what is Synchrotron radiation sources

Synchrotron radiation sources are a class of high-brightness electromagnetic radiation sources generated by high-energy electrons moving in curved trajectories in magnetic fields. They exhibit characteristics such as high brightness, broad spectral range, excellent collimation, and tunable polarization, making them widely applicable in materials science, life sciences, chemistry, physics, and other fields. Based on developmental stages and technical features, synchrotron radiation sources are primarily categorized into four generations, with free electron lasers (FELs)—based on stimulated emission—often regarded as an extension of synchrotron radiation. Below is a detailed introduction:

I. First-Generation Synchrotron Radiation Sources (1960s–1970s)

Characteristics: Early facilities utilized "by-products" of particle physics accelerators (e.g., circular electron-positron colliders or proton synchrotrons) without dedicated optimization for synchrotron radiation. They had relatively low brightness.
Representative Facilities:

• Tantalus (USA, 1961): The first dedicated synchrotron radiation      experimental station.

• ACO (France, 1965): A small electron synchrotron.

• DESY (Germany, early stages): Initially used for synchrotron      radiation research.

II. Second-Generation Synchrotron Radiation Sources (1980s–1990s)

Characteristics: Dedicated storage rings designed specifically for synchrotron applications. By reducing electron beam emittance (a key parameter measuring beam quality), brightness was significantly enhanced, marking the first "specialized" generation.
Representative Facilities:

• NSLS (National Synchrotron Light Source, USA, 1982): Energy      range 0.8–2.5 GeV.

• Photon Factory (PF, Japan, 1982): Energy 2.5 GeV.

• BESSY I (Germany, 1982): Energy 0.8 GeV (later upgraded to      BESSY II).

III. Third-Generation Synchrotron Radiation Sources (1990s–Present)

Characteristics: Incorporation of insertion devices (e.g., undulators, wigglers) to enhance radiation output, combined with superconducting magnets and low-emittance storage rings, leading to a 1–2 orders-of-magnitude increase in brightness compared to second-generation sources. These now represent the mainstream.
Representative Facilities:

• APS (Advanced Photon Source, USA, 1996): Energy 7 GeV,      dominated by insertion devices.

• ESRF (European Synchrotron Radiation Facility, Europe, 1994):      Energy 6 GeV (upgraded to ESRF-EBS in 2020, entering the fourth      generation).

• SPring-8 (Japan, 1997): Energy 8 GeV, one of the highest-energy      storage rings globally.

• SSRF (Shanghai Synchrotron Radiation Facility, China, 2009):      Energy 3.5 GeV, China’s first third-generation source.

IV. Fourth-Generation Synchrotron Radiation Sources (2020s–Present)

Characteristics: Based on diffraction-limited storage ring (DLSR) design, aiming to reduce electron beam emittance to the "diffraction limit" (~10 pm·rad or lower), resulting in X-ray beam spots comparable to their wavelength (~0.1 nm). Brightness is 1–2 orders of magnitude higher than third-generation sources, enabling finer structural characterization and time-resolved studies.
Representative Facilities:

• ESRF-EBS (Europe, upgraded in 2020): Emittance reduced from 100      pm·rad to ~10 pm·rad.

• ALS-U (Advanced Light Source Upgrade, USA, operational 2024):      Energy 1.5 GeV, emittance <10 pm·rad.

• HEPS (Beijing High-Energy Photon Source, China, operational      2025): Energy 6.8 GeV, emittance <10 pm·rad, China’s first      fourth-generation source.

• PETRA IV (Germany, operational 2026): Energy 6 GeV, emittance      <10 pm·rad.

V. Free Electron Lasers (FELs: An Extension of Synchrotron Radiation)

Characteristics: Based on stimulated emission principles, FELs utilize high-energy electron beams passing through periodic magnetic structures (undulators) to generate coherent X-rays. Their peak brightness exceeds that of synchrotron radiation by ~10⁹ times, with pulse durations as short as femtoseconds (fs) or even attoseconds (as), enabling studies of ultrafast processes.
Representative Facilities:

• LCLS (Linac Coherent Light Source, USA, 2009): Hard X-ray FEL.

• European XFEL (Europe, 2017): Covers infrared to hard X-rays;      the world’s longest undulator.

• SACLA (SPring-8 Angstrom Compact Free Electron Laser, Japan,      2012): Hard X-ray FEL.

• SXFEL (Shanghai Soft X-ray FEL, China, 2021): Soft X-ray FEL.

• HXFEL (Hard X-ray FEL, China, under construction): Energy      coverage 0.1–30 keV.

Summary

Synchrotron radiation sources have evolved from early "by-products" of particle accelerators to fourth-generation diffraction-limited storage rings, continuously pushing the boundaries of brightness and performance. Free electron lasers, as an extension, extend research frontiers to ultrafast and transient processes with their unparalleled brightness and ultrashort pulses. Different generations of sources complement each other, collectively supporting cutting-edge scientific research.