A half-mile-long tunnel under Menlo Park, Calif., has become colder than most of the universe because of a particle accelerator that slams electrons together here on Earth. Using the X-ray free-electron laser at the Department of Energy's SLAC National Accelerator Laboratory – part of an upgrade project to the Linac Coherent Light Source (LCLS) called LCLS II – scientists chilled liquid helium to -456 °F (-271 °C). That is just 2 kelvins above absolute zero, the coldest possible temperature at which all particle movement ceases. That frosty environment is crucial for the accelerator because at such low temperatures, the machine becomes superconducting, meaning it can boost electrons through it with near zero energy loss. Even empty regions of space aren't this cold, as they are still filled with the cosmic microwave background radiation, a remnant from shortly after the Big Bang that has a uniform temperature of -454 °F (-271 °C or 3 K). 

“The next-generation, superconducting accelerator of the LCLS-II X-ray free-electron laser has reached its operating temperature of two degrees above absolute zero,” Andrew Burrill, director of SLAC's Accelerator Directorate, said in an interview with “Live Science.” LCLS-II can now accelerate electrons at one million pulses per second, which is a world record, Burrill added. 

“This is four orders of magnitude, more pulses per second than its predecessor, LCLS, meaning that we will have sent more X-rays to users [who aim to utilize them in experiments] than LCLS has done in the past 10 years,” Burrill said. 

This is one of the last milestones that LCLS-II needs to achieve before it can go on to produce X-ray pulses that are, on average, 10,000 times brighter than those created by its predecessor. This should help researchers probe complex materials in unprecedented detail. The high-intensity, high-frequency laser pulses enable researchers to see how electrons and atoms in materials interact with unprecedented clarity. This will have numerous applications, from helping to reveal “how natural and man-made molecular systems convert sunlight into fuels, and thus how to control these processes, to understanding the fundamental properties of materials that will enable quantum computing,” Burill said. 

Creating the freezing climate inside the accelerator took some work. To keep the helium from boiling away, the team needed super-low pressures. Eric Fauve, director of the cryogenic division at SLAC, told “Live Science” that at sea level, pure water boils at 212 °F (100 °C), but this boiling temperature varies with pressure. For example, in a pressure cooker, the pressure is higher, and water boils at 250 °F (121 °C), while the reverse is true at altitude, where pressure is lower and water boils at a lower temperature.

“For helium, it is very much the same. At atmospheric pressure, helium will boil at 4.2 kelvins; however, this temperature will decrease if the pressure decreases,” Fauve said. “To lower the temperature to 2 kelvin, we need to have a pressure of just 1/30 of atmospheric pressure.”

To achieve these low pressures, the team uses five cryogenic centrifugal compressors, which compress the helium to cool it and then let it expand in a chamber to lower the pressure, making it one of the few places on Earth where 2 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 from an engine turbo-compressor. 

“While spinning, the impeller accelerates the helium molecules, creating a vacuum at the center of the wheel where molecules are sucked and generating pressure at the periphery of the wheel where molecules are ejected,” he said. 

Compression forces the helium to take its liquid state, but the helium escapes into this vacuum where it expands rapidly, cooling as it does so. In addition to its ultimate applications, the ultracold hydrogen created at LCLS-II is a scientific curiosity in itself. 

“At 2.0 kelvin, helium becomes a superfluid called helium II that has extraordinary properties,” Fauve said. “For instance, it conducts heat hundreds of times more efficiently than copper, and it has such low viscosity — or resistance to flow — that this can’t be measured.” 

For LCLS-II, 2 K is as low as temperatures are expected to go. “Lower temperatures can be achieved with very specialized cooling systems that can reach a fraction of a degree above absolute zero,” Burrill said. “But this particular laser doesn't have the ability to reach those extremes.” 

Originally published on “Live Science.”

Image: The LCLS-II accelerator, where temperatures 2 K above absolute zero have been achieved. Credit: Greg Stewart/SLAC National Accelerator Laboratory