April 26, 2000


Brookhaven Lab Research Associate Spins Thesis Into Prize

UPTON, NY - Mei Bai, a research associate at the U.S. Department of Energy's Brookhaven National Laboratory, has won the Outstanding Doctoral Thesis Research in Beam Physics Award from the American Physical Society (APS). Bai will receive her award at the April meeting of the APS in Long Beach, California, which begins on April 29.

The award recognizes her "work in the theory, experimental demonstration, and clear explanation of a method using an RF dipole for overcoming intrinsic spin resonances in polarized proton acceleration." In other words, Bai has come up with a way to keep the protons in an accelerating beam all spinning with their axes pointing in the same direction, or polarized, despite interferences from some of the magnets that make up the accelerator. This is important for experiments that aim to find out how protons get their spin, which will soon get under way at Brookhaven Lab.

To explain her technique, Bai first describes how the accelerator's focusing magnets interfere with the protons' alignment: As protons orbit around the accelerator, they oscillate naturally, both horizontally and vertically. Because of these oscillations, the proton spin gets "kicks," or pushes, from the magnets that focus the beam -"like a child being pushed on a swing at the pace of the swing's natural frequency," she says. These kicks add up, and, eventually, the protons' spin alignment is destroyed.

"You can't get rid of the magnetic field," says Bai. "An accelerator is one big magnetic field - that's the way it works." But Bai has come up with a way to compensate for the magnets' effects.

Her method, surprisingly, uses another type of magnet, an RF (radio-frequency) dipole, to produce even stronger vertical oscillations in the beam and enhance the kicks on the protons' spin. The key, she says, is to turn the magnet on slowly, so it gives the protons incrementally bigger pushes to gradually increase their oscillations. Eventually, the strengthened kicks from the focusing magnets completely reverse the polarization of the protons, which is exactly what Bai wants. "We don't care if they are all pointing up or all pointing down, as long as they are all pointing in the same direction," she explains. Then she turns off the RF dipole magnet in the same gradual manner to reverse the process. The method works, she says, without disturbing the distribution of the beam as a whole.

Bai's thesis represents two and a half years of work - and a lot of fun - at BNL, she says. She came to the Lab in 1996 from Indiana University, where she was doing her graduate work with S. Y. Lee, a former Brookhaven physicist. It was Lee who steered her toward Brookhaven to pursue her thesis work with the polarized proton beam at the Alternating Gradient Synchrotron (AGS).

"It was a big surprise," Bai says of winning the prize. "I feel very lucky to have the opportunity to work here, and I got so much help from so many people."

Thomas Roser, head of Brookhaven's Accelerator Division, who was one of those helpers, says, "Mei's work has made it possible to reliably accelerate polarized beam at the AGS. Her techniques of using an RF dipole magnet will also be used in the Relativistic Heavy Ion Collider (RHIC) for polarized beam acceleration and for studying non-linear beam dynamics. In fact, her use of an RF dipole has so many applications that it may soon become a standard diagnostic tool at large colliders."

The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.


Note to local editors: Mei Bai lives in Middle Island, New York.

Last updated 5/28/99 by Public Affairs