Jefferson Lab photos by various photographers, including Aileen Devlin/Greg Adams/Kandice Carter
Particle accelerator scientists have made the measurements, crunched the numbers and consolidated the data on the performance of the brightest electron accelerator for nuclear physics research
NEWPORT NEWS, VA – It’s a paper that’s been more than four decades in the making. A new study has re-benchmarked the main particle accelerator at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility. It has captured the accelerator’s original and upgraded operating parameters and describes in detail its main systems and subsystems, capabilities and limits.
The new study was led by Geoffrey Krafft, a member of Jefferson Lab’s Center for Advanced Studies of Accelerators and director of the Accelerator Education Program, as well as a Jefferson Lab Professor in Old Dominion University’s Center for Accelerator Science. The 63-page paper focuses on Jefferson Lab’s Continuous Electron Beam Accelerator Facility, also known as CEBAF.
“No accelerator like CEBAF has ever been built before. So, telling how it was built and how it works, it's not trivial,” he said.
A unique machine for exploring the atom’s nucleus
Forty years ago, in August 1983, Congress approved initial funding for research, development and design for what would soon become the Continuous Electron Beam Accelerator Facility. The particle accelerator’s purpose was to deliver beams of energetic electrons for nuclear physics experiments. Each of these electrons would be sent through the machine to be accelerated – ramped up to higher speeds and stuffed full of additional energies of up to 4 billion electron-Volts (4 GeV). These electrons would then be sent into a target, where they would interact with the target’s protons and neutrons to reveal the details of these particles’ structures and interactions inside ordinary matter.
As the 1980s wore on and the facility began taking shape, it was clear that it would be a new particle accelerator like no other.
For one, the design of the machine was radically altered in 1985. Originally, the design called for a room-temperature copper-based linear accelerator (and pulse stretcher ring) with an operating energy of 4 GeV. But by 1985, the facility’s first director, Hermann Grunder, decided to throw out that conservative design to try something new: an accelerator powered by superconducting radiofrequency technology. It was hoped that this material would enable the machine to reach even higher energies and also would make the accelerator upgradeable in the future.
SRF technology is built on a material called niobium, which becomes superconducting when it is cooled to near absolute zero. This allows SRF accelerators to use niobium SRF accelerator cavities to propel particle beams with much less energy lost to heat. Unlike the existing copper-based particle accelerators of the time, an SRF accelerator can deliver these beams nearly continuously, allowing researchers to conduct the same experiment many times over without the accelerator overheating.
The technology had shown promise in multiple university research labs, but it had never been built into a large-scale machine. The facility moved ahead with the new design, and by 1988, construction had begun at the site.
CEBAF had begun to take shape.
The machine was completed in December 1993 with the placing of its last section of accelerator, called a cryomodule. The complete machine would house two plus a one-quarter cryomodule in the machine’s injector, where the electron beams were formed. Another 40 of these cryomodules were lined up end to end in two linear accelerators (20 in each linac), which were connected by two arcs. This racetrack design was 7/8 of a mile around, and it would allow electrons to travel through the machine up to five times, gaining additional energy with each pass, before being directed into an experimental hall for research.
By 1995, CEBAF was already taking data in for nuclear physics experiments, some of which had been conceived nearly two decades prior. Less than two years later, all three of CEBAF’s experimental halls – Halls A, B and C – were up and running.
By 2001, Krafft and his accelerator scientist colleagues Christoph Leemann (who would later become Jefferson Lab’s second director) and David Douglas had gathered enough information to publish a benchmark paper on CEBAF. “The Continuous Electron Beam Accelerator Facility: CEBAF at the Jefferson Laboratory” was published in the Annual Review of Nuclear and Particle Science and has served as the resource for all things CEBAF ever since. This technical journal article provided nuclear and accelerator physicists with everything they needed to know about CEBAF, including its key technologies, operating specifications and demonstrated capabilities.
