SLAC’s new ultra-cold X-ray laser sharpens our view of how nature works

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One of the coldest places in the universe is in Menlo Park.

Thirty feet below the Peninsula’s graceful foothills, a new superconducting particle accelerator at SLAC National Accelerator Laboratory has been cooled to 456 degrees below zero Fahrenheit — just four degrees above the elusive “absolute zero,” the ultimate abyss of cold.

Each time temperatures are pushed a little lower, new scientific discoveries are made. With this new Big Chill, energy drawn from whizzing electrons can produce X-ray pulses that are much faster and 10,000 times brighter, on average, than SLAC’s current laser.

The X-rays will act like a super powerful microscope, revealing rare and fleeting chemical events, atomic details of biological molecules and the strange world of quantum mechanics.

Inside SLAC’s long tunnel, operated by Stanford for the U.S. Department of Energy, it’s colder than Mars (-195°F), Uranus (- 371°F) or even Pluto (-400°F.) By comparison, Antarctica is a balmy -128°F.  The tunnel is only two degrees warmer than the coldest spot in the universe, the Boomerang Nebula in the distant constellation Centaurus, at -458°F degrees.

“This is a huge accomplishment,” said Andrew Burrill, director of SLAC’s Accelerator Directorate. With faster and more frequent pulses, the laser lets scientists gather more data, enabling studies that were previously inconceivable, he said. “It’s a big increase in the amount of science that you can do in a short period of time.”

In smaller lab-based settings, scientists have achieved even colder temperatures.

But SLAC’s project — located under Interstate 280 — is big.  As long as seven football fields, it’s a half-mile stretch of a two-mile tunnel. The project brings new distinction to Menlo Park, a town better known for its sprawling Facebook headquarters and historic ties to Ken Kesey, Jerry Garcia and Bob Weir.

Aerial photo of SLAC Cryoplant. (Matt Beardsley/SLAC National Accelerator Laboratory)
Aerial photo of SLAC Cryoplant. (Matt Beardsley/SLAC National Accelerator Laboratory) 

The extreme cold helps the accelerator, through which the electrons travel, become “superconducting” — when electricity flows without resistance – so the electrons can reach high speeds very efficiently.

To achieve the low temperature, SLAC needed lots of liquid helium. Normally, helium is a gas, useful for things like blowing up birthday balloons. But if cooled, it becomes a liquid. When the helium liquid is used to bathe the superconductor, all electrical resistance suddenly vanishes.

This meant a big remodel of SLAC’s 13-year-old Linac Coherent Light Source laser. Crews removed part of the old copper accelerator and installed a series of different compressors to cool down the helium.

SLAC’s compressors work just like an ordinary freezer – but they’re much, much bigger.

“It’s almost like a small silo that you’d see on a farm,” said Burrill.

The new accelerator’s insulation acts like a Thermos. It creates a vacuum, so things stay cold.

How do scientists take the tunnel’s temperature? Not with old mercury thermometers, or even the newer alcohol-based ones. Their instruments are high-tech, with special sensors the size of a thumbtack.

Scientists say there’s no risk the insulation would accidentally leak, forcing Peninsula residents to reach for hats, scarves and parkas. But even if it did, liquid helium turns into a gas when it warms – and would simply float off into the atmosphere.

Inside the LCLS cryoplant. This cryoplant will cool helium to 2 kelvin (colder than outer space). The liquid helium is used to cool LCLS acccelerator cryogenic module to superconducting capacity. (Olivier Bonin/SLAC National Accelerator Laboratory)
Inside the LCLS cryoplant. This cryoplant will cool helium to 2 kelvin (colder than outer space). The liquid helium is used to cool LCLS acccelerator cryogenic module to superconducting capacity. (Olivier Bonin/SLAC National Accelerator Laboratory) 

On April 15, the new accelerator reached its final temperature of minus 456.07 – or 2 degrees Kelvin — for the first time.

“This is the first time any X-ray laser of this size has been built in the U.S.,” said Burrill. The effort was a collaboration with four other national labs – Lawrence Berkeley Lab, Virginia’s Jefferson Lab and Argonne and Fermilab, both in the Chicago area — and Cornell University. Germany has something comparable, called European XFEL, tunneled beneath the city of Hamburg.

Now the new accelerator is ready for initial operations.

Its electrons will move close to the speed of light, on a beam that follows a zigzag path. If everything is aligned just right, according to SLAC, the electrons will emit the world’s most powerful bursts of X-rays.

These X-rays will let researchers to take snapshots of materials and biological systems at the atomic level.

In just a few hours, the accelerator “will produce more X-ray pulses than the current laser has generated in its entire lifetime,” said Mike Dunne, director of the Linac Coherent Light Source. “Data that once might have taken months to collect could be produced in minutes.”

Above ground, basking in Wednesday’s warm sunshine, Menlo Park residents marveled at the project.

“It’s spectacular that we’re using these brains of ours to such great wondrous purpose, increasing our appreciation of what’s out there,” said resident Charles Ogle. “It’s such good news — that there are bright people exploring important new realms.”

Mayor Betsy Nash called the project “cool, in so many ways. As a long-time supporter of scientific research, Menlo Park is proud to be the home of SLAC whose work builds foundations for future technologies, as well as many of today’s life science and technology companies.”

First cryomodule weld for LCLS-II, April, 2019. (Robert Kish/SLAC National Accelerator Laboratory)
First cryomodule weld for LCLS-II, April, 2019. (Robert Kish/SLAC National Accelerator Laboratory) 


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