“That factor is such a big change, it’s going to revolutionize the types of science that we can do,” said Stephen Streiffer, Argonne Associate Laboratory Director for Photon Sciences and Director of the APS.
“We’ll be able to look at the structure of materials and chemical systems in the interior of things — inside a turbine blade or a catalytic reactor — almost down to the atomic scale. We haven’t been able to do that before. Given that vast change, we can only dream about the science we’re going to do.”
In December, DOE approved the technical scope, cost estimate and plan of work for an upgrade of APS.
The APS upgrade has been in the works since 2010. The upgrade will reveal a new machine that will allow its 5,500 annual users from university, industrial, and government laboratories to work at a higher spatial resolution, or to work faster with a brighter beam (a beam with more X-rays focused on a smaller spot) than they can now.
“Sometimes it’s called the ultimate X-ray microscope,” said Dennis Mills, Argonne Deputy Associate Laboratory Director for Photon Sciences. It will open opportunities for closer-up views of materials and, for experiments involving the evolution of phenomena over time, the collection of data at more rapidly occurring intervals.
In operation since 1996, APS’s electron storage ring, measuring two-thirds of a mile in circumference, is large enough to encircle a major-league baseball stadium. Its beams are a billion times more powerful than the X-rays at a doctor’s office. The 7 giga-electron-volt (GeV) facility is one of the world’s highest-energy synchrotron radiation sources.
“An important aspect of what we do with the APS isn’t big science. It’s small science at a very large scale,” said Streiffer. Fifty to 60 experiments get underway and generate data every day.
Scientists from across the country and around the world use the powerful, versatile, invisible light of the APS to study the arrangements of molecules and atoms, probe the interfaces where materials meet, determine the interdependent form and function of biological proteins, and watch nanoscale chemical processes. They need these capabilities to develop better ways to use energy, sustain the nation’s technological and economic competitiveness, and push back the ravages of disease.
A Nobel history
Research at the APS has contributed to two Nobel Prizes in chemistry, both pertaining to protein crystallography, a method used to investigate the features of proteins.
The 2009 Nobel Prize in chemistry went to Venkatraman Ramakrishnan, Thomas A. Steitz, and Ada E. Yonath for their study of the structure and function of the ribosome. All three laureates completed key aspects of their work at the APS. Ribosomes work as protein factories in all organisms, from humans to bacteria. The improved knowledge about ribosomes, especially in bacteria, opened a new avenue of medical research as scientists worked to identify antibiotics that can interfere with bacterial protein synthesis.
In 2012, Brian Kobilka shared the Nobel Prize in chemistry for work he had done on G-protein-coupled receptors, or GPCRs, a large family of proteins that play a key role in how cells interact with their environments. In his research, Kobilka passed extremely bright X-rays through crystallized proteins. By watching how the X-rays scattered, he revealed the three-dimensional structure of the proteins in great detail.
Many of today’s pharmaceuticals target GCPRs. Kobilka performed a study at APS that led to the first discovery of the structure of a human GPCR. Kobilka and other scientists bring their protein crystals to the APS with the goal of defining their structures.
“That can then help us understand the function of the molecule, which is what the biologists really want to know,” Mills said. Getting the proteins to crystallize is often difficult, sometimes resulting in rather small protein crystals. But the APS offers tightly focused beams. “You can use a small crystal at the APS and still get the data that you just couldn’t get at most other places.”
Bend, refocus, repeat
The lattice of bending magnets currently used at the APS causes the electron beam to spread horizontally. But after the APS upgrade, a more advanced magnetic lattice, called the multi-bend achromat, will gently bend the beam, refocus it, and bend it more — over and over again until the horizontal beam’s size and shape become much smaller than what the facility currently produces.
“Right now, the APS operates with a beam that very much looks like a flat pancake,” Streiffer said. “It’s large in the horizontal dimension and rather small in the vertical dimension.”
In its typical operational mode, the APS beam measures approximately 15 microns in diameter vertically (much less than the diameter of a human hair) and 207 microns horizontally. But after the upgrade, the beam will measure approximately 10 microns in diameter vertically and 20 microns horizontally.
“By installing this more complicated magnetic lattice — the multi-bend achromat, which achieves this bend, refocus, bend, refocus, bend, refocus — you make the horizontal dimension of the beam similar to the vertical dimension, which improves its properties very substantially,” Streiffer said.
The theory behind multi-bend achromats is well established, but the challenge for photon scientists is making them work in practice. Streiffer compared the current APS magnetic lattice to a sedan and the proposed upgrade to a sports car. The sedan is affordable and reliable; the sports car offers higher performance, but also is more temperamental.
Each new generation of light source has involved something of a leap into the unknown. The same applies to multi-bend achromats, a fourth-generation synchrotron radiation technology.
“It’s only been in the last five or 10 years that computational techniques for simulating accelerators have evolved to where people are becoming convinced they can actually make this high-performance lattice work,” Streiffer said.
The APS upgrade involves a technique called swap-out injection to introduce beams into a storage ring. Swap-out is the next outgrowth of the current technology’s top-up injection technique. In top-up mode, the weakest of the decaying electron bunches receive an added charge as they circulate, while all existing bunches stay in place.
“That works in third-generation machines. In fourth-generation machines, where you’re really pushing the performance, that doesn’t work so well,” said Michael Borland, Associate Director of Argonne’s Accelerator Systems Division and an Argonne Distinguished Fellow. “You get rid of the bunch that’s there and you inject a new one. You swap in a full-current bunch for a weakened one.”
Fewer donuts and sandwiches
The APS upgrade will require scientists to remove the current machine, install the new one, and re-open the facility to users, all within a year.
“To achieve this goal, we are simulating the process of commissioning the new machine,” Borland said.
Vadim Sajaev, group leader in accelerator operations and physics at the APS, has developed a complex simulation code that will evaluate 100 or more possible configurations of the machine and consider potential operational differences that could emerge between its intended design and its actual construction.
Using this simulation, Borland said, “We can make fairly confident statistical predictions about the performance of the machine after we’ve commissioned it. We also plan to apply this software to our existing machine.” In that exercise, they will pretend to commission the existing machine as a way to further test and develop the software.
“Usually commissioning is a bunch of physicists in a control room with donuts and sandwiches for weeks getting the machine to work,” Borland said. “We need to use more sophisticated technology to help us do that more quickly.”
APS physicists have also been optimizing their magnet configurations through advanced computer simulations that emulate biological evolution. They were among the first to apply the technique to accelerator design on a large scale. In 2010, Borland received 36 million processor hours from the U.S. Department of Energy to conduct simulations based on the technique at the Argonne Leadership Computing Facility, which houses one of the world’s fastest supercomputers.
“We continue to use Argonne facilities for that sort of calculation,” said Borland.
The calculations were performed using something called a multi-objective genetic algorithm, which starts with a given configuration of magnets in a possible accelerator. The simulations then determine how easy it will be to inject an electron beam into the accelerator, and how long the beam will last once it arrives.
“Then you make basically random tweaks to different parameters of the magnets and re-evaluate, to ask ‘How good is that configuration?’” Borland said.
After simulating an entire generation of a hundred or so randomizations from the starting point, the algorithm will find the ones that perform the best.
“Then you run the simulation on those so-called ‘rank one’ configurations. Basically, you take different properties from those configurations and you mix them together using random numbers. Then you re-evaluate all those ‘children’ of the previous generation and repeat. Eventually, the algorithm can find solutions that you never would have thought of yourself and that you couldn’t really get from any more deterministic technique,” Borland said.
The APS team has used this method both to improve the existing accelerator and to design new ones.
“It sometimes finds things that don’t seem right. And then you look in more detail and understand it did something really clever that you didn’t consider.”
Fine tuning a sticky note
Other simulation codes also help Argonne scientists understand and tune the APS electron beam’s properties. The continuously evolving ELEGANT code, which Borland and collaborators developed, has become a mainstay of accelerator design and simulation at APS and other light sources worldwide. These include the world’s first free-electron laser, the Department of Energy’s Linac Coherent Light Source at SLAC National Accelerator Laboratory.
“The basic equations that govern the electron beams are pretty simple. You can probably write them on a sticky note,” Borland said. Nevertheless, the configurations to create high-quality electron beams are quite complicated.
“You need simulation codes to help improve the electron beam’s quality,” Borland said. The outcome: APS beams that shine more brightly than ever.
The APS upgrade is funded by the U.S. Department of Energy’s Office of Science.
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