A brilliant new light shines in Grenoble, France, where the European Synchrotron Radiation Facility (ESRF) reopened 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 reborn synchrotron, dubbed the Extremely Brilliant Source (EBS), opened to general users in late August after being closed since December 2018 for the rebuilding process.
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.
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.
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.
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.
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.
************
Image – The ring-shaped synchrotron building in Grenoble, France, is unchanged, but physicists have replaced the machine inside with a radical new design. Courtesy of ESRF/P. Jayet.
For more information:
ESRF
