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

The physics community is rallying around CERN's Large Hadron Collider, which is finally operational after a major renovation and a years-long hiatus. But it isn't the only scientific gadget that has received fresh vigor. Another one is being finished over 6,000 kilometres distant, on the opposite side of the world.

The SLAC National Accelerator Laboratory, located south of San Francisco, is home to the LCLS, a powerful laser that allows scientists to peek inside molecules using X-rays.“The way to think about a facility like LCLS is really as a super-resolution microscope,” says the facility's director, Mike Dunne.

Now, LCLS has just completed a major update known as LCLS-II, which reduces the laser to a few degrees above absolute zero.

Giving a particle accelerator new life

A particle accelerator was located in SLAC's tunnel half a century ago. While most particle accelerators today hurl their targets in circles, this one was precisely straight. It had to be more than 2 kilometers long to accelerate electrons for smashing. It was the "longest building in the world" for decades after it debuted. (The tunnel, a mile-long straight line cut into the hillsides, is so unique that pilots use it for navigation.)

When it first went online in 1966, the Stanford Linear Accelerator was a technological wonder. In the decades afterwards, the particle physics research undertaken there has resulted in no less than three Nobel Awards in physics. By the twenty-first century, though, it had become somewhat of a relic, having been eclipsed by other accelerators at CERN and elsewhere that could smash particles at much greater energy and observe things Stanford couldn't.

But the 2-mile-long structure persisted, and SLAC installed it with a new machine in 2009: the Linac Coherent Light Source (LCLS).

The LCLS is an example of an X-ray free-electron laser apparatus (XFEL). Although it is a laser, it has little in common with the portable laser pointers that kitties love. These use electronic components such as diodes to generate a laser beam.

An XFEL, on the other hand, is much more like a particle accelerator. In reality, the laser's initial stage is to accelerate an electron beam to near the speed of light. The electrons are then forced to zig-zag through a series of magnets in quick switchbacks. The electrons' immense energy is released as X-rays as a result of this process.

This can generate a variety of electromagnetic waves, ranging from microwaves to ultraviolet to visible light. However, scientists favor X-rays. This is because X-rays have wavelengths similar to atoms, allowing scientists to peek into molecules when focussed in a strong beam.

LCLS is not like most other X-ray sources in the globe. The California beam functions similarly to a strobe light. “Each flash captures the motion of that molecule in a particular state,” Dunne explains.

LCLS was initially capable of firing 100 flashes per second. This enabled scientists to create, for example, a video of a chemical process as it occurred. They could observe atom bonds forming and breaking, as well as the formation of new molecules. It might soon be capable of producing movies with frame rates thousands of times quicker.

Chilling a laser

LCLS employed copper structures to accelerate electrons in its first generation. However, boosting the overall power of the machine was stretching the boundaries of that copper. “The copper just is pulling too much current, so it melts, just like when you fuse a wire in your fuse box,” Dunne explains.

There is a workaround: the strange quantum phenomenon known as superconductivity.

When a material is cooled below a specific critical temperature, its electrical resistance reduces to almost nothing. Then you can cause current to flow continuously without losing energy to its surroundings in the form of heat.

“It gets really hard to support these cryogenic systems that cool to very low temperatures,” says Georg Hoffstaetter, a scientist at Cornell University who worked on the subject previously. There are superconducting materials that can operate at somewhat lower temperatures, but none of them can operate in areas hundreds of feet long.

This difficulty could have intimidated a smaller operation, but SLAC erected a warehouse-sized refrigerator at one end of the building. The accelerator is cooled to -456°F using liquid helium.

Superconductivity also has the advantage of making the setup more energy-efficient; huge physics facilities are infamous for requiring the same amount of electricity as small nations. “The superconducting technology in itself is, in a way, a green technology, because so little of the accelerator power gets turned into heat,” adds Hoffstaetter.

When the changes are completed, the new and enhanced LCLS-II will be capable of delivering up to a million pulses per second, rather than only 100.

What to do with a million frames per second

According to Dunne, there are essentially three major areas where the beam can enhance research. For example, the X-ray beam can assist scientists in determining how to speed up operations while using less material, perhaps leading to more ecologically friendly industrial processes or more efficient solar panels.

For example, the instrument can help biologists with drug discovery by examining how medicines affect enzymes in the human body that are difficult to investigate using conventional approaches.

For a third, the beam can assist materials scientists in better understanding how a material may react under severe conditions such as an X-ray onslaught. Scientists may also utilize it to create new materials, such as even better superconductors for future physics machines like this one.

The miles-long facility that houses SLAC’s Linac Coherent Light Source X-ray free-electron laser.

Of course, there's a catch. As with each substantial improvement to a machine like this one, physicists must learn how to use their new tools. “You’ve sort of got to learn how to do that science from scratch,” Dunne explains. “It’s not just what you did before… It’s an entirely new field.”

One issue that scientists will have to address is how to handle the data that the laser generates: one terabyte every second. It's already a challenge for huge institutions, and it's just going to become worse if networks and supercomputers can't keep up.

Nonetheless, physicists' ardour for advancement has not waned. Scientists are already planning another upgrade for the laser, which will raise its energy and allow it to delve much deeper into the world of atoms.