JLab Undergoes a Renascence

This is a photo of the seven-cell, high-gradient version of a CEBAF accelerator cavity.

Renascence: (n) a rebirth, Renaissance

Accelerator technology is undergoing a renaissance at Jefferson Lab. A new section of the accelerator, called Renascence, features accelerator cavity technology that will provide the backbone of the 12 GeV Upgrade.

The workhorses of Jefferson Lab's CEBAF are the accelerator cavities. The cavities store the radiofrequency waves used to energize electrons for nuclear physics experiments. While the cavities have proven their mettle over a decade of service, accelerator scientists are researching ways to improve the technology for future accelerators, such as Jefferson Lab's 12 GeV Upgrade.

According to Charlie Reece, deputy director of the Institute for Superconducting Radiofrequency Science and Technology, there have been two competing ideas for improving the efficiency of the cavities. Both involve changing the shape of the doughnut-style sections of the cavities.

Whipping Cavities into Shape

CEBAF accelerator cavities are superconducting and made of niobium, which allows the cavities to efficiently energize electrons at an operating temperature near absolute zero. Because the cavities contain no moving parts, their efficiency is determined by their shape and their surface quality.

This graphic shows the differing cavity cell shapes of the original CEBAF five-cell design, the high-gradient seven-cell design and the low-loss (C100) design. The low-loss design will be used for the 12 GeV Upgrade. CAD drawing by Jim Henry, Engineering Division

"Subtle changes in the shape have beneficial effects," Reece explains.

Researchers pored over CEBAF operations data to see how well the original CEBAF cavities, a design that originated at Cornell University, accelerated electrons and what improvements could be made.

"Historically, the performance-limiting problem for CEBAF was field-emission," Reece says. This problem is caused by tiny nubbins of material on the inside surface of the cavity. These nubbins roguishly spew out wayward electrons that absorb the radiofrequency energy. The wayward electrons then plow into the cavity wall, heating it and essentially shorting out the cavity.

One way to prevent this problem is to minimize the electric fields these nubbins are exposed to. "If that's what you're concerned about, you build a cavity that has the minimum surface electric fields on the wall, compared to the field that's accelerating the beam. That design is the high-gradient cavity shape," Reece explains.

Though superconducting at low temperatures, the cavities generate some heat during operation. If the cavities generate more heat than the super-cold liquid helium circulating around the cavities can carry away, the cavities lose their superconductivity. So, researchers sought a cavity shape to minimize the cavity-generated heat. The low-loss cavity shape provides the same acceleration, with about 20 percent less heat, Reece says.

Renascence the Cryomodule

Both the high-gradient and low-loss cavity designs are featured in Renascence. Renascence is the third of a series of test cryomodules built since CEBAF began operation. Installed in the CEBAF accelerator for testing in 2007, Renascence is a modular section of the accelerator that contains eight cavities and much of the associated equipment needed to operate them.

There are five so-called, high-gradient cavities in Renascence and three low-loss. Of those, three high-gradient cavities are not performing well. "One has a tuner problem, one of them we believe was damaged in high-pressure rinse processing, and the window leaks on another one," Reece says. "The rest are performing quite nicely."

Pictured here is a portion of the endgroup components for a Renascence cavity during the machining process.

The researchers also used the cryomodule to test other new concepts.

For instance, the cryomodule contains new HOM couplers. Higher-order modes are akin to the overtone of harmonic frequencies that you hear when you pluck a chord on a guitar. Accelerator cavities store radiofrequency energy by vibrating much like a guitar string. Like the string, they also can resonate at extra harmonic frequencies, which interfere with the cavity's operation. HOM couplers damp those harmonics. The current CEBAF cryomodules use coupler technology developed at Cornell, while the new couplers were designed at DESY and adapted for use at JLab.

"The amount of HOM damping in the original CEBAF cavities was extremely generous, but we found that we could get by with much less. So we designed the damping to meet the new cavities' specification," Reece explains. "Unfortunately, some last-minute changes to the couplers to address a cooling problem created a damping problem in one of the Renascence cavities. This caused some operational issues last year, but also presented the accelerator physicists a rare opportunity to thoroughly study what happens when the damping is not quite strong enough."

In addition, design changes also were made to accommodate the newer cavities, which are longer. The original CEBAF cavities contained five cells. Newer designs use seven, so some equipment between the cavities was streamlined.

Other new technologies tested in Renascence include new gaskets, flanges, a new clamp developed at Jefferson Lab, refinements to insulation in the cryomodule and   other improvements.

This is a computer-aided design drawing of a CEBAF Upgrade cryomodule. The cavities (orange) reside inside helium vessels (teal and pink). Also shown is a liquid helium delivery pipe (grey), structural supports (yellow and red) and fundamental power input couplers (flat orange pipes entering module). CAD drawing by Jim Henry, Engineering Division

Renascence at Work

Although Renascence has exhibited some problems, it was among the best-performing cryomodules in the accelerator until its removal in March due to input radiofrequency window leaks into the beam line, even with only five of eight cavities running. Reece says that cryomodule designers have learned a lot from Renascence and their testing has provided invaluable information.

"This Renascence module was an extremely fruitful prototype for Jefferson Lab; it was extremely valuable. The 12 GeV Upgrade Project will be much more robust because of what we have been able to learn from it," Reece concludes.

Plans are being developed to rework the Renascence cryomodule after CEBAF is strong enough to support the 6 GeV physics program without it. Perhaps it will emerge yet again with a new lease on life – living up to its name.