Early in 2023, electrons flying at nearly the speed of light in a tunnel beneath California will produce the brightest X-rays ever seen on Earth, allowing us to examine atoms and molecules in unprecedented detail.
These record-breaking X-rays will be produced at the SLAC National Accelerator Laboratory, which has upgraded its Linac Coherent Light Source (LCLS) X-ray laser to be the fastest and brightest in the world. While the LCLS produced about 100 X-ray pulses each second, LCLS-II will increase that number to 1 million, with X-rays that are about 10,000 times brighter.
At the heart of the machine are electrons moving through a 3-kilometre-long metal tube. The SLAC researchers knock them out of a copper plate with ultraviolet light, then use devices that emit intense microwave pulses to push the electrons to near the speed of light.
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Once up to speed, the electrons pass through thousands of magnets in rows a few millimetres apart that are arranged with alternating magnetic poles. These alternating poles wiggle electrons back and forth, producing X-rays in predictable, narrow pulses with controllable energies 1 million times every second. The pulses can be used to image inside materials, just like a medical X-ray, but 1 trillion trillion times brighter.
The upgrade was necessary as more powerful and frequent X-ray pulses make more informative images, says at SLAC.
鈥淚t鈥檚 like a super microscope. X-rays have a wavelength about the size of an atom, so you can image individual atoms within a molecule with high precision,鈥� he says. 鈥淲e can see timescales on which atoms make friends with each other, on which chemical bonds are made or broken. It鈥檒l be like watching a movie of molecules evolving.鈥�
This will be useful for studies ranging from fundamental physics to designing solar panels and developing drugs, says , also at SLAC.
Key to the LCLS-II upgrade was refurbishing the tunnel that guides electrons through the magnets by switching the copper lining with a niobium one, cooled to about -271掳C to make it a superconductor. Without this, the upgraded laser would have been so energetic that it would have melted the tunnel.
Researchers assembled about 700 metres of niobium pieces in labs across the US, then had to transport them to California before welding them together without any contamination or errors. They also had to arrange the magnets with exquisite precision, because even the smallest mistake in alignment would have been massively amplified by the fast-moving electrons. 鈥淟CLS-II is a miracle of engineering as well as science,鈥� says Dunne.
The team began sending electrons through the niobium tunnel in September 2022 and hopes to produce the first record-breaking X-rays in February or March 2023. 鈥淚t will be a tool that will give us so much more information than we can get with other methods. I think everyone should want to come to experiment here,鈥� says Cordones-Hahn.
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