LCLS-II Upgrade Will Boost Power of World’s Brightest X-ray Laser

Construction is underway on a major upgrade to LCLS (Linac Coherent Light Source), a unique X-ray laser at the Department of Energy’s SLAC National Accelerator Laboratory. The project, known as LCLS-II, will add a second X-ray laser beam that’s 10,000 times brighter, on average, than the first one and fires 8,000 times faster, up to a million pulses per second.

LCLS-II will greatly increase the power and capacity of LCLS for experiments that sharpen our view of how nature works on the atomic level and on ultrafast timescales. The Department of Energy formally approved construction of the project in March with favorable “Critical Decisions 2 and 3 (CD-2/3)” and its Office of Science will provide $1 billion in funding.

“LCLS-II will take X-ray science to the next level, opening the door to a whole new range of studies of the ultrafast and ultrasmall,” says Mike Dunne, LCLS director. “This will tremendously advance our ability to develop transformative technologies of the future, including novel electronics, life-saving drugs and innovative energy solutions.”

When LCLS opened six years ago as a DOE Office of Science User Facility, it was the first light source of its kind—a unique X-ray microscope that uses the brightest and fastest X-ray pulses ever made to provide unprecedented details of the atomic world.

Hundreds of scientists use LCLS each year to catch a glimpse of nature’s fundamental processes in unprecedented detail. Molecular movies reveal how chemical bonds form and break; ultrafast snapshots capture electric charges as they rapidly rearrange in materials and change their properties; and sharp 3-D images of disease-related proteins provide atomic-level details that could hold the key for discovering potential cures.

LCLS-II, The Next Leap for X-ray Science. Video: SLAC

The new X-ray laser will work in parallel with the existing one, with each occupying one-third of SLAC’s two-mile-long linear accelerator tunnel. Together they will allow researchers to make observations over a wider energy range, capture detailed snapshots of rapid processes, probe delicate samples that are beyond the reach of other light sources and gather more data in less time, thus greatly increasing the number of experiments that can be performed at the facility.

“The upgrade will benefit X-ray experiments in many different ways and I’m very excited to use the new capabilities for my own research,” says Peter Weber, a professor at Brown University who co-led an LCLS study that used X-ray scattering to track ultrafast structural changes as ring-shaped gas molecules burst open in a chemical reaction vital to many processes in nature. “With LCLS-II, we’ll be able to bring the motions of atoms much more into focus, which will help us better understand the dynamics of crucial chemical reactions.”

Like the existing facility, LCLS-II will use electrons accelerated to nearly the speed of light to generate beams of extremely bright X-ray laser light. The electrons fly through an undulator, a series of magnets that forces them to travel a zigzag path and give off energy in the form of X-rays.

But the way those electrons are accelerated will be quite different, and give LCLS-II much different capabilities. At present, electrons are accelerated down a copper pipe that operates at room temperature and allows the generation of 120 X-ray laser pulses per second. For LCLS-II, crews will install a superconducting accelerator. It’s called “superconducting” because its niobium metal cavities conduct electricity with zero loss when chilled to -456°F. Accelerating electrons through a series of these cavities allows the generation of an almost continuous X-ray laser beam with pulses that are 10,000 times brighter, on average, than those of LCLS and arrive up to a million times per second.

In addition to a new accelerator, LCLS-II requires a number of other cutting-edge components, including a new electron source, two powerful cryoplants that produce refrigerant for the niobium structures and two new undulators to generate X-rays.

To make this major upgrade a reality, SLAC has teamed up with four other national labs—Argonne National Laboratory (CSA CSM), Berkeley Lab, Fermi National Accelerator Laboratory (CSA CSM) and Thomas Jefferson National Accelerator Facility (CSA CSM)—and Cornell University, with each partner making key contributions to project planning as well as to component design, acquisition and construction.

“We couldn’t do this without our collaborators,” says John Galayda, head of the LCLS-II project team. “To bring all the components together and succeed, we need the expertise of all partners, their key infrastructure and the commitment of their best people.”

Engineers at Fermilab, for example, are working on a cryogenic distribution system and a prototype cryomodule for LCLS-II. The work is modeled on the European XFEL project at DESY, and the prototype uses cavities from the International Linear Collider (ILC) R&D program. Engineers at Fermilab prepared these cavities for assembly into SRF prototype cryomodules at both Fermilab and the Thomas Jefferson National Accelerator Facility.

LCLS-II cavity string with 300 mm return pipe. Image: Fermilab, Jay Theilacker

LCLS-II cavity string with 300 mm return pipe. Image: Fermilab, Jay Theilacker

At the core of the cryomodule will be eight 1.3 GHz SRF cavities fabricated from ~3 mm thick niobium sheet. Fermilab engineers have already assembled a cavity string in an onsite clean room and attached a 300 mm return pipe, thermal shielding and associated helium pipes above it. The 300 mm pipe will provide a return path for the low-pressure helium flowing off the cryomodule’s 2K bath and will also act as a strong back to support the entire cavity string.

Unlike XFEL and ILC, LCLS-II will be a near-continuous wave X-ray laser (delivering 1,000,000 pulses per second), and will thus have higher heat load, more dynamic losses and increased refrigeration requirements. Fermilab engineers, therefore, chose to modify the ILC SRF cavities using nitrogen doping and other techniques in order to reduce the cryogenic losses.

The Fermilab team also had to find a way to improve the system’s magnetic shielding. Two layers of shielding have been installed on cavities in the string. A high magnetic field can increase the heat load and deteriorate RF performance. Next up, the Fermilab team will install more instrumentation and cold mass components, including tuners, before placing the string into a vacuum vessel. Work on the prototype is expected to be completed in June 2016, with tests beginning thereafter.

Meanwhile, engineers at SLAC are now clearing out the first third of the linac to make room for the superconducting accelerator, scheduled to begin operations in the early 2020s. In the meantime, LCLS will continue to serve the X-ray science community, except for a construction-related, six-month downtime in 2017 and a 12-month shutdown extending from 2018 into 2019.