|Let There Be LUX|
|Lynn Yarris, [email protected]|
With femtosecond spectroscopy, scientists are able to study the motion of atoms in molecules during physical, chemical, and biological reactions. With attosecond spectroscopy, they could study the motion of electrons in those atoms during such reactions. Femtosecond and, ultimately, attosecond spectroscopic studies with x-rays are among the many intriguing promises of a large new accelerator facility that scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have conceptualized, called LUX, for linac-based ultrafast x-ray source.
"LUX is a concept to produce ultrashort x-ray pulses in a highly refined manner for experiments across all fields of the physical, chemical and biological sciences," says Steve Leone, a chemist with Berkeley Lab's Chemical Sciences Division.
Leone and John Corlett, a physicist with Berkeley Lab's Accelerator and Fusion Research Division (AFRD), are two of the leaders spearheading a plan under which Berkeley Lab would design and build LUX as a U.S. Department of Energy national user facility.
The facility would be optimized to produce substantial fluxes of x-rays at pulse lengths ranging from 50 to 200 femtoseconds, with a goal of even shorter pulse lengths in the future. It would benefit many of the researchers now using the bright beams of x-ray and ultraviolet light at the Advanced Light Source (ALS), as well as those who expect to use the nanofabrication capabilities at the forthcoming Molecular Foundry.
"There is a tremendous grass-roots effort growing in ultrafast x-ray science," says Leone, an expert in the study of chemical reaction dynamics, who oversees operation of the chemical dynamics beamline at the ALS. "The experimental interest is both national and international, and LUX will provide the best opportunity to achieve many of the most critical experimental goals."
For molecules and atoms, time is measured in femtosecond increments. A femtosecond is one millionth of a billionth of a second, which is to one second what one second is to about 30 million years. The development of pulsed lasers that flash femtosecond blinks of visible, infrared or ultraviolet light has enabled scientists to take stop-action photo snapshots which can record the dynamics of chemical bond-breaking during molecular reactions.
However, to get analogous photos of the structural changes that take place during these reactions requires x-rays. Otherwise, clear spectroscopic features of atoms are not observable, and structural details of molecules are blurred.
"Lasers perform all of the ultrafast experiments we know of today,
but they can't -- and aren't likely to ever -- produce the substantial
fluxes of x-rays needed for the diffraction experiments that can give
us structural information," says Leone. "LUX is designed to
make femtosecond x-ray diffraction experiments possible."
The primary component of the proposed LUX facility would be a 2.5 GeV (billion electron volt) recirculating superconducting linac, or linear accelerator. This linac would feature a photocathode gun, an injector linac, and a 50-meter-long main linac, with four outward spiraling recirculating rings, plus multiple beam lines and experimental end-stations that would be coupled to lasers. The entire facility would occupy an area of about 150 by 50 meters, bigger than a football field. Construction would take about six years; the estimated total cost of the project is $380 million in 2003 dollars.
"LUX is the latest in Berkeley Lab's history of ultrafast x-ray source developments," says Corlett, who leads the Beam Electrodynamics Group at Berkeley Lab's Center for Beam Physics. Corlett was involved in an earlier experiment in which femtosecond x-ray pulses were generated at the Beam Test Facility off the ALS's 50 MeV linac, and in more recent work in which femtosecond x-ray pulses were obtained off the ALS's primary electron beam. In these previous efforts, however, the x-ray pulses produced were low in flux and tunability.
"A recirculating linac is potentially a brighter source of x-rays than a synchrotron and provides an electron bunch repetition rate that is well suited to pump-probe dynamics experiments," Corlett says. "With LUX, we will use short electron bunch lengths to produce ultrashort x-ray pulses that have high flux per pulse. This will give us spatially and temporally coherent radiation in the soft x-ray regime."
While LUX will be optimized for the production of 50 to 200 femtosecond x-ray pulse-lengths, Leone and Corlett say the prospects for shortening those pulse-lengths are excellent. This would enable LUX to drop down into the realm of the attosecond, a span of time almost incomprehensibly brief. There are more attoseconds in one minute than there have been minutes in the entire history of the universe.
Attoseconds are the time scale of electron motions -- it takes an electron about 24 attoseconds to orbit a hydrogen nucleus. With attosecond x-ray spectroscopy capabilities, it is possible that scientists could steer the motion of electrons and control not only chemical reactions but even the emission of light.
Says Leone, "The x-ray region is where attosecond time dynamics will be achieved. This should have a major impact on the study of electronic dynamics."
Funding for LUX has so far come from within Berkeley Lab, under its Laboratory Directed Research and Development program. The concept of LUX has been presented to members of DOE's Basic Energy Sciences Advisory Committee, where it was well received. Corlett says that at least two to three more years of R&D are needed before a formal project proposal is submitted to DOE.
More about LUX, a recirculating linac user facility