In 2009, the lab repurposed one-third of its original 1960s-era copper accelerator to feed an electron beam into LCLS, the first laser of its kind that produces rapid pulses of “hard” or high-energy X-rays for innovative imaging experiments. Another one-third of that original copper linac has now been cleared to make room for the arrival of the new superconducting cryomodules for LCLS-II.
LCLS-II will be the nation’s only X-ray free-electron laser facility, generating the brightest X-ray pulses ever made to provide research teams with unprecedented details of the atomic world. Research goals include examining the details of complex materials with unparalleled resolution; uncovering rare and transient chemical events; studying how biological molecules perform life’s functions; and peering into the strange world of quantum mechanics by directly measuring the internal motions of individual atoms and molecules.
“It required years of effort from large teams of engineers and scientists in the United States and around the world to make the arrival of the first cryomodule at SLAC a reality,” says John Galayda, SLAC’s project director for LCLS-II. “And it marks an important step forward as we construct this innovative machine.”
The cryomodules, when lined up end to end, will make up the bulk of the superconducting accelerator that will power the massive three-mile-long machine. Fermi National Accelerator Laboratory (CSA CSM) is set to provide 22 of the modules, with the rest contributed by the Thomas Jefferson National Accelerator Facility (CSA CSM). The units will all be painted “international orange” as a tribute to the Golden Gate Bridge, SLAC’s Bay Area neighbor.
The cryomodules for LCLS-II are based on those recently commissioned for the European X-ray Free-Electron Laser. Engineers at Fermilab and Jefferson Lab tweaked the design of those cryomodules to tailor the equipment for LCLS-II. The teams also greatly improved the quality of the cavities through a technique called nitrogen doping to produce cavities that generate less heat at the coldest temperatures. These tweaks, according to Fermilab, will reduce energy loss, make a much brighter laser possible, lower the cryogenic load and double the quality factor compared to the previous state of the art. LCLS-II will be the first large-scale implementation of these latest technical advances.
“LCLS-II represents an important technological step which demonstrates that we can build more efficient and more powerful accelerators,” says Nigel Lockyer, Fermilab director. “This is a major milestone for our accelerator program, for our productive collaboration with SLAC and Jefferson Lab and for the worldwide accelerator community.”
Inside each cryomodule is a string of eight niobium cavities. The element is a common material for superconductors, and the cavities are made with an extremely pure version to minimize any electrical loss. “They’re assembled like a ship in a bottle,” says Marc Ross, a SLAC accelerator physicist who is leading the development of the cryomodules.Three nested layers of cooling equipment, with each successive layer lowering the temperature until it reaches 2 K, surround the cavities. SLAC engineers and industry partners are building a cryoplant that will contain the compressors, pumps and liquid helium needed to sustain this temperature (see T. Peterson, “The LCLS-II Cryogenic System,” Cold Facts Vol. 33, No.4.).
“The difference between room temperature and a few Kelvin creates a problem, one that manifests as vibrations in the cryomodule,” says Genfa Wu, a Fermilab scientist working on LCLS-II. “And vibrations are bad for linear accelerator operation.”
In initial tests of the prototype cryomodule at Fermilab, scientists found vibration levels that were higher than specification. To diagnose the problem, the team used geophones—the same kind of equipment that can detect earthquakes—to rule out external vibration sources, determining that the cause was inside the cryomodule.
Fermilab engineers then made a number of changes to the modules, including adjusting the path of the flow of liquid helium. The changes worked, according to the team, substantially reducing vibration levels to a tenth of what they were originally, and have been successfully applied to subsequent cryomodules.Fermilab scientists and engineers also addressed unwanted magnetic fields in the cryomodule, ones that could have otherwise reduced operating efficiency. Engineers at the lab have tested seven cryomodules, plus one built and previously tested at Jefferson Lab. Each of these, along with the modules yet to be built and tested, will be transferred by truck to SLAC in the months and years to come.
The microwave energy needed for the accelerator will be produced with solid-state amplifiers in an aboveground facility and then fed through waveguides to the cryomodules, housed 30 feet below ground. There, the electric field generated by the microwaves will build strength within the niobium cavities inside each cryomodule.
Scientists and engineers time the oscillating voltage in each cavity to the rhythm of electron bunches passing through the cavities—the electrons get a boost of energy and accelerate. By the time the electrons pass through all 37 cryomodules they’ll be traveling at nearly the speed of light.
After the electrons reach this high speed, they’ll pass through a series of strong magnets, called undulators, that will bounce the electron beam back and forth to generate an X-ray laser beam that’s 10,000 times brighter than the current LCLS, moving from 120 pulses per second to 1 million pulses per second—far beyond any other facility in the world.“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.”
Engineers from Lawrence Berkeley National Laboratory—with significant design contributions by Argonne National Laboratory (CSA CSM)—created the advanced “electron gun” to inject electrons into the accelerator and the specialized undulators used to generate the X-rays.
When the oscillating voltage in each cavity is timed to the rhythm of electron bunches passing through the cavities, the electrons get a boost of energy and accelerate. “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 Ross. “Superconductivity allows the cavities to accelerate the electrons in a steady, continuous wave without interruption, and with extremely high efficiency.”
The LCLS-II superconducting accelerator will work in parallel with the original. SLAC researchers say having two laser beams will open up entirely new types of studies of the quantum world, informing the development of materials with novel characteristics. Construction on LCLS-II began last year and scientists there expect that the Department of Energy user facility will open to researchers from around the world in the early 2020s. ■