Empty Space in CEBAF Helps Make Upgrade Possible

The empty space in the CEBAF linear accelerator sections will be filled with 10 new cryomodules as part of the 12 GeV Upgrade, doubling the energy of JLab's electron beam from 6 Giga-electron Volts to 12 GeV.

Jefferson Lab's Continuous Electron Beam Accelerator Facility is set to undergo its first major upgrade, a $310 million project to enhance the facility's research capabilities. To make it possible, designers are filling in "extra space" in the linear accelerators or straight sections of the machine with additional accelerator components.
                       
But where did that extra space come from?

According to Andrew Hutton, associate director of the Accelerator Division, the space is the result of a trade-off made nearly 25 years ago when the accelerator was being constructed.

"Somewhere along the way, which is often the case in a construction project, money started to become tight," Hutton says.

So, scientists explored ways to cut costs. One idea called for using fewer accelerating components, called cryomodules, in the machine. At that time, cryomodules cost around $1 million each. They contain the equipment that "accelerate" the electrons, essentially giving them more energy.

The accelerator design then under construction called for sending the electrons through the two linear sections of cryomodules up to four times: a four-pass machine. But to use fewer cryomodules, it was suggested that an additional row of bending magnets be added, making the accelerator a five-pass machine.

Bending magnets are required because of the accelerator’s racetrack-shape. The powerful magnets bend the paths of the electrons as they pass through the turns, or arcs.

"After detailed cost and technical analyses, it was found that five-pass would be a cheaper option. But by then, the tunnel contract had already been let and the footprint of the racetrack defined," Hutton explained.

Consequently, the accelerator tunnel was built to the longer, four-pass specifications, and an additional row of bending magnets was added to each arc. That serendipitous outcome now makes possible the machine's next incarnation with the installation of 10 new high-performance cryomodules.

"This additional space, which was for many years sitting there with no real reason, became the defining way of moving to 12 GeV," Hutton said.

For more information on the 12 GeV Upgrade, visit: http://www.jlab.org/12GeV/public.html

By Kandice Carter
Science writer