How to make an X-ray laser that’s colder than space

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The physics community is rallying around CERN’s Large Hadron Collider. It has just come online after a long upgrade and a yearlong pause .. This isn’t the only science machine that can literally receive new energy. Nearly 6,000 miles away, on the other side of the globe, another one is undergoing its final touches.

The SLAC National Accelerator Laboratory is south of San Francisco and houses a large laser called LCLS. This allows scientists to use X-rays for observing molecules. “A facility like LCLS can be thought of as a super-resolution microscope,” says Mike Dunne , the facility’s director.

LCLS just completed a major upgrade–LCLS-II–that lowered the laser to just a few degrees above absolute 0.

Giving an accelerator new life

A half-century ago, SLAC had a tunnel that housed a particle accelerator. While most particle accelerators send their quarry spinning in circles, this accelerator was straight. It had to be more than 2 miles long to bring electrons up speed for smashing. For decades after it opened, it was the “longest building in the world.” (The tunnel is so distinctive, a miles-long straight line carved into foothills, that pilots use it for wayfinding. )

When it came online in 1966, this so-called Stanford Linear Accelerator was an engineering marvel. In the following decades, the particle physics research conducted there led to no fewer than three Nobel prizes in physics. But by the 21st century, it had become something of a relic, surpassed by other accelerators at CERN and elsewhere that could smash particles at far higher energies and see things Stanford couldn’t.

But that 2-mile-long building remained, and in 2009, SLAC outfitted it with a new machine: the Linac Coherent Light Source (LCLS).

LCLS (X-ray-free-electron-laser) is an example of such an apparatus. It is a laser but it doesn’t share much with the handheld laser pointers that excite kittens. These laser beams are created using electronic components, such as diodes.

AnXFEL has much more in common than a particle accelerator. This is actually the laser’s first stage. It accelerates a beam electrons to very close to the speed of light. Next, the electrons are forced to zig-zag through a series of magnets. The electrons then release their immense energy as X-rays.

How to make an X-ray laser that’s colder than space
The electron gun that’s the source of the beam. Marilyn Chung/Berkeley Lab via SLAC

Doing so can create electromagnetic waves ranging from visible light to ultraviolet to microwaves. However, scientists prefer X-rays. Because X-rays have wavelengths about the same size as atoms and can be focused in a powerful beam to allow scientists to see inside molecules.

[Related: Scientists are putting the X factor back in X-rays]

LCLS differs from other X-ray sources around the world. The California beam functions like a strobe lamp. Dunne says that each flash captures the motions of a particular molecule in a particular state.

LCLS could originally shoot 100 flashes per second. Scientists could then create a movie of chemical reactions as they happen. They could observe how bonds between atoms formed and broke down and create new molecules. It could soon be able make movies with frames rates thousands of times faster.

Chilling a laser

In its initial iteration, LCLS used copper structures in order to accelerate its electrons. However, increasing the machine’s power was exceeding the limits of the copper. Dunne says that the copper is melting because it is pulling too much current.

There’s a way around that: the bizarre quantum effect called superconductivity.

When a material is lowered below a critical temperature, its electrical resistivity drops to almost nothing. This allows current to flow functionally indefinitely without losing energy to the surrounding environment, such as heat.

LCLS was not the first laser to use this technology. The problem is that getting to that temperature–typically just a few degrees above absolute zero–is no small feat.

[Related: Scientists found a fleeting particle from the universe’s first moments]

“It gets really hard to support these cryogenic systems that cool to very low temperatures,” says Georg Hoffstaetter, a physicist at Cornell University who had previously worked on the technology. Although superconducting materials can operate at temperatures slightly lower than those of the original, they are not able to work in spaces of hundreds of feet.

A smaller facility might have been overwhelmed by this challenge but SLAC constructed a large refrigerator at one end. It uses liquid helium to cool the accelerator down to -456degF.

Superconductivity also has the bonus of making the setup more energy-efficient; large physics facilities are notorious for using as much electricity as small countries do. Hoffstaetter says that superconducting technology is in a way a green technology because so little accelerator power is converted into heat.

When the upgrades are finished, the new and improved LCLS-II will be able to deliver not just 100 pulses a second, but as many as a million.

What do you do with a million frames per minute

Dunne states that the beam can be used to advance science in three areas. One, the Xray beam can be used to help chemists find out how to make reactions run faster with less material. This could lead to more environmentally-friendly industrial processes or more efficient solar cells.

Another use of the tool is to aid biologists in drug discovery, which examines how pharmaceuticals affect enzymes in the body. This is a difficult task that can be done using other methods.

A third benefit is that the beam can be used to help materials scientists understand how a material behaves under extreme conditions such as an Xray barrage. It can also be used by scientists to design new substances, such as superconductors that will help build future physics machines.

SLAC's Linac Coherent Light Source X-ray free-electron laser is housed in this building.
The miles-long facility that houses SLAC’s Linac Coherent Light Source X-ray free-electron laser. SLAC National Accelerator Laboratory

There’s a catch. Like any major upgrade to a machine, physicists must learn how to use the new tools. Dunne says, “You’ve got to learn how you do that science from scratch.” “It’s not what you did before…It is an entirely new field .”

Scientists will have to figure out how to deal with the laser’s data: one terabyte every second. It’s already a hurdle that large facilities face, and it’s likely to get even more acute if networks and supercomputers can’t quite keep up.

This hasn’t dampened physicists enthusiasm for enhancement. Scientists are already plotting yet another update for the laser, set for later in the 2020s, which will boost its energy and allow it to probe even deeper into the world of atoms.

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