Digital Self-Excited Loop Makes Accelerator Tuning a Breeze

Jean Delayen
Accelerator Division

If you've ever plunked a few notes on a dusty piano, you may have noticed some that were off-key. Those notes can regain their bell-like quality with a simple tuning of the piano strings. Similarly, components in Jefferson Lab's accelerators, called cavities, also require regular tuning to function properly. Now, JLab accelerator staffers are perfecting a technology that will take most of the time and guesswork out of tuning cavities after the planned 12 GeV upgrade.

The technology was originally developed by Jean Delayen, a principal scientist in JLab's Accelerator Division. When Delayen was a graduate student, scientists were having a tough time producing reliable cavities for certain accelerators. "There were small machines, mostly to accelerate heavy particles. Not electrons like here, but protons and things like that," Delayen recalls. "The accelerating structures to do that were very complicated and were really shaking a lot."

At JLab, cavities are the structures that shape the energy waves used to accelerate electrons. They do so by maintaining waves of energy that electrons can "surf" through the accelerator. To accelerate a beam of particles, each cavity must maintain a stable wave, and all cavities must maintain the same wave.

"If you have lots of cavities shaking independently of each other, the beam going through wouldn't know what to do. The accelerator just wouldn't work," Delayen adds.

A stable wave is initiated at JLab with a generator that produces radiofrequency waves. The radiofrequency waves are forced into each cavity and tweaked until the cavity sustains a stable wave. Then all of the cavities are gently tuned to the same wave.

While this method works now, it won't work as well when the energy of the machine is doubled to 12 billion electron volts (GeV). It's expected that the 80 new cavities added to the machine for the 12 GeV Upgrade will not only be more difficult to tune but will also take much longer. To prevent that, accelerator staff turned to the technology Delayen developed as a student.

Engineering Division staff engineers (from left) Curt Hovater, John Musson, Trent Allison and Tomasz Plawski are working with Jean Delayen to take Delayen’s cavity-tuning technology digital.

Delayen's technology establishes the radiofrequency wave differently. Instead of using a generator to produce the waves, energy is simply fed into the cavities until they produce a stable radiofrequency wave on their own. "The cavity chooses the frequency as opposed to it being decided by the radiofrequency generator," Delayen explains.

The radiofrequency waves are amplified and fed back into the cavities, creating what Delayen calls a self-excited loop. Once cavities sustain their own waves, they can each be gently tuned to the same wave.

"It's another way of operating and driving superconducting cavities, which is inherently stable," Delayen continues. He says some older accelerators have already successfully used the analog version of this technology.

Now, Delayen and JLab Engineering Division staff engineers Trent Allison, Curt Hovater, John Musson and Tomasz Plawski are taking the technology digital.

"We developed an algorithm that emulates an analog self-excited loop. It's a piece of software that's downloaded onto a processor. The digitization makes it a lot easier to control," Hovater comments. "We've taken the technology and made it smaller, more compact, and easier to change and mass produce."

Hovater says the digital self-excited loop will save accelerator operators time and effort every time they bring the accelerator back online, whether from a scheduled down or an unexpected trip. In addition, it is easier to service and adapt to changing conditions.

"From our standpoint, it makes things a lot easier, because you can change certain aspects of these algorithms quickly and easily by simply changing the software," Hovater explains.

The energy waves for the new cavities to be installed for the 12 GeV Upgrade will be controlled using the new technology. "If we have a requirement five years into the operations of 12 GeV that requires a subtle change to the field control, we can apply that simply by changing the algorithm that's inside the control system. It's a quick download," he continues.

by Kandice Carter