As a graduate student in the 1970s, Wolfgang Eberhardt conducted his first experiment at the old DESY synchrotron in Hamburg, Germany. A so-called “parasitic” operation on a high-energy physics ring, it had tremendous difficulties, including highly erratic, fluctuating pulses.
“Nowadays,” he says, “nobody would even consider doing experiments under those conditions.” Nevertheless, photon science at the time was new, so “whatever you looked at, whatever you studied, it was pretty much for sure that nobody had seen that before. It was very exciting.”
Eberhardt, a Professor of Physics at the Technical University of Berlin and a Leading Scientist at Helmholtz Zentrum Berlin (HZB) and the Center for Free-Electron Laser Science (CFEL) at DESY, is currently wrapping up an extended visit to the ALS, working on furthering our basic understanding of organic solar-cell materials.
In addition to serving as Scientific Director of BESSY (the synchrotron in Berlin) from 2001–2008, Eberhardt has published over 300 refereed papers on topics ranging from the development of angle-resolved photoemission, to femtosecond magnetization dynamics, to scattering and holography with coherent synchrotron radiation. He is an internationally respected and sought-after expert in the generation and application of synchrotron radiation and has served on a long list of advisory boards, councils, and committtees across Europe, the US, and Asia. In 2008, he co-chaired with Franz Himpsel a DOE BESAC workshop on “Next-Generation Photon Sources for Grand Challenges in Science and Energy.”
Now, with a diffraction-limited ALS upgrade (ALS-U) on the horizon, Eberhardt agreed to give a special ALS seminar this month on “Diffraction Limited Storage Rings and Free Electron Lasers—Why Do We Need Both?” He pointed out that synchrotron science at storage rings has been fantastically successful, but attention has recently turned somewhat to the construction of free-electron laser facilities. “There’s lots of science, very exciting science out there,” Eberhardt said in an interview following the seminar, “and the science cannot be mapped uniquely onto one specific facility.” While this message came through in the 2008 BESAC workshop report, he said, “the distinction—what can you do better with what facility—that is still a learning process.”
Eberhardt summarized his current understanding of that distinction: “If you want to look at processes, if you want to look at function of materials, you need to do movies, consecutive interrogations of the same object and see how it evolves under certain conditions. How a reaction progresses, for example. So you can’t take one snapshot and know it all—what happens. This is clearly the realm of the new diffraction-limited storage rings.”
On the other hand, “if you want to have snapshots at the best determined time to investigate and freeze a state of a system, then that’s the free-electron laser, because that allows you to get really the best time resolution. But it only allows you to get a single picture—or maybe a double picture—it doesn’t allow you to really do a full movie,” before the sample is destroyed or changed by the x-ray pulses.
Ultimately, achieving the right balance between ring sources and linac sources is a many-dimensional problem that can’t be captured in a simple two-dimensional matrix of science areas vs. photon attributes. You also have to take into account timing, budgets, risks, and changes in technology over time.
“When the ALS was built,” says Eberhardt, “it was the most modern, most fantastic machine that existed in the world. All the scientists came from everywhere to do experiments. It was really fantastic.” Now, more modern, diffraction-limited x-ray facilities are being built, such as MAX IV in Sweden. While there are always certain risks involved whenever you try something totally new, with respect to the MAX IV design, “right now it looks like these risks can be handled and the challenges can be met.”
Eberhardt is quick to note that the ALS has not stood still, but has been continuously upgraded. “The machine has improved as much as it can be in the normal program, the instrumentation is continually being upgraded,” he says. “But right now, it’s due for a major upgrade, this refurbishment, simply because accelerator design has advanced. And that’s what the ALS-U is about.”
Eberhardt cautions against simple calculations about cost per experiment. “What is the Higgs boson worth?” he asks. “Certainly in physics, chemistry, there are many different Higgs bosons around.” When pressed on the question, Eberhardt cited the example of high-temperature superconductivity. “It’s a very strange mechanism; to really pin that down and say why are all these materials behaving that way, that would be very exciting.” On the question of whether we really need both diffraction-limited storage rings and free-electron laser facilities or can we manage with just one or the other, Eberhardt is clear: “My firm position on that one is you really need both, because they are really complementary.”