Scientists produced X-ray pulses ten times more powerful than ever before

The new system could open up a whole host of new research avenues.
Chris Young
An artist's impression of SLAC’s LCLS-II X-ray laser.
An artist's impression of SLAC’s LCLS-II X-ray laser.

Greg Stewart/SLAC National Accelerator Laboratory 

Scientists at the Department of Energy's SLAC National Accelerator Laboratory developed a new method to push the limits of the lab's Linac Coherent Light Source (LCLS) X-ray free-electron laser (XFEL), a press statement reveals.

The team adapted a Nobel Prize-winning technique used to create superpowerful optical laser pulses called chirped pulse amplification (CPA). In doing so, they were able to produce X-ray pulses ten times more powerful than ever before.

Enhancing an incredibly powerful laser system

The team, who published their findings in the Physical Review Letters journal on Nov. 18, were able to increase the power of the X-ray pulses tenfold while staying within the LCLS's existing free-electron laser infrastructure.

"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, Li continued, "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."

LCLS's laser uses an atomic-resolution camera allowing it to capture images of tiny changes in molecules within fractions of a second. The powerful laser is, therefore, a great research tool for medicine, astrophysics, biology, and potentially a number of other fields.

One problem scientists have encountered with LCLS, however, is that increasing the laser's power can make the timing of the laser pulses inconsistent, which prevents it from capturing accurate images.

Due to this problem, "in the past decade of XFEL laser experiments, more than 90 percent 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."

Adapting a Nobel Prize-winning method

The team of researchers used a process called asymmetric Bragg reflection to implement the CPA process to the laser. CPA was invented in the 1980s by physicists Donna Strickland and Gérard Mourou from the University of Rochester. In 2018, they won the Nobel Prize in Physics for their work. They originally devised CPA to increase the power of optical lasers by stretching the duration of an energy pulse before passing it through an amplifier. A compressor then reverses the stretching, resulting in a superpowerful ultra-short pulse.

Li said he and his team "realized that asymmetric Bragg reflections can be used to implement the CPA mechanism. Then, our X-ray optics team and accelerator physics team worked together to optimize the design based on simulations with realistic beam parameters."

Using state-of-the-art computer modeling, the researchers designed a new CPA method that generates a high-power X-ray pulse within the beam parameters of free-electron lasers — meaning it can be utilized with existing technology.

"Our new system shows we can produce terawatt, femtosecond hard X-ray pulses with existing free-electron laser facilities," Li said. Next, he continued, "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."

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