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The ring-shaped synchrotron building in Grenoble, France, is unchanged, but physicists have replaced the machine inside with a radical new design.


Rebirth of leading European facility promises revolutionary advances in x-ray science

A brilliant new light shines in Grenoble, France, where officials at the European Synchrotron Radiation Facility(ESRF) last week announced the reopening of their completely rebuilt x-ray source. The ring-shaped machine, 844 meters around, generates x-ray beams 100 times brighter than its predecessor and 10 trillion times brighter than medical x-rays. The intense radiation could open up new vistas in x-ray science, such as imaging whole organs in three dimensions while resolving individual cells.

“The light is back at ESRF,” said the lab’s director general, Francesco Sette, at an 8 July online press conference. The reborn synchrotron, dubbed the Extremely Brilliant Source (EBS), will open to general users in late August, but since April, researchers have used its intense beams to study SARS-CoV-2, the virus responsible for the COVID-19 pandemic, and the disease’s impact on the body. And the EBS is lighting the way for others, as the United States, Japan, and a dozen other countries develop similar machines.

A synchrotron is a ring-shaped accelerator that boosts charged particles such as electrons to high energies and near–light speed. Just as a wet rag flings droplets of water if you twirl it over your head, the circulating electrons radiate photons, including x-rays if the electrons have enough energy. In the 1950s, scientists began to siphon x-rays from electron accelerators built for particle physics experiments. Dedicated x-ray synchrotrons followed in the 1980s, employing magnets called wigglers to shake the electrons as they whirl around, causing them to produce more x-rays. In the 1990s, better synchrotrons debuted with magnets called undulators that shake the circulating electrons more harmoniously and effectively.

Around the world dozens of synchrotrons of different energies now crank out light ranging from longer ultraviolet wavelengths to short “hard” x-rays, serving more than 55,000 users annually in fields ranging from chemistry and material sciences to geology and art history. Blasts of the ultraintense x-rays can probe the arrangement of atoms in a protein crystal even as they destroy it. Of the more than 166,000 3D protein structures biologists have deduced so far, more than 120,000 were determined at synchrotrons.

The trick to brightening ESRF’s x-rays was to shrink the machine’s already microscopic electron beam even further, says Pantaleo Raimondi, director of ESRF’s accelerator and source division. The new machine will circulate a ribbonlike beam 2 micrometers high and 20 micrometers wide, one-thirtieth as wide as the old beam. To squeeze it, ESRF researchers followed a tack invented in the early 2000s and implemented at the lower energy MAX IV synchrotron in Sweden, which turned on in 2016.

In a synchrotron, magnets called dipoles sandwich the tubular vacuum chamber through which the electrons travel, supplying the vertical field that bends the particles’ trajectory around the ring. The dipoles bend electrons slightly different amounts depending on their energies, causing the electron beam to spread. To keep it focused, more complex magnets called quadrupoles fit between the dipoles and act like lenses. But a quadrupole that focuses the electron beam horizontally spreads it vertically and vice versa, so the beam expands and contracts like an accordion as it circulates. MAX IV physicists realized they could reduce those oscillations by replacing longer dipoles with a larger number of shorter ones and more quads.

There was a catch, however. The quads also focus electrons differently depending on their energy. Even more complex magnets called sextupoles can correct for that effect. But if the beam is already narrow, the sextupoles work inefficiently and the scheme conks out before reaching maximal compression. MAX IV researchers sidestepped that problem by reducing the energy of their electrons, but ESRF couldn’t do that and still produce hard x-rays. In 2008, Raimondi and colleagues found a way out of the dilemma: They would arrange the gauntlet of magnets so that the electron beam would spread out briefly as it passed through the sextupoles.

After shutting down their machine in December 2018, ESRF workers replaced almost all of its components in just 13 months at a cost of €150 million. Whereas the original machine had two long dipoles in each of its 32 segments, or arcs, the new one has seven, plus 24 other magnets. All told, more than 1000 new magnets were installed in the same doughnut-shaped hall as before. “The body of the car remains the same, but we took out the old motor and put in the engine of a Ferrari,” Raimondi says.

The rebuilt machine should open up qualitatively new windows in x-ray science, says Harald Reichert, ESRF’s director for research in the physical sciences. Hard x-rays can penetrate materials far more deeply than lower energy x-rays, and the new machine’s intense x-ray beams will enable it to study samples up to 1 meter thick. So, scientists could scan an engine block and then zoom in on material defects with near atomic resolution, Reichert says.

Because the x-ray photons emerge from such a tiny electron beam, they should oscillate in unison like those in laser light, accentuating the wavelike nature of the x-ray beam. That enhanced coherence gives the new ESRF a big advantage for imaging. When researchers shine an x-ray beam through a sample, variations in the material will delay the wave front of the coherent x-rays to different degrees, creating a mottled intensity pattern on a distant detector. From many such patterns, researchers can extract a detailed 3D image of the sample.

For example, neuroscientists striving to map the individual neurons and their interconnections in the mouse brain must now cut a brain into fine slices, scan each slice with an electron microscope, and then use massive computer power to link the slices. With ESRF’s penetrating, coherent beams, researchers may be able to do the same thing far faster in an intact mouse brain. “If they can do that, it’s going to have a huge impact on the community,” says Eva Dyer, a computational neuroscientist at the Georgia Institute of Technology. “It’s supercool.”

ESRF will have a few years to press its advantage. Its nearest competitor, the Advanced Photon Source (APS) at Argonne National Laboratory in Illinois, will undergo a similar yearlong rebuild in 2022, says APS Director Stephen Streiffer. “We see the two machines not so much as competitors but as sister facilities,” Streiffer says. “There’s plenty of discoveries to go around.”

The RIKEN Spring-8 laboratory in Japan and the German Electron Synchrotron laboratory are also planning rebuilds, and China is designing a brand-new x-ray facility.

Researchers are already exploiting ESRF’s new capabilities to help confront the COVID-19 pandemic. Several projects are studying the molecular structure of the SARS-CoV-2 virus, and another is using the new synchrotron to image lungs damaged by COVID-19, says Jean Susini, ESRF’s director for research for life sciences. “In fact, the EBS’s first light was used in COVID-19 research.”