Every day, in dozens of synchrotrons around the globe, electrons are whipped around in circular storage rings to provoke them into emitting X-rays, useful for imaging materials, identifying chemical-reaction products and determining crystal structures.
But photon scientists do not want just any old storage ring. For more than a decade, they have dreamt of ‘ultimate’ storage rings — ones that use specialized magnets to produce X-ray beams that are as tightly focused as theory allows.
Now, researchers at the largest US synchrotron, the Advanced Photon Source (APS) at the Argonne National Laboratory in Illinois, are taking steps to develop this technology. In the process, they hope to leapfrog several international facilities that have a head start.
In Sweden, ultimate-storage-ring technology is being pioneered at MAX IV, a 528-meter-circumference synchrotron in Lund. Scientists there first sought to increase the intensity and brightness of the synchrotron’s X-ray light in 2006 by focusing electron beams more tightly. The design relied on groups of seven magnets, known as multi-bend achromats, that could be used in as many as 20 places around the ring to nudge the paths of electrons back and forth until they lined up more-or-less perfectly. Machine director Mikael Eriksson recalls that when he toured US light sources to describe the project, “few believed it”.
Eriksson now has believers. In a report posted online on 29 August, researchers at the Argonne lab describe how they are hoping to upgrade the 1.1-kilometer-circumference APS with multi-bend achromats (see go.nature.com/asxrqb). “There’s a new technology that has come along and it’s pretty revolutionary,” says APS director Brian Stephenson. Current storage rings have at most double-bend achromats, which contain two magnets rather than seven. Physicists had thought that including more magnets would make the beam unstable by bending it too much and introducing too many fluctuations. But the work at MAX IV showed that very compact magnets enable bending paths that are short enough to stop fluctuations from building up.
The US Department of Energy, which funds the APS, still needs to approve the plan. In July, one of the department’s advisory committees suggested that US labs were being left behind while other countries push towards ultimate storage rings. The committee had also recommended pursuing a next-generation X-ray laser, useful for making ‘molecular movies’ of chemical reactions, among other things (see Nature 500, 13–14; 2013). But such a laser would have limitations: its strongly peaked light pulses would destroy delicate materials. Ultimate storage rings, by contrast, satisfy a need for more gradually peaked pulses of light.
Researchers say that these storage rings could revolutionize X-ray imaging by making it possible to map evolving chemical processes. Current X-ray sources are not bright enough to track changes in materials with nanometer and nanosecond resolution, because there are not enough coordinated photons in the beams. Ultimate storage rings would change that. “A whole class of new problems opens up,” says Paul Evans, a materials scientist at the University of Wisconsin–Madison. For example, he says that the rings could be used to investigate what happens chemically and electrically at the interface between materials inside a battery as it runs out.