|
| An electron beam travels through a niobium cavity – a key component of SLAC’s LCLS-II X-ray laser. Credit: Greg Stewart/SLAC National Accelerator Laboratory |
If scientists want to push the boundaries of, say, an X-ray laser, they may
need to create some new technology. But occasionally there's no need to
reinvent the wheel. Instead, scientists simply come up with a new way to use
it.
Now, researchers at the Department of Energy's SLAC National Accelerator
Laboratory have done just that in an effort to push the capabilities of the
lab's Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL).
By adapting a technique for modern, superpowerful optical laser pulses
called chirped pulse amplification (CPA), the SLAC team has designed a
system capable of producing X-ray pulses ten times more powerful than
before—all while staying within the LCLS's existing free-electron laser
infrastructure.
The team published their results in Physical Review Letters on November 18.
"Current X-ray laser pulses from free-electron lasers have a peak power of
roughly 100 gigawatts, and usually with a complex and stochastic structure,"
said Haoyuan Li, a postdoctoral scholar at SLAC and Stanford University and
lead author of the new study.
With chirped pulse amplification for X-rays, "we've shown that we can
achieve very impactful beam parameters of greater than 1 terawatt peak power
and a pulse duration of about 1 femtosecond at the same time."
Even the best laser has its limits
LCLS works like an atomic-resolution camera, taking snapshots of the most
minute changes in molecules and materials within a tiny fraction of a
second. The ultrabright, ultrafast X-ray pulses it produces are of great
interest for many applications and scientific research in fields as diverse
as the dynamics of biological molecules, studying astrophysics in the
laboratory, and observing how photons interact with matter.
However, increasing the power of the laser can make the timing of the laser
pulses inconsistent. That inconsistency creates in turn a distorted or
inaccurate image of what's happening with the system—something scientists
desperately want to circumvent. Existing solutions to that problem
significantly reduce laser power, limiting what researchers can do.
Because of these restrictions, "in the past decade of XFEL laser
experiments, more than 90% of experiments used the X-ray source like an
ultrafast flashlight," said Diling Zhu, senior scientist at SLAC and senior
coauthor of the study. "Very few really used it as a 'laser' in the sense of
how we use optical lasers. We are just starting to learn how to manipulate
the X-ray beam like we have done for decades with optical lasers."
Chirping X-rays
CPA was originally designed for increasing the power of optical lasers, and
it works by stretching the duration of an energy pulse before it passes
through an amplifier and finally a compressor that reverses the stretching
done in the first step. The result is a super intense, clean, and
ultra-short pulse.
Physicists Donna Strickland and Gérard Mourou from the University of
Rochester invented CPA in the 1980s and received the 2018 Nobel Prize in
Physics for their work. While CPA has revolutionized high energy pulse
generation for optical lasers, the technique has proved difficult to adapt
for X-ray wavelengths, Li said.
Through designing and implementing crystal optics systems for Angstrom
wavelengths, Li and his colleagues learned how X-rays were reflected and
dispersed from a crystal in a process called asymmetric Bragg reflection.
"We then realized that asymmetric Bragg reflections can be used to implement
the CPA mechanism," Li said. "Then our X-ray optics team and accelerator
physics team worked together to optimize the design based on simulations
with realistic beam parameters."
X-ray pulses within reach
Through detailed numerical modeling, the researchers designed a CPA method
for generating high intensity hard X-ray pulses within the beam parameters
of existing free-electron lasers. Other designs for such powerful hard X-ray
pulses rely on overly optimistic parameters that are out of reach with
current technology.
"Our new system shows we can produce terawatt, femtosecond hard X-ray pulses
with existing free-electron laser facilities," including LCLS at SLAC, Li
said.
The next step is to build the system, which will be a significant
engineering effort. "We would like to experimentally demonstrate that we can
build the required stretcher and compressor that meet the system design
specifications, starting with a miniature prototype," Li said.
The team hopes to continue their efforts, Zhu said. "Adapting the lessons
from many exciting, elegant optical laser technologies to X-ray wavelengths
could lead us to brighter X-ray laser sources in the future," he said.
Reference:
Haoyuan Li et al, Femtosecond-Terawatt Hard X-Ray Pulse Generation with
Chirped Pulse Amplification on a Free Electron Laser, Physical Review
Letters (2022).
DOI: 10.1103/PhysRevLett.129.213901