Robert P Crease visits the High Energy Photon Source near Beijing, which will be – when it opens later this year – the most advanced fourth-generation synchrotron-light source
Leading light The High Energy Photon Source (HEPS), due to start operating in December 2025, will be the world’s most advanced synchrotron light source of its type. (Courtesy: IHEP)"> 3D model of the High Energy Photon Source (HEPS) synchrotron building
Leading light The High Energy Photon Source (HEPS), due to start operating in December 2025, will be the world’s most advanced synchrotron light source of its type. (Courtesy: IHEP)
I’m standing next to Yang Fugui in front of the High Energy Photon Source (HEPS) in Beijing’s Huairou District about 50 km north of the centre of the Chinese capital. The HEPS isn’t just another synchrotron light source. It will, when it opens later this year, be the world’s most advanced facility of its type. Construction of this giant device started in 2019 and for Yang – a physicist who is in charge of designing the machine’s beamlines – we’re at a critical point.
“This machine has many applications, but now is the time to make sure it does new science,” says Yang, who is a research fellow at the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS), which is building the new machine. With the ring completed, optimizing the beamlines will be vital if the facility is to open up new research areas.
[China’s High Energy Photon Source
China’s next big thing: a new fourth-generation synchrotron facility in Beijing](https://physicsworld.com/a/chinas-next-big-thing-a-new-fourth-generation-synchrotron-facility-in-beijing/)
From the air – Google will show you photos – the HEPS looks like a giant magnifying glass lying in a grassy field. But I’ve come by land, and from my perspective it resembles a large and gleaming low-walled silver sports stadium, surrounded by well-kept bushes, flowers and fountains.
I was previously in Beijing in 2019 at the time ground for the HEPS was broken when the site was literally a green field. Back then, I was told, the HEPS would take six-and-a-half years to build. We’re still on schedule and, if all continues to run as planned, the facility will come online in December 2025.
Lighting up the world
There are more than 50 synchrotron radiation sources around the world, producing intense, coherent beams of electromagnetic radiation used for experiments in everything from condensed-matter physics to biology. Three significant hardware breakthroughs, one after the other, have created natural divisions among synchrotron sources, leading them to be classed by their generation.
Along with Max IV in Sweden, SIRIUS in Brazil and the Extremely Brilliant Source at the European Synchrotron Radiation Facility (ESRF) in France, the HEPS is a fourth-generation source. These days such devices are vital and prestigious pieces of scientific infrastructure, but synchrotron radiation began life as an unexpected nuisance (Phys. Perspect. 10 438).
Classical electrodynamics says that charged particles undergoing acceleration – changing their momentum or velocity – radiate energy tangentially to their trajectories. Early accelerator builders assumed they could ignore the resulting energy losses. But in 1947, scientists building electron synchrotrons at the General Electric (GE) Research Laboratory in Schenectady, New York, were dismayed to find the phenomenon was real, sapping the energies of their devices.
Where it all began Synchrotron light is created whenever charged particles are accelerated. It gets its name because it was first observed in 1947 by scientists at the General Electric Research Laboratory in New York, who saw a bright speck of light through their synchrotron accelerator’s glass vacuum chamber – the visible portion of that energy. (Courtesy: AIP Emilio Segrè Visual Archives, John P. Blewett Collection)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2025/03/2025-03-Crease-Chinese-synchrotron-General-Electric.jpg">First site of synchrotron radiation at General Electric Research Laboratory
Where it all began Synchrotron light is created whenever charged particles are accelerated. It gets its name because it was first observed in 1947 by scientists at the General Electric Research Laboratory in New York, who saw a bright speck of light through their synchrotron accelerator’s glass vacuum chamber – the visible portion of that energy. (Courtesy: AIP Emilio Segrè Visual Archives, John P. Blewett Collection)
Nuisances of physics, however, have a way of turning into treasured tools. By the early 1950s, scientists were using synchrotron light to study absorption spectra and other phenomena. By the mid-1960s, they were using it to examine the surface structures of materials. But a lot of this work was eclipsed by seemingly much sexier physics.
High-energy particle accelerators, such as CERN’s Proton Synchrotron and Brookhaven’s Alternating Gradient Synchrotron, were regarded as the most exciting, well-funded and biggest instruments in physics. They were the symbols of physics for politicians, press and the public – the machines that studied the fundamental structure of the world.
Researchers who had just discovered the uses of synchrotron light were forced to scrape parts for their instruments. These “first-generation” synchrotrons, such as “Tantalus” in Wisconsin, the Stanford Synchrotron Radiation Project in California, and the Cambridge Electron Accelerator in Massachusetts, were cobbled together from discarded pieces of high energy accelerators or grafted onto them. They were known as “parasites”.
Early adopter A drawing of plans for the Stanford Synchrotron Radiation Project in the US, which became one of the "first generation" of dedicated synchrotron-light sources when it opened in 1974. (Courtesy: SLAC – Zawojski)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2025/03/2025-03-Crease-Chinese-synchrotron-SSRP-plans.png">April 1974 drawing of plans for the first Stanford Synchrotron Radiation Project (SSRP) beam lines.
Early adopter A drawing of plans for the Stanford Synchrotron Radiation Project in the US, which became one of the “first generation” of dedicated synchrotron-light sources when it opened in 1974. (Courtesy: SLAC – Zawojski)
In the 1970s, accelerator physicists realized that synchrotron sources could become more useful by shrinking the angular divergence of the electron beam, thereby improving the “brightness”. Renate Chasman and Kenneth Green devised a magnet array to maximize this property. Dubbed the “Chasman–Green lattice”, it begat a second-generation of dedicated light sources, built not borrowed.
Hard on the heels of Synchrotron Radiation Light Source, which opened in the UK in 1981, the National Synchrotron Light Source (NSLS I) at Brookhaven was the first second-generation source to use such a lattice. China’s oldest light source, the Beijing Synchrotron Radiation Facility, which opened to users in Beijing early in 1991, had a Chasman–Green lattice but also had to skim photons off an accelerator; it was a first-generation machine with a second-generation lattice. China’s first fully second-generation machine was the Hefei Light Source, which opened later that year.
By then instruments called “undulators” were already starting to be incorporated into light sources. They increased brightness hundreds-fold, doing so by wiggling the electron beam up and down, causing a coherent addition of electron field through each wiggle. While undulators had been inserted into second-generation sources, the third generation built them in from the start.
Bright thinking Consisting of a periodic array of dipole magnets (red and green blocks), undulators have a static magnetic field that alternates with a wavelength λu. An electron beam passing through the magnets is forced to oscillate, emitting light hundreds of times brighter than would otherwise be possible (orange). Such undulators were added to second-generation synchrotron sources – but third-generation facilities had them built in from the start. (Courtesy: Creative Commons Attribution-Share Alike 3.0 Unported license)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2025/03/2025-03-Crease-Chinese-synchrotron-undulator.png">
Bright thinking Consisting of a periodic array of dipole magnets (red and green blocks), undulators have a static magnetic field that alternates with a wavelength λu. An electron beam passing through the magnets is forced to oscillate, emitting light hundreds of times brighter than would otherwise be possible (orange). Such undulators were added to second-generation synchrotron sources – but third-generation facilities had them built in from the start. (Courtesy: Creative Commons Attribution-Share Alike 3.0 Unported license)
The first of these light sources was the ESRF, which opened to users in 1988. It was followed by the Advanced Photon Source (APS) at Argonne National Laboratory in 1995 and SPring-8 in Japan in 1999. The first third-generation source on the Chinese mainland was the Shanghai Synchrotron Radiation Facility, which opened in 2009.
In the 2010s, “multi-bend achromat” magnets drastically shrank the size of beam elements, further increasing brilliance. Several third generation machines, including the APS, have been upgraded with achromats, turning third-generation machines into fourth. SIRIUS, which has an energy of 3 GeV, was the first fourth-generation machine to be built from scratch.
Next in sequence The Advanced Photon Source at the Argonne National Laboratory in the US, which is a third-generation synchrotron-light source. (Courtesy: Argonne National Laboratory)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2025/03/2025-03-Crease-Chinese-synchrotron-APS-storage-ring-magnet-upgrade.png">A test module of magnets
Next in sequence The Advanced Photon Source at the Argonne National Laboratory in the US, which is a third-generation synchrotron-light source. (Courtesy: Argonne National Laboratory)
Set to operate at 6 GeV, the HEPS will be the first high-energy fourth-generation machine built from scratch. It is a step nearer to the “diffraction limit” that’s ultimately imposed by the way the uncertainty principle limits the simultaneous specification of certain properties. It makes further shrinking of the beam possible – but only at the expense of lost brilliance. That limit is still on the horizon, but the HEPS draws it closer.
The HEPS is being built next to a mountain range north of Beijing, where the bedrock provides a stable platform for the extraordinarily sensitive beams. Next door to the HEPS is a smaller stadium-like building for experimental labs and offices, and a yet smaller building for housing behind that.
Staff at the HEPS successfully stored the machine’s first electron beam in August 2024 and are now enhancing and optimizing parameters such as electron beam current strength and lifetime. When it opens at the end of the year, the HEPS will have 14 beamlines but is designed eventually to have around 90 experimental stations. “Our task right now is to build more beamlines” Yang told me.
Looking around
After studying physics at the University of Science and Technology in Hefei, Yang’s first job was as a beamline designer at the HEPS. On my visit, the machine was still more than a year from being operational and the experimental hall surrounding the ring was open. It is spacious unlike of many US light sources I’ve been to, which tend to be crammed due to numerous upgrades of the machine and beamlines.
As with any light source, the main feature of the HEP is its storage ring, which consists of alternating straight sections and bends. At the bends, the electrons shed X-rays like rain off a spinning umbrella. Intense, energetic and finely tunable, the X-rays are carried off down beamlines, where are they made useful for almost everything from materials science to biomedicine.
New science Fourth-generation sources, such as the High Energy Photon Source (HEPS), need to attract academic and business users from home and abroad. But only time will tell what kind of new science might be made possible. (Courtesy: IHEP)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2025/03/2025-03-Crease-Chinese-synchrotron-frontis.jpg">Illustration of the High Energy Photon Source (HEPS) synchrotron
New science Fourth-generation sources, such as the High Energy Photon Source (HEPS), need to attract academic and business users from home and abroad. But only time will tell what kind of new science might be made possible. (Courtesy: IHEP)
We pass other stations optimized for 2D, 3D and nanoscale structures. Occasionally, a motorized vehicle loaded with equipment whizzes by, or workers pass us on bicycles. Every so often, I see an overhead red banner in Chinese with white lettering. Translating, Yang says the banners promote safety, care and the need for precision in doing high-quality work, signs of the renowned Chinese work ethic.
We then come to what is labelled a “pink” beam. Unlike a “white” beam, which has a broad spread of wavelengths, or a monochromatic beam of a very specific colour such as red, a pink beam has a spread of wavelengths that are neither broad nor narrow. This allows a much broader flux – typically two orders of magnitude more than a monochromatic beam – allowing a researcher fast diffraction patterns.
Another beamline, meanwhile, is labelled “tender” because its energy falls between 2 keV (“soft” X-rays) and 10 keV (“hard” X-rays). It’s for materials “somewhere between grilled steak and Jell-O” one HEPS researcher quips to me, referring to the wobbly American desert. A tender beam is for purposes that don’t require atomic-scale resolution, such as the magnetic behaviour of atoms.
Three beam pipes pass over the experimental hall to end stations that lie outside the building. They will be used, among other things, for applications in nanoscience, with a monochromator throwing out much of the X-ray beam to make it extremely coherent. We also pass a boxy, glass structure that is a clean room for making parts, as well as a straight pipe about 100 m long that will be used to test tiny vibrations in the Earth that might affect the precision of the beam.
Challenging times
I once spoke to one director of the NSLS, who would begin each day by walking around that facility, seeing what the experimentalists were up to and asking if they needed help. His trip usually took about 5–10 minutes; my tour with Yang took an hour.
But fourth-generation sources, such as the HEPS, face two daunting challenges. One is to cultivate a community of global users. Nearby the HEPS is CAS’s new Yanqi Lake campus, which lies on the other side of the mountains from Beijing, from where I can see the Great Wall meandering through the nearby hills. Faculty and students at CAS will form part of academic users of the HEPS, but how will the lab bring in researchers from abroad?
The HEPS will also need to get in users from business, convincing companies of the value of their machine. SPring-8 in Japan has industrial beamlines, including one sponsored by car giant Toyota, while China’s Shanghai machine has beamlines built by the China Petroleum and Chemical Corporation (Sinopec).
Yang is certainly open to collaboration with business partners. “We welcome industries, and can make full use of the machine, that would be enough,” he says. “If they contribute to building the beamlines, even better.”
The other big challenge for fourth-generation sources is to discover what new things are made possible by the vastly increased flux and brightness. A new generation of improved machines doesn’t necessarily produce breakthrough science; it’s not like one can turn on a machine with greater brightness and a field of new capabilities unfolds before you.
Going fouth The BM18 beamline on the Extremely Brilliant Source (EBS) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The EBS is a dedicated fourth-generation light source, with the BM18 beamline being ideal for monitoring very slowly changing systems. (Courtesy: ESRF/Stef Candé)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2025/03/2025-03-Crease-Chinese-synchrotron-ESRF-PXL-scaled.jpg">Model volcano at the BM18 beamline
Going fouth The BM18 beamline on the Extremely Brilliant Source (EBS) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The EBS is a dedicated fourth-generation light source, with the BM18 beamline being ideal for monitoring very slowly changing systems. (Courtesy: ESRF/Stef Candé)
Instead, what can happen is that techniques that are demonstrations or proof-of-concept research in one generation of synchrotron become applied in niche areas in the next, but become routine in the generation after that. A good example is speckle spectrometry – an interference-based technique that needs a sufficiently coherent light source – that should become widely used at fourth-generation sources like HEPS.
For the HEPS, the challenge will be to discover what new research in materials, chemistry, engineering and biomedicine these techniques will make possible. Whenever I ask experimentalists at light sources what kinds of new science the fourth-generation machines will allow, the inevitable answer is something like, “Ask me in 10 years!”
Yang can’t wait that long. “I started my career here,” he says, gesturing excitedly to the machine. “Now is the time – at the beginning – to try to make this machine do new science. If it can, I’ll end my career here!”