The University Record, June 7, 1993

Siberian snake more potent than expected

By Sally Pobojewski
News and Information Services

The Siberian Snake, a series of magnets wrapped around the beam pipe of a particle accelerator, has turned out to be even more potent than expected in curing the “depolarization” problems of beams of spinning protons. The snake was designed to eliminate magnetic disorientations called depolarizing resonances that occur whenever a beam of spinning protons is being accelerated.

The new experimental results from the Indiana University Cyclotron Facility Cooler Ring were published in the April 26 issue of Physical Review Letters. The project was funded by the National Science Foundation and the Department of Energy.

The snake has performed very well on IUCF’s medium-energy proton accelerator since it first was tested there in 1989. It keeps all the protons in the beam spinning in one direction (called a polarized beam); this control of the spin direction allows experiments that investigate the fundamental particles of the universe.

Still, no one was sure whether the snake could cure all depolarization problems at very high-energy accelerators like the Superconducting Supercollider (SSC), where the many depolarizing resonances are so strong that they sometimes overlap. So a team of scientists built a special high-frequency magnet to create an artificial depolarizing resonance that overlapped with one of the Indiana accelerator’s natural disturbances. When the Siberian Snake was turned on, it completely overcame both overlapping resonances and kept all the protons spinning in one direction.

“Accelerator theory cannot yet deal with the overlapping depolarizing resonances, so we did not know what would happen,” said project leader Alan D. Krisch, professor of physics. “Fortunately, the Siberian Snake was powerful enough to overcome completely both overlapping resonances and all their interference effects.

“It is important to have very high-energy spin-polarized proton beams because spin may give us a special handle on understanding the inner structure of the proton,” Krisch explained. “We think that the proton is made up of objects called ‘quarks.’ Some high-energy experiments suggest that spin is especially important in violent collisions of protons, which directly probe these small objects inside each proton.

“Therefore we feel that it’s important to do experiments where we can control the spin at much higher energy proton accelerators like Fermilab and the SSC in the United States and UNK in Russia. We hope that these experiments will give us some direct information about how the quarks interact with each other and how spin affects their interaction.”

The Siberian Snake, first suggested in 1974 by Russian theorists Yaroslav S. Derbenev and Anatoly M. Kondratenko of the Novosibirsk Laboratory, works by reversing the spin of each proton in a beam every time the proton travels once around the accelerator ring, Krisch said.

Thus, he explained, if any unwanted magnetic disturbance in the ring changes a proton’s spin direction, the same disturbance will have exactly the opposite effect after the snake reverses the proton’s spin direction during its next trip around the ring. This reversal forces each disturbance to cancel itself.

Siberian Snakes already are being installed on five or six medium-energy accelerators in laboratories around the world, he noted.

Individuals from the U-M who also participated in the experiment were Rachid Baiod, C.M. Chu, Ernest Courant, Yaroslav Derbenev, Jeffery Duryea, Andrei Koulsha, Michiko Minty, Teamour Nurushev, Richard Phelps, Dimitri Shoumkin and Victor Wong.

Others were David Caussyn, S.Y. Lee, Thomas Rinckel, Franz Sperisen, Edward Stephenson and Barbara von Przewoski from Indiana University; Chihiro Ohmori from Tokyo University; Paul Pancella from Western Michigan University; Lazarus Ratner from Brookhaven National Laboratory; and Uli Wienands from TRIUMF in Vancouver, Canada.