Hollywood’s finest tradition is to follow up a smash hit with a much-anticipated sequel, and so it will be with the Department of Energy’s Jefferson Lab and its Continuous Electron Beam Accelerator Facility. On May 18, 2012, CEBAF shut down after a long and highly successful 17-year run, during which scientists completed more than 175 experiments in the exploration of the nature of matter. A little over a year from now, CEBAF will ramp up to begin commissioning new hardware. In 2015, CEBAF will return with double the energy and a host of other enhancements, providing scientists with unprecedented precision and reach for studies of the particles and forces that build our visible universe.
In the wee hours of May 18, the last crew to run the 6 GeV CEBAF accelerator caught a glimpse of the future: Its newest section of hardware had operated for an hour at its full design energy for the first time. That meant that, although the machine was about to be shut down for well over a year for an upgrade, the core acceleration technologies that would make the upgrade a reality had just passed an ultimate test.
“That run was the culmination of a decade's worth of effort,” says Leigh Harwood, associate project manager for accelerators in the upgrade.
The May test focused on new sections of accelerator, called cryomodules, and their associated systems.
“We did it literally years ahead of when we were supposed to have demonstrated it. So that's why it was a big deal,” says John Hogan, a staff engineer in the lab’s SRF Institute. Hogan is responsible for the final design, parts acquisition, manufacturing, initial testing and installation of the new cryomodules, called C100s.
C100s take shape
At the core of a cryomodule are eight cavities: hollow metal cylinders with seven connected compartments that roughly resemble a stack of doughnuts. These cavities are designed to harness energy that is pumped into them, focusing it onto, and thus accelerating, a thin beam of electrons.
The cavities, which are made of a metal called niobium, become superconducting at just a few degrees above absolute zero, allowing energy to flow through them without resistance. The cavities are encased in a helium vessel, where they are bathed in a constant stream of liquid helium to keep them cold. The vessels are cocooned with a variety of insulating material, including aluminized mylar, the stuff of shiny party balloons.
Tuners are attached to the ends of the cavities, which allow for adjustment of the cavities’ ability to harness the energy pumped into them. Other mechanical necessities, wires and parts are suspended in a space frame, and the entire assembly is then encased in the outer stainless steel shell, sometimes called the vacuum vessel.
“From a single cavity to a finished cryomodule is easily six months,” Hogan says. “A cavity alone takes two months to get through the whole qualification process. And once you have eight cavities qualified, you build them into a string. Once the string is complete, it takes nominally three months to build it out from a string to a complete cryomodule.”
Once complete, a cryomodule is three feet in diameter and 30 feet long. It weighs in at roughly 12,000 pounds and contains at least 1,000 major parts, many of which were designed at Jefferson Lab and are now patented technologies.
Test success
Much hard work testing and installing the new cyromodule paid off on the morning of May 18. Approaching an hour after midnight, the team tentatively ramped the module up to its full design specification and ran it there for a minute. It successfully ran at its full specification, imparting 108 megavolts of power to the electron beam (C100 refers to this ~100 MV design specification). In comparison, the average original CEBAF cryomodules impart just 20 MV of power to the electron, with the best original module reaching just 32 MV.
Then came the real test: could the cryomodule run at full specification, while also delivering the most demanding beam ever required of CEBAF for experiments in two halls simultaneously?
The team attempted the one-hour test three times in quick succession. The third proved the charm. At 2:55 a.m., the cryomodule had achieved an hour of stable running at full specification.
"This test has demonstrated that the integrated cryomodule plus microwave-power system can successfully deliver the performance that was envisioned in 2001, and which is needed for the planned nuclear physics research program in the 12 GeV era," says Leigh Harwood, associate project manager for the 12 GeV Upgrade project. "We've demonstrated that there are no fundamental design problems that we overlooked. We now know we can rely on this critical new technology."
Arne Freyberger, head of accelerator operations, agrees. “This was a significant accomplishment, which I've already said, and it took a lot of work. It also took a lot of cooperation with the users as well, the scientists. And so, I thank them for their patience. And I think it shows that we can do more than one thing at once here, and do it all exceptionally well,” he says.
The upgrade continues
Now, plans for the debut of CEBAF's much-anticipated sequel are underway. The CEBAF accelerator was shut down at 8:18 a.m. on May 18 for the Long Shutdown. Over the next year-plus, Jefferson Lab staffers will be busy preparing CEBAF—the accelerator, the experimental halls, the cryogenics system and all other related systems—for operations at 12 GeV.
The racetrack-shaped CEBAF originally operated with about 20 cryomodules in each of its two straight sections. For the upgrade, each straight section will get an additional five cryomodules, for a total of 10 new modules. All of the components of the new radiofrequency system, along with the many other support components, will also be installed. Many of the magnets in the accelerator, used to control and steer the electron beam, are being refurbished so they can handle electrons at higher energies. The cryogenic system, needed to keep the cavities superconducting, is also undergoing a major upgrade and maintenance.
So far, six cryomodules have been completed, and the rest are in various stages of construction. This fall, Harwood expects that half of the new cryomodules will be installed. By next fall, the machine and all its systems will be gearing up for the first, weeks-long commissioning run of all of the new modules and systems.
“The effort that was put in from November to May in understanding all the control issues to get the cryomodules working correctly, that effort was very productive and resulted in these modules reaching their design energy,” says Arne Freyberger, head of accelerator operations. “In November 2013, when we come to commission this machine, we're going to be so much further up the learning curve in how to run this machine. The C100s are the major new component to the CEBAF accelerator. There will be new magnets and there will be new power supplies for magnets, but those are not as challenging to commission as a new cryomodule. So to get this far up the learning curve is a tremendous advance.”
A version of this article appeared in DOE Pulse on Sept. 24, 2012.