for various applications – such as generating brilliant synchrotron radiation. After use, the electron bunches are directed back to the superconducting linear accelerator, where they release almost all their remaining energy. This energy is then available for accelerating new electron bunches.
Electron source: photocathode
A crucial component of the design is the electron source. Electrons are generated by illuminating a photocathode with a green laser beam. The quantum efficiency, as it is referred to, indicates how many electrons the photocathode material emits when illuminated at a certain laser wavelength and power. Bialkali antimonides exhibit particularly high quantum efficiency in the region of visible light. However, thin films of these materials are highly reactive and therefore very sensitive, so they only work at ultra-high vacuum.
Manufacturing process modified
A HZB team headed by Martin Schmeißer, Dr. Julius Kühn, Dr. Sonal Mistry, and Prof. Thorsten Kamps has now greatly improved the performance of the photocathode so it is ready for use with bERLinPro. They modified the manufacturing process for the photocathodes of cesium- potassium-antimonide on a molybdenum substrate. The new process delivers the desired high quantum efficiency and stability. Studies showed that the photocathodes do not degrade, even at low temperatures. This is a critical prerequisite for operation within a superconducting electron source, where the cathode must be operated at temperatures far below zero.
High quantum efficiency
The physicists were able to demonstrate this performance with detailed studies: Even after its transport via the photocathode transfer system and introduction into the photo injector of the SRF, the quantum efficiency of the photocathode was still about five times higher than necessary to achieve the maximum electron-beam current needed for bERLinPro.
Milestone for bERLinPro
“An important milestone for bERLinPro has been reached. We now have the photocathodes available to generate the first electron beam from our SRF photoinjector at bERLinPro in 2019“, says Prof. Andreas Jankowiak, head of the HZB Institute for Accelerator Physics.
Published in Physical Review Accelerators and Beams (2018): "Addressing challenges related to the operation of Cs-K-Sb photocathodes in SRF photoinjectors"; M. A. H. Schmeisser, S. Mistry, H. Kirschner, S. Schubert, A. Jankowiak, T. Kamps, J. Kühn.
doi:10.1103/PhysRevAccelBeams.21.113401
- A new way to control the magnetic properties of rare earth elementsThe special properties of rare earth magnetic materials are due to the electrons in the 4f shell. Until now, the magnetic properties of 4f electrons were considered almost impossible to control. Now, a team from HZB, Freie Universität Berlin and other institutions has shown for the first time that laser pulses can influence 4f electrons- and thus change their magnetic properties. The discovery, which was made through experiments at EuXFEL and FLASH, opens up a new way to data storage with rare earth elements.
- BESSY II shows how solid-state batteries degradeSolid-state batteries have several advantages: they can store more energy and are safer than batteries with liquid electrolytes. However, they do not last as long and their capacity decreases with each charge cycle. But it doesn't have to stay that way: Researchers are already on the trail of the causes. In the journal ACS Energy Letters, a team from HZB and Justus-Liebig-Universität, Giessen, presents a new method for precisely monitoring electrochemical reactions during the operation of a solid-state battery using photoelectron spectroscopy at BESSY II. The results help to improve battery materials and design.
- HZB magazine lichtblick - the new issue is out!In his search for the perfect catalyst, HZB researcher Robert Seidel is now getting a tailwind – thanks to a ERC Consolidator Grant. In the cover story, we explain why the X-ray source BESSY II plays an important role for his research.