Each of the eight cylindrical segments in the string (see picture) contains a niobium accelerating cavity - a structure that harnesses the energy that the machine uses to accelerate electrons. The cavities were manufactured by RI Research Instruments GmbH at a cost of about $81,000 each. The cavities are the first to be tested, cleaned and assembled into a string.
Leigh Harwood, associate project manager for the 12 GeV Upgrade project, says the niobium cavities differ from those currently in use in Jefferson Lab's CEBAF accelerator.
"The cell shape used in the original cryomodules was developed at Cornell in the early 1980s," Harwood says. "The new cell shape leverages the subsequent progress that has been made in preparation of the niobium surfaces."
Since the original machine was finished in 1993, great progress has been made by JLab's SRF Institute, and others, in understanding the factors that affect the performance of niobium cavities, including novel cell shapes, cleaning and assembly.
"With the current processes, higher fields can be reached with lower generation of heat. Higher surface electric fields can also be used. This permits the shape to be adjusted to reduce the amount of heat produced by an additional one third, reducing the load on the helium refrigerator for a given amount of accelerating field," Harwood says.
The 12 GeV Upgrade cavities, called C100 cavities, are also longer. The original cavities contained five cells, while the new C100s each have seven. This will permit greater acceleration of the electrons in the space available for installation in Jefferson Lab's CEBAF accelerator.
In all, Jefferson Lab has purchased 86 niobium cavities for the 12 GeV Upgrade project. Of those, 84 have been delivered. The best of these cavities (80 in total) will be distributed among 10 new cryomodules for the CEBAF accelerator. The first new cryomodules to be completed are planned for installation this summer. The rest will be installed in a year-long installation period that begins in May 2012. After the installation period, the accelerator will enter a commissioning phase for the new and upgraded equipment and facilities.