Superconducting X-ray laser in California reaches temperatures colder than in space

(ORDO NEWS) — The particle accelerator that slams electrons together here on Earth has reached a temperature colder than in space.

Using an X-ray free electron laser at the Department of Energy’s SLAC National Accelerator Laboratory part of the Linac Coherent Light Source (LCLS) upgrade project called LCLS II scientists have cooled liquid helium to minus 456 degrees Fahrenheit (minus 271 degrees Celsius), or 2 kelvins. .

That’s just 2 kelvins above absolute zero, the lowest temperature at which particle motion stops.

This freezing environment is very important for the accelerator because at such low temperatures the machine becomes superconductive, meaning it can drive electrons through itself with virtually zero energy loss.

Even the empty regions of space are not as cold as they are still filled with cosmic microwave background radiation, a remnant from shortly after the Big Bang that has a uniform temperature of minus 454 F (minus 271 C), or 3 K.

“The next-generation X-ray free electron laser superconducting accelerator LCLS-II has reached an operating temperature of 2 degrees above absolute zero,” Andrew Burrill, director of SLAC’s accelerator directorate, told Live Science.

The LCLS-II is now ready to start accelerating electrons at 1 million pulses per second, a world record, he added.

“That’s four orders of magnitude more pulses per second than its predecessor, LCLS, which means that in just a few hours, we’ll send more X-rays to users [who want to use them in experiments] than LCLS has done in the last 10 years,” Burrill said. .

This is one of the final steps the LCLS-II must go through before it can produce X-ray pulses that are, on average, 10,000 times brighter than those produced by its predecessor.

This will help researchers study complex materials in unprecedented detail. High-intensity and high-frequency laser pulses allow researchers to see how electrons and atoms in materials interact with unprecedented clarity.

This will have a number of applications, from helping to uncover “how natural and artificial molecular systems convert sunlight into fuel, and therefore how to control these processes, to understanding the fundamental properties of materials that will enable quantum computing,” Burrill said. .

Related articles: 10 cosmic mysteries that the Large Hadron Collider can unravel

Creating a frosty climate inside the accelerator required some effort. For example, to keep the helium from boiling away, the team needed ultra-low pressure.

Eric Fauve, director of SLAC’s Cryogenics Division, told Live Science that at sea level, pure water boils at 212 F (100 C), but that boiling point varies with pressure.

For example, in a pressure cooker the pressure is higher and the water boils at 250 F (121 C), while at high altitude the opposite is true: the pressure is lower and the water boils at a lower temperature.

“It’s the same with helium. At atmospheric pressure, helium boils at a temperature of 4.2 kelvins, but as the pressure decreases, that temperature decreases,” Fauw said.

“To bring the temperature down to 2.0 Kelvin, we need to have a pressure of only 1/30 of atmospheric pressure.”

To achieve this low pressure, the team uses five cryogenic centrifugal compressors that compress the helium to cool it and then allow it to expand in a chamber to reduce the pressure, making it one of the few places on Earth where 2.0 K helium can be produced in large quantities. scales.

Fauve explained that each cold compressor is a centrifugal machine equipped with a rotor/impeller similar to an engine turbocharger.

“As the impeller rotates, it accelerates the helium molecules, creating a vacuum at the center of the wheel where the molecules are sucked in, and creating pressure at the periphery of the wheel where the molecules are [ejected],” he said.

The compression causes the helium to become liquid, but the helium escapes into this vacuum, where it rapidly expands, cooling as it does so.”

Beyond its end use, the ultracold hydrogen produced by LCLS-II is a scientific curiosity in itself.

“At 2.0 Kelvin, helium turns into a superfluid called helium II, which has extraordinary properties,” says Fauve. For example, it conducts heat hundreds of times more efficiently than copper and has such a low viscosity – or resistance to flow – that it’s impossible to measure, he added.

For LCLS-II, 2 Kelvin is the lowest temperature.

“Lower temperatures can be achieved with very specialized cooling systems that can reach temperatures a fraction of a degree above absolute zero, where all movement stops,” Burrill said.

But this particular laser does not have the ability to reach such extreme values, he said.

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