A new method for generating monochromatic light in storage rings

When the ultrafast electrons are deflected, they emit light – synchrotron radiation. This is used in so-called storage rings, in which magnets push the particles into a closed path. This light is longitudinally incoherent and consists of a wide spectrum of wavelengths.

Its high brilliance makes it an excellent tool for materials research. Monochromators can be used to select individual wavelengths from the spectrum, but this reduces the radiant power by many orders of magnitude to values ​​of only a few watts.

But what if the storage ring were instead to deliver monochromatic, coherent light with a power of several kilowatts, analogous to a high-powered laser? Physicist Alexander Chao and his PhD student Daniel Ratner found an answer to this challenge in 2010: if the bunches of electrons orbiting in the storage ring are shortened by the wavelength of the light they emit, the emitted radiation becomes coherent and therefore a million times stronger. .

“You have to know that the electrons in the storage ring are not homogeneously distributed,” explains Arnold Kruschinski, Ph.D. student at HZB and the main author of the paper. “They move in clusters with a typical length of about a centimeter and a distance of around 60 centimeters. This is six orders of magnitude larger than the microclusters proposed by Cha.”

Chinese theorist Xiujie Deng defined a set of settings for a particular type of ring accelerator, isochronous or “low-alpha” rings for the Steady-State Micro-Bunching (SSMB) project. After interacting with the laser, they form short clusters of particles that are only one micrometer long.

As early as 2021, a research team from HZB, Tsinghua University and PTB demonstrated that it works in a proof-of-principle experiment. They used the Metrological Light Source (MLS) at Adlershof – the first storage ring ever designed for low-alpha operation. The team was now able to fully validate Deng’s theory for generating micro-clusters in large-scale experiments. “For us, this is an important step on the way to a new type of SSMB radiation source,” says Kruschinski.

However, HZB project manager Jörg Feikes is sure that it will be some time until then. He sees some parallels between SSMB and the development of free-electron lasers.

“After initial experiments and decades of development work, this idea turned into a kilometer-long superconducting accelerator,” he says. “This kind of development is very long-term. It starts with an idea, then a theory, and then there are experimenters who gradually realize it, and I think SSMB will develop in the same way.”