New method for generating monochromatic light in storage rings

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

Its high brightness 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 a storage ring provided monochromatic, coherent light of several kilowatts, analogous to a high-power laser? Physicist Alexander Chao and his PhD student Daniel Ratner found an answer to this challenge in 2010: if the bunches of electrons spinning in a storage ring become shorter than the wavelength of the light they emit, the emitted radiation becomes coherent and for therefore millions of times more powerful. .

“You have to know that the electrons in a retaining ring are not homogeneously distributed,” explains Arnold Kruschinski, Ph.D. student at HZB and main author of the paper. “They move in bunches with a typical length of about one centimeter and a distance of about 60 centimeters. That’s six orders of magnitude more than the micro-bunches proposed by Chao.”

Chinese theorist Xiujie Deng has defined a set of settings for a specific type of circular accelerator, isochronous or “low-alpha” rings, for the Steady-State Micro-Bunching (SSMB) project. After interacting with a laser, these create short bunches of particles that are only one micrometer 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 Metrological Light Source (MLS) at Adlershof – the first storage ring ever designed to operate with alpha low. The team has now been able to fully verify Deng’s theory of micro-cluster generation in extensive 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 confident that it will take 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 evolved into a kilometer-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 realize it, and I think SSMB will develop in the same way.”