The World’s Brightest X-Ray Laser Is Nearing Completion

LCLS-II will be able to produce images of atoms a million times a second.
Christopher McFadden

Scientists at the Department of Energy's SLAC National Accelerator Laboratory are putting the finishing touches on what will become the world's brightest laser. Called the Linac Coherent Light Source II (LCLS-II), it will be 10,000 times brighter than the brightest laser before itself, once it becomes operational. 

Currently, under construction about 30 feet (9 meters) underground close to Stanford University, the laser promises to help physicists unlock some of the fundamental unknowns about our universe. The laser apparatus extends for about 2 miles (3.2 km) in a specially excavated tunnel.

Its predecessor, Linac Coherent Light Source I (LCLS-I) went live in 2009 and is able to create a beam capable of 120 light pulses a second. LCLS-II, however, crushes this record by being able to produce 1 million pulses per second. 

"I think it's absolutely fair to say that the LCLS-II will usher in a new era of science," Dr. James Cryan, a staff scientist at SLAC told CNET in an exclusive tour of the new facility

The new laser is "capable of making pulses all the way down below a femtosecond. A femtosecond is to a second as the second is to the age of the universe." This enormously rapid pulsing ability will enable scientists to conduct experiments never before thought possible. 

To this end, LCLS-II will operate a bit like a microscope but with atomic-level resolution. At the heart of the device is a powerful particle accelerator that will speed up charged particles and channel them into a very powerful beam.

This beam will then run through a series of alternating magnets (called an undulator) to produce X-rays. Scientists can use those X-rays to create what they call "molecular movies". You can think of these as snapshots of atoms or molecules in motion capturing each "image" within a few quadrillionths of a second and strung together like a film. 

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The predecessor of the new laser, while slower at taking these snapshots, enabled scientists from around the world to make very interesting and important discoveries in the past. This has included things like observing chemical reactions as they happened, demonstrating the behavior of atoms inside stars, and producing live snapshots detailing the process of photosynthesis. The ability to take images at femtosecond intervals with the new laser will, according to Andrew Burrill, SLAC associate lab director, be a very real gamechanger. 

"If you think about a strobe light that goes off 120 times, you see one image. If it goes off a million times in a second, you get a much different image. So you can create a much better movie," he says.

The new laser is a whole different beast from its predecessor

The speed at which the LCLS-II is able to take snapshots is not the only difference between the new laser and its predecessor.

While both devices accelerate electrons to nearly the speed of light, they each do it in a different way. The LCLS-I, for example, pushes electrons down a copper pipe at room temperature. This is fine for short bursts, but not ideal for continuous operation. 

This is where the new laser comes into its own, however. Continuous operation of the kind needed for the LCLS-II generates a lot of heat. Conventional copper cavities, as used in LCLS-I would remove too much of this heat, so engineers turned to a new superconducting accelerator. 

The new material consists of dozens of 40-foot-long (12 m) devices called cryomodules designed to run at two degrees above absolute zero (-456 degrees Fahrenheit). They're kept at operating temperature by a massive cryogenics plant above ground.

All this kit will enable, according to Cryan, SLAC scientists to answer fundamental questions like, "How does energy transfer happen inside molecular systems? How does charge transfer happen?"
"Once we understand some of these principles, we can start to apply them to understand how we can do artificial photosynthesis, how can we build better solar cells," he continues.

SLAC scientists hope that be able to turn the new laser on in the New Year, and hope to generate the first X-rays in summer. It will be interesting to see what secrets of the universe LCLS-II will unveil in 2022 and beyond.

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