Scientists from Helmholtz-Zentrum Berlin (HZB), Tsinghua University, and Germany’s national metrology institute Physikalisch-Technische Bundesanstalt (PTB), are building the foundation for a future source for coherent UV radiation, known as steady-state microbunching (SSMB). According to the researchers, SSMB could provide a way to generate coherent synchrotron radiation at an electron storage ring in order to supply kilowatt (kW)-level average power radiation in the extreme UV (EUV) regime. As semiconductor manufacturers ask for shorter wavelengths to etch structures of smaller scale, SSMB could meet the power level demands for lithography applications that cannot be met by established accelerator technologies. It also could provide applications in various fields of science and industry with ultrahigh-brilliance X-ray radiation at high repetition rates. 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. Courtesy of HZB/ Communications Physics. When ultrafast electrons are deflected, they emit synchrotron radiation that can be used in storage rings where the particles are magnetically forced onto a closed path. This longitudinally incoherent light consists of a broad spectrum of wavelengths and its high degree of 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. In 2010, researcher Alexander Chao, a member of the current research team, showed that 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 more powerful. Researcher Xiujie Deng, also a member of the current team, defined settings for the SSMB for a specific type of circular accelerator with low-alpha rings. These rings create short particle bunches that are only one μm long after they interact with a laser. “You need to know that the electrons in a storage ring are not homogeneously distributed,” researcher Arnold Kruschinski said. “They move in bunches with a typical length of about a centimeter and a distance around 60 centimeters.” This is six orders of magnitude greater than the microbunches proposed in the 2010 research, Kruschinski said. In 2021, the researchers validated the settings created by Deng, using what they believe to be the first storage ring designed for low-alpha operation. Through extensive experiments, the team has now fully validated Deng’s theory for generating microbunches. “For us, this is an important step on the way to a new type of SSMB radiation source,” Kruschinski said. Project Manager Jörg Feikes (left) and researcher Arnold Kruschinski (right) in the control room of BESSY II and the MLS. Courtesy of Ina Helms/HZB. The team’s systematic studies are performed in an ongoing proof-of-principle experiment, where microbunching is generated from an energy modulation imposed by a 1064-nm laser. The results achieved so far confirm the dependence of the microbunching process on laser modulation amplitude, and the accuracy of the theoretical description of the microbunching mechanism. The results further show that the influence of transverse-longitudinal coupling dynamics corresponds to the team’s theoretical expectations, and can be manipulated with a high degree of accuracy. The SSMB scheme with the most potential to achieve high-power EUV radiation is based on using transverse-longitudinal coupling dynamics to generate efficient microbunching. Starting from an electron storage ring with a high repetition rate in the megahertz (MHz) range, SSMB could potentially use an optical laser modulator to replace the radio frequency cavity as the main longitudinal focusing element, creating persistent microbunches in the circular accelerator. High average power coherent radiation could be produced in this way. With a suitable higher-harmonic generation scheme, the wavelengths of the generated radiation could reach the EUV regime, yielding an EUV radiation source with a power level suitable for lithography. SSMB could also serve as a source of high-brightness, narrow-bandwidth UV radiation for angle-resolved photoemission spectroscopy. Although efforts have been made to improve the capability of free electron lasers (FELs) to generate ultrashort, high peak-power radiation pulses up to the hard X-ray regime, FELs are not yet able to supply high average power radiation at kW level at short wavelengths. SSMB has the potential to fill this gap. The confirmation of key parts of the SSMB theory establishes a solid footing, the team believes, for continuing the proof-of-principle efforts toward the goal of constructing a prototype SSMB synchrotron radiation light source. Preparations for the next phase of the SSMB experiment are ongoing. HZB project manager Jörg Feikes believes that it will take some time before SSMB is introduced as an actual radiation source for EUV. He sees some parallels between the development of SSMB and the development of FELs. “After initial experiments and decades of development work, this idea turned into kilometer-long, superconducting accelerator,” he said. "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.” The research was published in Nature Communications Physics (www. doi.org/10.1038/s42005-024-01657-y).
