China’s next big thing

Bright future A computer graphic of China’s new High Energy Photon Source – the country’s latest synchrotron facility. It will have up to 70 beamlines when it opens by the end of 2025. (Institute of High-Energy Physics)

Work has just begun on China’s first fourth-generation synchrotron-radiation source. Robert P Crease gets a sneak preview of what’s in store

At the entrance to the Institute of High Energy Physics (IHEP) in Beijing stands a shiny metal sculpture on a plinth. From a distance, it looks like a face with two spirally eyes – one black, one white – framed by strands of hair shooting off in two directions. It’s almost like something Picasso might have created, had he only curved plastic pipes to work with.

Dong Yuhui, a scientist and administrator at the institute, set me straight when I visited IHEP in June this year. “What you see is the yin and yang symbol representing the inseparable opposites that compose all things,” he explained. So what I thought were eyes were, in fact, the beginnings of each symbol nestled together, and the strands of hair were the tails.

It’s an image, Dong added, that nicely fits the Beijing Electron Positron Collider (BEPC), which has been running at IHEP since 1988. “You bring yin and yang together and you make lots of things – electrons and positrons, other bits of matter,” he told me. The sculpture, he continued, was based on a concept by Tsung Dao Lee, the Chinese-born theorist who shared the 1957 Nobel Prize for Physics with Chen Ning Yang for their work on parity violation.

A lively man who punctuates his conversation with jokes and laughter, Dong is currently the director of IHEP’s Multidisciplinary Research Center. But he has also taken on a new job as vice-manager of China’s newest synchrotron – the High Energy Photon Source (HEPS). The facility is the fourth synchrotron to be built in China, and its groundbreaking took place on 29 June.

Round and round

Synchrotron radiation has nothing intrinsically to do with synchrotrons. It’s a consequence of well-understood laws of classical electrodynamics, according to which any charged particle radiates energy as it accelerates, just as electrons do as they travel round a circular accelerator. The name comes from the fact that the magnetic field that bends the particles around increases with time, being “synchronized” to their increasing kinetic energy.

The phenomenon was first observed in the late 1940s by scientists at the General Electric (GE) Research Laboratory in Schenectady, New York, which then had a world-class programme of accelerator and solid-state research. A small GE synchrotron there happened to be built with a glass vacuum chamber, allowing the scientists to see the radiated light, and the connection with synchrotron devices became cemented into the name of the light itself.

Synchrotron radiation was initially regarded as a nuisance. That’s because, beyond a certain point, any additional energy put into the electrons would be promptly radiated away. It seemed that synchrotron light would limit the size – and hence power – of electron accelerators. But over the next decade, physicists realized it could potentially be used as a source of intense and finely tunable X-rays for diffraction, spectroscopy, imaging and other purposes.

Experimentalists at the Stanford Linear Accelerator Center (SLAC) in California and elsewhere began to use electron-storage rings that had been abandoned by high-energy physicists – or borrowed them when not in use. “The experimenters were parasites on the high-energy physicists,” says Dong, who is equally at home talking about the history of synchrotrons as about the BEPC itself. You could say it was a case of a “bug” being turned into a tool.

New generation

Machines like those at SLAC were the first generation of light sources. But synchrotron-light users wanted their own dedicated machines, ideally with beams that are slimmer in size than those available from high-energy machines built by high-energy physicists. Narrow, more focused electron beams could produce brighter X-ray beams, with “brightness” being a key parameter linked to the intensity of the beam and how it diverged.

In the 1970s two accelerator physicists at Brookhaven National Laboratory – Renate Chasman and Ken Green – devised a magnet array specifically to maximize brightness. The first accelerators built with the resulting Chasman–Green lattice were the second generation of synchrotron-light sources. These included Brookhaven’s own National Synchrotron Light Source (NSLSI), which fired up in the 1980s, as well as the Synchrotron Radiation Facility in Daresbury, UK.

But as China emerged on the global scientific stage, it too was keen to get in on the act. It therefore built the Beijing Synchrotron Radiation Facility (BSRF) – the country’s first such light source when it opened in 1991. The BSRF uses a modified Chasman–Green lattice, but gets its electrons from the BEPC. “It’s a first-generation machine with second-generation beam parameters,” said Dong, laughing at the unusual hybrid nature of the facility.

Still operating, the BSRF is limited compared to other synchrotrons, of which there are now more than 50 around the world. In particular, it has only 14 beam lines. That’s far fewer than, say, the European Synchrotron Radiation Facility in Grenoble, France, which has more than 40 beamlines, or the NSLS I, which had nearly 80. Still, the BSRF is actively used. Dong himself received his PhD in condensed-matter physics based on work he did there in 1995 – one of the first experiments at the facility.

Yin and yang This sculpture outside the Institute for High Energy Physics in Beijing alludes to its accelerator bringing opposites – electrons and positrons – together. (Institute of High-Energy Physics)

Fourth and final?

By the mid-1990s, a third generation of synchrotron light sources had arrived, built with long straight sections to accommodate instruments called “wigglers” and “undulators”. These devices, which had been developed from the 1960s onwards, improved brightness by using a series of magnets in a straight section of the accelerator to oscillate the electron beam, making it give off even more light.

After the BSRF, two more synchrotron sources were built in China. There was a “proper” second-generation facility in Hefei, followed by a third-generation lab in Shanghai. The HEPS will, however, be the country’s first fourth-generation synchrotron source – and one of only a handful of such facilities around the world. It will have even brighter beams using a still more advanced magnet array called a multi-bend achromat.

But the new technology wasn’t the only challenge. Finding a site for the HEPS was hard too, Dong told me. Planners wanted a location that would be near Beijing so that experimentalists didn’t have far to travel. However, the device also had to be built on relatively uninhabited land with a stable rock base. It took four years before a location was found in Beijing’s northeast region, next to the Jingmi diversion canal, which brings drinking water from Miyun to the city.

The HEPS will be 1.3 km in circumference, have 60–70 beam lines with more than 90 experimental stations, and is expected to be completed by the end of 2025. “I am in charge of all the beamlines,” Dong says. “I have to decide what kinds of beamlines need to be built, what experiments go where for every station, and get everything in under budget – all in six-and-a-half years!”

From plants to proteins

Once the new machine is complete, the BSRF will probably be shut down. But for now, it is still highly active, and Dong took me round to show off the kind of research it supports. When I was there, the BEPC was operating in its high-energy physics mode, as it does for about 75% of the time. Some BSRF beamlines work during the high-energy physics mode, but the strong shielding let us walk freely around.

As we entered the BSRF building, a loud, piercing siren went off to signal that the BEPC was being filled with electrons, meaning that there were restrictions on access to the injection area. Dong, who had become accustomed to such painful noises over almost three decades, didn’t flinch. Mercifully, the alarm ceased after about a minute.

The BSRF is 240 m in circumference, and its experimental stations are all located near ports in a sector outside the ring. Though there are only 14 stations, the facility supports 1800 to 2000 users a year, nearly all from China. There were pipes and equipment draped in aluminum foil, and hutches with posters displaying descriptions of the station’s research.

“This one is XFAS,” Dong said as we walked around, “that one diffraction, and over there is X-ray fluorescence.” As at other synchrotrons, the user profile has changed over the years. At first, he said, most users were condensed-matter physicists. Then, protein crystallographers took an interest as they realized how valuable synchrotron X-rays were for solving protein structures – provided they had big enough samples to put in the beam, that is.

Protein crystallography and a few other uses in the life sciences led to an upsurge in biological scientists at light sources. A few years ago, however, solving protein structures began to be taken over by cryogenic electron microscopy (cryo-EM), which uses standalone instruments that do the job better than synchrotrons. You don’t need as much sample to solve a protein structure with a cryo-EM, which slowed the expansion of protein crystallographers among synchrotron users.

Dong and I then stopped at the X-ray fluorescence station, where a poster showed pictures of three different kinds of plants studied at the station. “At this port they are studying environmental pollution,” Dong explained, indicating research into chromium uptake in one of the plants. “This plant is from an area in the south of China heavily polluted by chromium, and the experimenters are trying to see if certain fungi can be used to draw chromium from the soil.”

A rice plant was also depicted on the poster. “This is from the Guizhou province, where there were mercury mines in the Han Dynasty about 3000 years ago,” Dong explained. “We know this because we can image it in the rice!” The mining ceased long ago, but the pollution is still present in dangerous amounts. The researchers had used the BSRF to find ways to reduce the toxicity of the mercury in rice, in particular exploring the use of selenium.

“Selenium combines with mercury and deposits it on the surface of the grain,” Dong explained. “Machines can then polish the surface to remove the grains. At this station they are locating exactly where the mercury and selenium combine.” Although the grain product is not for human or animal consumption, it can be used to produce industrial alcohol. “In highly populated areas,” Dong said, “you have to make the best use of all arable land.”

Like other light sources, the BSRF serves academic and industrial users, the latter being allowed to use the facility without a charge as long as they publish their results; if they don’t, they have to pay the full fee. But the work of pharmaceutical firms at the facility revealed certain differences between Western and Chinese light sources.

In the West, firms such as Novartis, Merck and Pfizer are big, powerful and rich enough to develop a drug from beginning to end. “These companies build beamlines at synchrotron radiation facilities and have experience in cooperating with the machine operators,” Dong said, with the most expensive parts of drug development being the clinical trials. “In China we don’t have such wealthy companies. Ours can only afford to do the ‘cheap’ stage of solving a structure and verifying how it functions with other molecules. They want to keep that testing a secret – sometimes they won’t even tell us what protein they are testing. This makes it harder to work together.”

The critical point

At the BSRF, Dong said, he was beginning to tire of the routine: “Users come, users go, research grows more complicated, researchers don’t know the details of the machine, and the operators have to help them. You have to pay less attention to the machine and more to the users’ needs.” He is also dismayed by the lack of developments in synchrotron radiation source technology. “There have been no new innovations in this century.”

For Dong, synchrotrons appear to be approaching a limit on possible brightness imposed by the optics – limits on the size of the beam spot and focusing. A few years ago, he said, fourth-generation synchrotron sources were even being called “ultimate storage rings”. But at the HEPS, special research centres will be built to connect research teams and the machine’s operators, improving the interactions. Dong also mentioned new ideas for the facilities, such as an energy recovery linac proposed by researchers at Cornell University. Hopefully, such innovations will secure a strong future for synchrotron sources in the decades ahead.