The world's most powerful X-ray laser is about to get far more powerful. Since it was first fired up in 2009, the Linac Coherent Light Source (LCLS) has helped scientists peer into the mysterious world of atoms and quantum physics, and now phase two is about to kick off. The first of 37 "cryomodules" has arrived at Stanford University's SLAC National Accelerator Laboratory, which will boost the speed and power of the facility.
Snapping high-resolution X-ray images on the molecular scale requires a "camera" that's 2 miles (3.2 km) long. It all starts at the injector, which blasts a copper plate with ultraviolet light to trigger a burst of electrons. These electrons are then accelerated down a long copper pipe until they reach the Undulator Hall, where powerful magnets introduce a wobble to their previously-straight trajectories. That side-to-side motion causes the electrons to throw off X-rays, and these signals are then amplified and sent to different parts of the facility for different experiments.
Currently, LCLS can fire off 120 X-ray pulses per second. That was once state-of-the-art, but the Stanford facility has since been eclipsed by the European X-ray Free Electron Laser (XFEL) in Germany, which can produce 27,000 pulses per second. LCLS-II is set to snatch back the crown though, boosting its output to a million pulses per second and amplifying the X-rays to 10,000 times the brightness they can currently reach.
This incredible power boost comes courtesy of a series of cryomodules, which are designed to be superconducting to boost the electrons with minimal energy loss. At the core of each cryomodule is a string of eight cavities made of extremely pure niobium, and these are encased in three layers of cooling equipment. While the copper pipe currently in use operates at room temperature, the cryomodules will chill the tube down to -456° F (-271° C), or just above absolute zero. That's as cold as the vacuum of space.
To give the electrons an energy boost as they pass through this incredibly cold environment, microwaves are first generated in solid-state amplifiers housed in an above-ground facility. The microwaves are then piped down into the cryomodules, where they power an oscillating electric field that resonates inside the niobium cavities. As this extremely high voltage oscillates, it's made to sync up with the rhythm of passing bunches of electrons, which transfers the energy to them, accelerating them with minimal losses. By the time the electrons pass through all 37 cryomodules, they'll be traveling at almost the speed of light.
"If a tuning fork – another type of resonator – had the same performance quality as one of these superconducting cavities, it would ring for well over a year," says Marc Ross, who's leading the development of the cryomodules. "Superconductivity allows the cavities to accelerate the electrons in a steady, continuous wave without interruption, and with extremely high efficiency."
The improvements aren't just limited to the cryomodules. The facility will be upgraded at both the beginning and the end of the process, with a new "electron gun" injecting the electrons into the accelerator and a refined undulator to get them wobbling and producing X-rays.
With a million X-ray pulses per second, LCLS-II will be able to gather far more data in much less time. And since it can capture events that occur on the scale of quadrillionths of a second, the upgrade will increase the chances of observing incredibly rare events of a biological, quantum, molecular or chemical nature.
"Within the space of just a few hours, LCLS-II will be able to produce more X-ray pulses than the current laser has delivered in its entire operations to date," says Mike Dunne, director of LCLS. "Data that would currently take a month to collect could be produced in a few minutes."
The first cryomodule arrived at SLAC on January 19, and the remaining 36 are due to arrive over the next 18 months, after intense testing of each one. The current plan is for LCLS-II to achieve first light in early 2020.
The video below gives an overview of the upgrades.
Source: Stanford University
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