Forty years after SLAC National Accelerator Laboratory (SLAC) researchers first extracted hard X-rays from the program’s Stanford Positron Electron Asymmetric Ring (SPEAR) accelerator, the laboratory has continued to produce cutting-edge research in the field while inspiring similar efforts around the world.
Initially, X-ray research wasn’t a planned application for SPEAR. The storage ring, 230 meters in circumference, was instead designed to collide electrons and positrons for particle physics experiments. However, researchers quickly saw the potential of a byproduct of the machine that had little use in atom-smashing but turned out to be otherwise highly valuable — synchrotron radiation, which contains hard X-rays.
“When high energy electrons curve in a circular path bent by magnetic fields, they emit X-rays much like mud flying off a spinning tire,” explained Herman Winick, who led the technical design of the follow-on Stanford Synchrotron Radiation Project (SSRP).
In 1968, four years before SPEAR’s completion, SLAC Director Wolfgang Panofsky gave researcher William Spicer permission to put the accelerator’s X-rays to use. Spicer and three other researchers from Stanford — Sebastian Doniach, Ingolf Lindau, and Piero Pianetta — subsequently set up a pilot project that aimed to extract the X-rays without breaking the chamber’s vacuum and thus interfering with the electron-positron collisions.
“A lot of people didn’t believe it would work,” Doniach said. “But when we managed to get the X-rays out and start looking at what we could do with them, it was pretty spectacular.”
By revealing the structure and properties of materials from proteins to superconductors at a previously unthinkable level of detail, the project opened the door to further breakthroughs.
“When you increase the power of something a million times, you see things you couldn’t dream of seeing before that,” Doniach said.
Spicer and Doniach founded SSRP by the end of 1973, backed by an initial $1.2 million from the National Science Foundation. Winick called the project a “very small, parasitic operation” at its inception and emphasized its growth over time.
“[A project] that started with a few people, one beam line and [a] few stations has now grown to a fully dedicated operation,” Winick said.
The Stanford Synchrotron Radiation Lightsource, an evolution of SSRP, gained exclusive usage of SPEAR in 1990. In 2003, the original accelerator was replaced by SPEAR3, which generates even more intense X-rays through a new ring of magnets and which has the ability to host researchers at 30 stations.
That capacity has consistently met with robust demand, with thousands of scientists submitting proposals for SPEAR3’s usage each year; in 2012, roughly 1,600 groups conducted experiments at SSRL. Over its lifetime, SSRL has facilitated research on subjects ranging from the development of HIV medications to the design of new solar cells.
Even as SPEAR3 continues to improve its capabilities, however, SLAC’s new Linac Coherent Light Source (LCLS) offers the opportunity for even greater breakthroughs, with the world’s first hard X-ray free-electron laser generating rays one billion times brighter than those of synchrotron machines.
Pianetta, SSRL’s current director, said that long-term plans include potentially turning SLAC’s much larger PEP storage ring into a synchrotron light source.
“At SLAC we’re always trying to think of something new,” he said.