Researchers keep beam on track

September 23, 1994

By Lynn Yarris,

The premier quality of light from the ALS springs from the high quality of the electron beam used to generate that light. To maintain that quality while the electron beam circles for hours through the ALS storage ring, a new "transverse feedback" system for rapidly identifying and correcting problems has been installed.

Designed by researchers in the Accelerator and Fusion Research Division's Center for Beam Physics (CBP), the ALS transverse feedback system eliminates horizontal and vertical fluctuations in the electron beam's path. Corrections are made so quickly that ALS users are always able to work with a perfectly stable beam.

The beam in the ALS storage ring is formed from discrete bunches of electrons rather than a continuous stream. As each bunch orbits around the ring, it comes under the influence of the electromagnetic fields of its neighbors. These "collective effects," which are also known as "coupled bunch phenomena," escalate until the bunches start to vibrate horizontally or vertically and the entire beam deviates transversely from its original orbital path. Coupled bunch problems also arise when an off-center bunch passes through radio frequency (rf) cavities--devices that use electromagnetic fields to energize electrons. The distorted motion of one bunch creates a transverse electromagnetic field inside the cavity that is transferred onto the bunches that follow.

"With multiple bunch beams like those in the ALS storage ring, once a problem starts, it quickly gets worse," says physicist John Corlett, who leads the CBP's Beam Electrodynamics Group. "Transverse motion can reach the point, in some accelerators, where the electron beam is lost. Though this has not happened in the ALS, a transverse flaw in the electron beam could eventually destroy the focus of the extracted light."

Walter Barry, the project lead engineer, together with Glen Lambertson, John Byrd, and Corlett, designed a feedback system that detects transverse motion at two points in the storage ring beam and applies a corrective transverse energy kick at a third point. This applied energy has the desired effect of straightening out the beam and restoring it to its original orbital path.

CBP director Swapan Chattopadhyay likens the action to what takes place on a pinball machine. "The system sees the beam start to oscillate and gives it a compensating flip of energy to steer it back on course," he explains.

The transverse feedback system at the ALS was tested with transverse motion detection at only one point in the storage ring. With the system off, deviations in the path of the storage ring beam were measured as high as 50 microns. With the feedback system on-line, the beam showed essentially no deviation.

"We know the system works," says Corlett. When the full complement of electronics is in place, he says, the transverse feedback system will be able to control the ALS beam when its storage ring is filled to capacity with 328 electron bunches.

In addition to transverse motion problems, multiple bunch particle beams in storage rings can also develop problems with longitudinal motion. These problems arise when the spacing between bunches of particles is altered so that bunches are either too close together or too far apart. Longitudinal distortions give beams a jittery motion that could prove especially troublesome for the B-factory proposed for the Stanford Linear Accelerator Center (SLAC). The B-factory is designed to create head-on collisions between beams of electrons and positrons that, like the storage ring beam of the ALS, are formed from discrete bunches of particles.

Corlett's group assisted a SLAC team headed by John Fox in the design and development of a longitudinal feedback system for the ALS. This system operates in a similar fashion to the transverse system, except that accelerating or retarding voltages are applied to individual bunches in a beam to correct spacing inequalities. Both systems are being used as prototypes for the B-factory.

"The bunch-to-bunch approach we take in designing our transverse and longitudinal feedback systems is extremely powerful in enabling us to correct coupled bunch phenomena," says Corlett. "Our goal is to stop any deviant motion before it becomes a problem."