In a recent study published in the journal Communications Physics, Physicist Alexander Chao and Doctorate Student Daniel Ratner from the Helmholtz-Zentrum Berlin discovered that if the electron bunches orbiting in a storage ring get shorter than the wavelength of the light they emit, the radiation they release becomes coherent and gains millions of times more power.
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. Image Credit: Helmholtz-Zentrum Berlin/ Communications Physics
Synchrotron radiation is the light released when ultrafast electrons are deflected. It is applied to “storage rings,” wherein the particles are driven onto a path that is closed by magnets. This light has a wide range of wavelengths and is longitudinally incoherent.
Due to its extreme brilliance, it is a great instrument for studying materials. Monochromators can be employed to select specific wavelengths from the spectrum, but doing so drastically lowers the radiant output to just a few watts.
Image Credit: D-Krab/Shutterstock.com
However, what if a storage ring could produce monochromatic, coherent light similar to a high-power laser, with outputs of several kw? Physicist Alexander Chao and his Doctorate Student Daniel Ratner discovered the solution to this puzzle in 2010.
You need to know that the electrons in a storage ring are not homogeneously distributed. They move in bunches with a typical length of about a centimeter and a distance of around 60 cm. That is six orders of magnitude more than the micro-bunches proposed by Chao.
Arnold Kruschinski, Ph.D. Student and Study Lead Author, Helmholtz-Zentrum Berlin
For the Steady-State Micro-Bunching project (SSMB), Chinese Theorist Xiujie Deng has defined a set of parameters for a particular kind of circular accelerator: the isochrone or “low-alpha” rings. These form tiny, 1-µm -long particle bunches after coming into contact with a laser.
In a proof-of-principle experiment conducted in 2021, the research team from HZB, Tsinghua University, and PTB has already shown that this is feasible. They made use of the Adlershof-based Metrology Light Source (MLS), the first storage ring ever made for low-alpha operation. Through numerous experiments, the team has finally been able to thoroughly validate Deng's theory for producing micro-bunches.
Kruschinski said, “For us, this is an important step on the way to a new type of SSMB radiation source.”
But until then, Jörg Feikes, Project Manager at HZB, is positive that it will take some time. He draws some comparisons between the development of free-electron lasers and the SSMB.
After initial experiments and decades of development work, this idea turned into a kilometer-long, superconducting accelerator. 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 that SSMB will develop in the same way.
Arnold Kruschinski, Ph.D. Student and Study Lead Author, Helmholtz-Zentrum Berlin
Journal Reference:
Kruschinski, A., et al. (2024) Confirming the theoretical foundation of steady-state microbunching. Communications Physics. doi.org/10.1038/s42005-024-01657-y.