Small powerhouses for very special light
A pulsed laser co-propagates with the electron beam through the MLS U125 undulator and imposes an energy modulation. The same undulator serves as a radiator on the following passes of the electron beam. The undulator radiation is detected by a fast photodiode, while the laser pulse is blocked from the detection path using an electro-optical switch. © HZB/ Communications Physics
Jörg Feikes and PhD student Arnold Kruschinski in the control room of BESSY II and the MLS. © Ina Helms / HZB
An international team presents the functional principle of a new source of synchrotron radiation in Nature Communications Physics. Steady-state microbunching (SSMB) allows to build efficient and powerful radiation sources for coherent UV radiation in the future. This is very attractive for applications in basic research as well in the semiconductor industry.
When ultrafast electrons are deflected, they emit light - synchrotron radiation. This is used in so called storage rings in which magnets force the particles onto a closed path. This light is longitudinally incoherent and consists of a broad spectrum of wavelengths. Its high brilliance makes it an excellent tool for materials research. Monochromators can be used to pick out individual wavelengths from the spectrum, but this reduces the radiant power by many orders of magnitude to values of a few watts only.
Size matters
But what if a storage ring were instead to deliver monochromatic, coherent light with outputs of several kilowatts, analogous to a high-power laser? Physicist Alexander Chao and his doctoral student Daniel Ratner found an answer to this challenge in 2010: if the electron bunches orbiting in a storage ring become shorter than the wavelength of the light they emit, the emitted radiation becomes coherent and therefore millions of times more powerful.
"You need to know that the electrons in a storage ring are not homogeneously distributed," explains Arnold Kruschinski, PhD student at HZB and lead author of the paper. "They move in bunches with a typical length of about a centimetre and a distance around 60 centimetres. That is six orders of magnitude more than the micro-bunches proposed by Alexander Chao." Chinese theorist Xiujie Deng has defined a set of settings for a specific type of circular accelerator, the isochrone or "low-alpha" rings, for the Steady-State Micro-Bunching project (SSMB). After interacting with a laser, these create short particle bunches that are only one micrometre long.
The research team from HZB, Tsinghua University and PTB already demonstrated that this works in a proof-of-principle experiment in 2021. They used the Metrology Light Source (MLS) in Adlershof - the first storage ring ever designed for low-alpha operation. The team has now been able to fully verify Deng's theory for generating micro-bunches in extensive experiments. "For us, this is an important step on the way to a new type of SSMB radiation source," says Arnold Kruschinski.
The long road to success
However, HZB project manager Jörg Feikes is certain that it will take some time until then. He sees some parallels between the SSMB and the development of free-electron lasers. "After initial experiments and decades of development work, this idea turned into kilometre-long, superconducting accelerator," he says. "Such developments are very long-term. It starts with an idea, then a theory, and then there are experimenters who gradually realise it and I think that SSMB will develop in the same way."
- Lithium-sulphur batteries with lean electrolyte: problem areas clarifiedUsing a non-destructive method, a team at HZB investigated practical lithium-sulphur pouch cells with lean electrolyte for the first time. With operando neutron tomography, they could visualise in real-time how the liquid electrolyte distributes and wets the electrodes across multilayers during charging and discharging. These findings offer valuable insights into the cell failure mechanisms and are helpful to design compact Li-S batteries with a high energy density in formats relevant to industrial applications.
- Self assembling monolayer can improve lead-free perovskite solar cells tooTin perovskite solar cells are not only non-toxic, but also potentially more stable than lead-containing perovskite solar cells. However, they are also significantly less efficient. Now, an international team has succeeded in reducing losses in the lower contact layer of tin perovskite solar cells: The scienstists identified chemical compounds that self-assemble into a molecular layer that fits very well with the lattice structure of tin perovskites. On this monolayer, tin perovskite with excellent optoelectronic quality can be grown, which increases the performance of the solar cell.
- Scrolls from Buddhist shrine virtually unrolled at BESSY IIThe Mongolian collection of the Ethnological Museum of the National Museums in Berlin contains a unique Gungervaa shrine. Among the objects found inside were three tiny scrolls, wrapped in silk. Using 3D X-ray tomography, a team at HZB was able to create a digital copy of one of the scrolls. With a mathematical method the scroll could be virtually unrolled to reveal the scripture on the strip. This method is also used in battery research.