A Fresh Slice of Electrons Feeds Enhanced X Rays
X-ray free-electron lasers (XFELs) can now generate x-ray pulses having nearly terawatts of power, if only for a minuscule fraction of a second. Such high energies and short durations are finding an increasing number of applications in atomic physics, chemistry, and biology. Now Alberto Lutman and colleagues at SLAC National Accelerator Laboratory’s Linac Coherent Light Source (LCLS) in California have produced soft x-ray pulses with record energy by using an improved version of a recently developed technique called “fresh slice.” The pulses, which are a few femtoseconds long, have energies of hundreds of microjoules—more than ten times larger than previous demonstrations at the same wavelengths.
XFELs such as the LCLS generate x-ray pulses through a lasing effect obtained by wiggling high-energy bunches of electrons along a path that is hundreds of meters long and transferring energy from the electrons to the x rays. In the fresh-slice scheme used by the authors, x rays are initially generated from the trailing end of an electron bunch. As the energy of that part of the bunch is depleted, the electron bunch is delayed by a few femtoseconds, so that the x-ray pulse overlaps with fresh electrons closer to the center of the bunch. The process is repeated in three stages, each of which shifts the bunch a bit more, so that it always presents a “fresh” electron slice to amplify the x-ray pulse.
The scheme also delivers a significant number of pulses whose spectra are dominated by a single wavelength. These pulses could generate “double core holes” in molecules—states in which two core electrons are extracted from the same molecule. According to theory, the spectra of such states could be sensitive probes of molecular structure.
This research is published in Physical Review Letters.
–David Harris
David Harris is a science writer based in Brisbane, Australia.