Researchers adapt Nobel Prize-winning method to design new ultra-powerful X-ray systems

If scientists want to push the limits of an X-ray laser, for example, they may need to create new technology. But sometimes you don’t have to reinvent the wheel. Instead, scientists are simply coming 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 laser (XFEL). By adapting a technique for modern, super-powerful optical laser pulses called chirped pulse amplification (CPA), the SLAC team designed a system capable of producing X-ray pulses ten times more powerful than before, while remaining in the existing free electron field of the LCLS. laser infrastructure.

The team published their results in Physical examination letters November 18.

“Current X-ray laser pulses from free-electron lasers have a peak power of about 100 gigawatts, and generally with a complex and stochastic structure,” said Haoyuan Li, postdoctoral researcher at SLAC and Stanford University. and lead author of the new study. .

With chirped pulse amplification for X-rays, “we have shown that we can achieve very punchy beam parameters of over 1 terawatt of peak power and a pulse width of around 1 femtosecond at the same time” .

Even the best laser has its limits

LCLS works like an atomic-resolution camera, taking snapshots of the tiniest changes in molecules and materials in the tiniest fraction of a second. The ultra-bright and ultra-fast X-ray pulses it produces are of great interest for many scientific applications and research in fields as diverse as the dynamics of biological molecules, the study of astrophysics in the laboratory and observation. of the interaction of photons with matter.

However, increasing the laser power can make the timing of the laser pulses inconsistent. This inconsistency in turn creates a distorted or inaccurate picture of what is happening with the system, which scientists are desperate to get around. Existing solutions to this problem drastically reduce the power of the laser, limiting what researchers can do.

Because of these restrictions, “over the past decade of XFEL laser experiments, more than 90 percent of experiments have used the X-ray source as a super-fast flashlight,” said Diling Zhu, senior scientist at the SLAC and co-lead author of the study. “Very few have actually used it as a ‘laser’ in the sense that we use optical lasers. We are just beginning to learn how to manipulate the x-ray beam as we have for decades with optical lasers.”

Shrill x-rays

CPA was originally designed to increase 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 which reverses the stretch performed 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 proven difficult to scale to X-ray wavelengths, Li said.

By designing and implementing crystal optics systems for Angstrom wavelengths, Li and his colleagues learned how X-rays were reflected and scattered from a crystal in a process called asymmetric Bragg reflection.

“We then realized that asymmetric Bragg reflections could be used to implement the CPA mechanism,” Li said. “Then our X-ray optics team and our accelerator physics team worked together to optimize simulation-based design with realistic beam parameters.”

X-ray pulses at your fingertips

Using detailed numerical modeling, the researchers devised a CPA method for generating high-intensity hard X-ray pulses within the beam parameters of existing free-electron lasers. Other equally powerful hard X-ray pulse designs rely on overly optimistic parameters that are out of reach with current technology.

“Our new system shows that 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 major engineering effort. “We would like to experimentally demonstrate that we can build the required stretcher and compressor that meets system design specifications, starting with a miniature prototype,” Li said.

The team hopes to continue its efforts, Zhu said. “Adapting the lessons of many elegant and exciting optical laser technologies to X-ray wavelengths could lead us to brighter X-ray laser sources in the future,” he said.