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

(ORDO NEWS) — The particle accelerator that pushes electrons together here on Earth has reached a temperature lower than in outer 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. .

It’s only 2 kelvin. above absolute zero, the lowest possible temperature at which particle motion stops.

This freezing environment is critical for the accelerator because at such low temperatures the machine becomes superconductive, meaning it can push electrons through it with only about zero energy loss.

Even 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, which has a uniform temperature of minus 454 F (minus 271 C), or 3 K.

“The superconducting accelerator of the new generation of X-ray free electron laser LCLS-II has reached an operating temperature of 2 degrees above absolute zero,” – Andrew Burrill, director of the SLAC accelerator. The director 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 will send more X-rays to users [who intend to use them in experiments] than LCLS has done in the last 10 years. “Barrill said.

This is one of the final milestones the LCLS-II must complete before it can produce X-ray pulses that are, on average, 10,000 times brighter than its predecessor.

This should help researchers examine complex materials with unprecedented precision. High-intensity, 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 discover “how naturally and how molecular systems are made to convert sunlight into fuel and therefore how to control these processes, to understanding the fundamental properties of materials that will make quantum computing possible,” Berill said. .

Creating an ice climate inside the accelerator required some work. For example, to prevent the helium from boiling off, the team needed ultra-low pressure.

Eric Fauve, director of SLAC’s cryogenics department, told Live Science that at sea level, pure water boils at 212°C. F (100 C), but this boiling point is pressure dependent.

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

“For helium, everything is the same. However, at atmospheric pressure, helium will boil at 4.2 kelvin; that temperature will drop if the pressure decreases,” Fauve said.

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

To achieve these low pressures, 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 on a large scale. .

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

“As the impeller spins, 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 liquefy, but the helium escapes into this vacuum, where it rapidly expands as it cools.

Beyond its end use, the supercold hydrogen produced at LCLS-II is of scientific interest in its own right.

“At 2.0 Kelvin, helium becomes superfluid, called helium II. , which has extraordinary properties,” Fov said. For example, it conducts heat hundreds of times more efficiently than copper and has such a low viscosity, or resistance to flow, that it can’t be measured, he added.

For LCLS-II, 2 Kelvin is as low as the temperature is expected to drop.

“Lower temperatures can be achieved with very specialized cooling systems that can reach fractions of a degree above absolute zero when all movement comes to a halt,” Burrill said.

But this particular laser is not capable of reaching those limits, he said.

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