Fermilab’s recently assembled prototype cryomodule for SLAC National Accelerator Laboratory’s future LCLS-II is entering its next phase. In late July, the cryomodule was moved to new, temporary lodgings at Fermilab’s Cryomodule Test Facility, where it is hooked up to a new test stand, also just completed.
Now that the new cryomodule and test stand are hitched, testing can begin. The cryomodule will be put through rigorous paces before it heads off to its final destination at SLAC later this year.
LCLS-II, or Linac Coherent Light Source II, is an upgrade to SLAC’s current LCLS, the world’s first hard-X-ray free-electron laser. Like the first LCLS, the more powerful LCLS-II will be used for a variety of experiments in a wide range of fields, such as physics, chemistry, biology and medicine.
Fermilab’s contribution to LCLS-II
Fermilab is contributing cryomodules to LCLS-II’s superconducting linear accelerator, which accelerates electron beams to increasingly higher energies in order to produce X-rays with laser-like qualities. The upgraded accelerator is the piece responsible for bringing more power to LCLS.
The roughly 12-meter-long cylindrical cryomodules, lined up end to end, are the major sections that make up the accelerator.
Fermilab designed the cryomodules for LCLS-II, and the prototype will guide the development of all the cryomodules being built for LCLS-II.
Thirty-five of them will operate at 1.3 gigahertz. Fermilab is building 17 of them, and Thomas Jefferson National Accelerator Facility in Virginia will provide the remaining 18. Fermilab is also building two 3.9-GHz cryomodules, which will help enhance the electron beam’s quality.
During the coming months, the Fermilab LCLS-II team will test each of the completed cryomodules produced here at the new test stand, called the Cryomodule Test Stand 1. Each will take roughly a month to test, except for the prototype, which, as the first, will necessarily undergo additional tests. The team will make measurements to characterize each cryomodule and its cavities to ensure they meet their performance specifications. Early in the test schedule, they will explore different schemes for bringing the cryomodule down to very cold temperatures to maintain its high performance.
“We want to learn as much as we can from the prototype,” said Elvin Harms, who leads the LCLS-II cryomodule testing at Fermilab.
Fermilab began constructing CMTS1 in August 2014 and the cryomodules early this year.
“It was really an aggressive schedule,” project leader Jerry Leibfritz said. “But things went quite smoothly, and we finished construction of CMTS1 on schedule, in less than two years”
Each cryomodule is composed of eight superconducting radio-frequency cavities, which look like shiny metallic beads on a rigid string, along with cooling equipment, structural support and various instruments. The particle beams pass through the center of the SRF cavities, which boost the particles to higher energies.
SRF technology will play a major role in generating the high-intensity electron beams needed for LCLS-II. The cavities’ superconductivity means that they can generate extremely high accelerating voltages with very little loss from electrical resistance, reducing the cost of powering the accelerator.
LCLS-II will deliver beams at a million pulses per second, forming a near-continuous beam 10,000 times brighter than the original. LCLS, which is based on older accelerator technology, generates an X-ray laser beam that pulses 120 times per second.
“To reach high energies in the future, everyone is saying you need something more energy-efficient,” Harms said. “Pretty much every new high-energy particle accelerator now employs or will be using SRF as the technology to give the particles their energy.”
SLAC turned to Fermilab to design the cryomodules because of Fermilab’s expertise in SRF technology.
“Fermilab, together with their collaborators at DESY, built an almost identical cryomodule in 2007,” said SLAC scientist Marc Ross, LCLS-II cryogenics systems manager. “The experience gained from that pre-prototype put them on the map.”
Fermilab scientists believe the investment has paid off.
“Fermilab made a choice to get into the SRF game,” Harms said. “We’ve built up our infrastructure and staff to become one of the world leaders.”
Standing up to the test
The recently completed test stand, CMTS1, takes the form of two stainless steel platforms bolted to the floor. The platforms support large cylinders that clamp onto both ends of the cryomodule, connecting it to its necessary systems through a network of cables, tubes and pumps.
CMTS1 will be used not only for LCLS-II but also for future projects, such as the Proton Improvement Plan II, or PIP-II. PIP-II is a proposed project to overhaul Fermilab’s particle accelerator complex and position the lab as the world leader for accelerator-based neutrino experiments.
The completion of CMTS1 makes Fermilab one of just a few labs in the country capable of testing SRF accelerator cryomodules, enhancing the lab’s standing as a leader in SRF.
“It really completes the package of SRF capabilities for Fermilab,” Leibfritz said.
The cold truth
Fermilab’s is also developing the cryogenic distribution system for the LCLS-II. In order to make the cavities superconductive, the LCLS-II team has to keep them at negative 456 degrees Fahrenheit, just above absolute zero. The cold also helps to expel magnetic flux, which can cause the cavities to lose some of their energy.
“The Tevatron was superconducting, so we have a lot of experience with cryogenics,” said Leibfritz, referring to the high-energy collider that operated at Fermilab from 1983 to 2011. “We have some of the best cryogenics experts in the world.”
Leibfritz expects the 17 main cryomodules to be finished with their tests and shipped off within the next two years. Once finished, the team will move on to the two 3.9-GHz cryomodules.
The goal for SLAC’s $1 billion project is to begin working with the new accelerator in the early 2020s.
“We spent the last two years building,” Leibfritz said. “Now it’s getting exciting.”