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Don't Rain on My Parade

 Groovy beam pipe insert averts electron cloud formation.

This insert, manufactured by SLAC and the company EMEGA, will be placed inside the KEK-B accelerator beam pipe to trap stray electrons with its triangular grooves. (Photo by Mauro Pivi. Click for larger image.)

As accelerators around the world increase in power, clouds become more and more of a problem. These are not thunder clouds sending lightning bolts at power supplies, but rather electron clouds: a haze of electrons that gathers in accelerator beam pipes and disrupts positron or proton beams. Now, researchers at SLAC and KEK are using the positron beam at KEK's B Factory in Japan to test one promising technique to clear these clouds.

Electron clouds begin to form when a stray electron strikes the interior wall of the beam pipe, producing two electrons. "In turn, those electrons strike the wall, and from two they become 4, then 8, then 16, then 32," said SLAC researcher Mauro Pivi. "And soon, an electron cloud forms." The cloud attracts the positively-charged beam traveling through the pipe, causing the beam to behave erratically—not ideal when the end goal is to very accurately collide two particle beams.

An insert without grooves (top) currently sits within the KEK-B accelerator beam pipe. (Photo courtesy of KEK. Click for larger image.)

To reduce the electron cloud problem SLAC researchers Pivi and Lanfa Wang, and KEK physicists Yusuke Suetsugu and Hitoshi Fukuma designed beam pipes with a grooved inner surface. This surface, which from the side looks rather like a row of triangular shark teeth, traps incoming electrons and keeps them—and their resulting cloud of electrons—ricocheting within the well of the groove until they have dissipated enough energy to be absorbed into the beam pipe wall.

This type of triangular groove is currently undergoing tests at KEK-B. Future experiments are planned at the Cornell Electron Storage Ring Test Accelerator and the European accelerator laboratory CERN. At KEK-B, a beam pipe without grooves has been installed into a short region of the accelerator. A detector just behind the pipe will monitor the number of electrons produced over the next two months of operation. Then, technicians will replace the smooth beam pipe with the triangular grooved version. After two additional months of data collection, Pivi and colleagues will compare the number of electrons detected with the smooth and grooved beam pipes.

The region in the KEK-B accelerator where the inserts are being tested. (Photo courtesy of KEK. Click for larger image.)

Previous experiments at SLAC's PEP-II proved that rectangular grooves work well to capture stray electrons in an environment with no magnetic field. Without a magnetic field, the electrons zip into the groove from all directions, and are trapped as they ricochet from side to side. The triangular grooves, Pivi said, should work in the magnetic field that surrounds the KEK-B test region. That's because in a magnetic field, the electrons spiral straight down into the groove along the field lines. Inside the groove, they collide with the sloped walls and become trapped, instead of simply bouncing off the flat bottom of a rectangular groove and spiraling back up into the center of the beam pipe.

"This research is very promising, and so is another method being developed at KEK that uses clearing electrodes," said Pivi. (Read about the clearing electrodes approach to the electron cloud problem in symmetry magazine.) SLAC has been leading the experimental effort on the electron cloud problem for future accelerators, including the International Linear Collider.

—Kelen Tuttle
SLAC Today, October 6, 2008