With their ultrashort, powerful pulses of high-energy radiation, x-ray free-electron lasers (XFELs) are unsurpassed in their ability to freeze-frame chemical reactions, interactions between drugs and target proteins, and electron dynamics. They are also huge and expensive: The Linac Coherent Light Source (LCLS), built on SLAC’s existing 3-km-long linear accelerator, cost more than $500 million, and the 3.4-km-long European XFEL, located near Hamburg, Germany, cost about €1.25 billion ($1.4 billion). Globally, only five XFELs have been built.
In March a consortium of institutions headed by Arizona State University (ASU) announced plans to design and build a more affordable, compact XFEL—or CXFEL—that could be housed at academic institutions, medical centers, and industrial facilities. Supported by a $91 million NSF grant and $80 million from ASU, the collaboration expects to complete the construction of a room-size CXFEL in five years at the Biodesign Institute on ASU’s Tempe campus.
The new instrument will have the potential to produce extremely short-duration x-ray pulses to access the attosecond regime, less than one millionth of a billionth of a second. That resolution will be able to image molecules as they interact at an atomic level.
The CXFEL will build on knowledge gained from ASU’s construction of a compact x-ray light source, or CXLS. Although not a laser, the CXLS, which began operating in February, produces a high-flux beam of hard x rays (in the range of 4–20 keV) in pulses of a few hundred femtoseconds. Those wavelengths are short enough to resolve the atomic structures of complex molecules.
The CXFEL will produce soft x rays, ranging from 0.3–2.5 keV, says ASU’s William Graves, the project’s principal investigator. Hard x-ray output is precluded by current limitations on performance, jitter, and stability, though Graves says he hopes to eventually build a hard x-ray version. CXFEL pulses will be extremely short, in the range of 0.5–10 fs, and the laser’s full coherence will allow tailoring the pulses to produce either two colors separated by attoseconds of delay or chirped pulses that can be further compressed.
The CXLS and CXFEL will share a common lab. The linac, high-brightness photoinjector, and magnets of CXLS will be reproduced for the CXFEL, Graves says, with the laser to be added. The NSF and university funding covers the cost of the CXLS and CXFEL, six end stations for experiments, and construction of the labs that they are housed in.
With a flux of nanojoules per pulse, the CXFEL’s output will be far below the millijoule flux of today’s XFELs. “You get something for your $1 billion,” Graves notes. Depending on the parameter, the LCLS provides thousands to many millions of times the brightness of the CXFEL, says Mike Dunne, LCLS director.
But the CXFEL will offer some advantages over its larger cousins. Existing XFELs utilize a phenomenon known as self-amplified spontaneous emission (SASE) to generate the same type of x rays that are produced by synchrotrons. Most lasers produce a coherent seed pulse that is amplified to high power, Graves says, but that is traditionally not possible for x-ray wavelengths. X-ray lasers instead amplify spontaneous emission through a sufficiently long undulator (LCLS’s is about 130 m).
The spontaneous emissions are essentially noise, though, and the x-ray pulses in a traditional XFEL fluctuate in energy, spectrum width, and phase, Graves says. In place of SASE, the ASU machine will generate coherent x-ray emission by arranging the electrons into packets that are coherent at x-ray wavelength scale. “So in that sense, we overcome some of the issues the big machines have,” he says. “I think of the CXFEL as a precision tool, where the big machines are a very powerful hammer.” The ASU technology also might be used to provide a coherent seed for the large XFELs, smoothing out the fluctuations that are inherent in SASE. “You could have your cake and eat it too,” Graves says.
Dunne says the CXFEL will broaden the field of researchers who can make use of ultrashort-pulse x rays. “This will inject new ideas, new scientific directions, and a growth of the field,” he says, because access to the world’s few high-end x-ray laser facilities is highly competitive. About 1000 unique users, approximately 30% of them new, access LCLS each year, Dunne says. The instrument can accommodate only one-fifth of qualified research proposals. An upgrade underway, known as LCLS-II, will double experimental capacity, and the increase in pulse rate from 120 Hz to 1 MHz will further increase throughput, he says.
The ultrafast x-ray pulses from the CXFEL will produce two sets of data: a map of molecular structures using coherent imaging methods and spectroscopy data that map how electrons flow within molecules.
Petra Fromme, director of ASU’s Biodesign Center for Applied Structural Discovery and scientific director of the CXFEL project, is a codeveloper of the process used at LCLS to rapidly determine the structure of proteins before they are destroyed by x-ray pulses. She plans to use the CXFEL to probe the fundamental steps by which plants convert sunlight to energy through photosynthesis, such as the flow of electrons in the water-splitting photosystem II protein complex. That understanding could help in developing artificial photosystems for renewable energy. Just last week, an international team reported in Nature that it had used the LCLS and a Japanese XFEL to capture for the first time in atom-scale detail what happens in the final moments leading up to the release of breathable oxygen by photosystem II. The data reveal an intermediate reaction step that had not been observed before.
Arvinder Sandhu, a University of Arizona physicist who leads the development of advanced laser technology for the project, says the short-pulse duration of the CXFEL will enable researchers to capture freeze-frame images of electron movements within complex systems that typically occur on attosecond to femtosecond scales. “With that experimental understanding, we can compare with theory to understand electronic correlations—interactions between electrons in a many-electron system,” Sandhu says.
X rays are key to probing dynamics because x-ray absorption provides a fingerprint of the molecule, Sandhu says. In a biomolecule, the carbon, nitrogen, and oxygen absorb at discrete energy levels in the soft x-ray region. XFELs, for example, could provide insights into the details of how photons entering the eye cause the protein rhodopsin to trigger the electrical activity in the brain that produces vision. “It’s a process we understand macroscopically, but microscopically, what is the time scale of the process, and what mechanisms enable the first few steps of the process?” says Sandhu.
Tabletop lasers don’t provide enough photons consistently for that type of investigation, Sandhu says. And researchers can’t get sufficient beam time on the large XFELs to answer all their questions. “The compact XFEL is an intermediate, university-based source which will allow us to dig deeper into these questions.”
