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Bates Accelerator has new racetrack for electrons

In a flash it was over, but scientists and engineers at the Bates Linear Accelerator Center knew they'd marked a milestone in the development of the latest tool at Bates to study particle physics.

In February the researchers successfully injected and stored the first beam of electrons in the South Hall Ring, a kind of "racetrack" for electrons with a circumference of 600 feet. They did so on the first try.

The achievement was a feat of engineering and science. Why? For one, "it's difficult to get particles to go around the ring repetitively without hitting the walls of the vacuum tube [they travel in]," said Professor Stanley Kowalski, director of Bates.

Key to guiding the electrons are about 200 magnets that surround the ring. And "the magnets had to be positioned so that their centers were within a tenth of a millimeter of a preferred position," Professor Kowalski said.

The electrons hurtle along at more than 99.99 percent of the speed of light through a tube that is two and a half inches in diameter. During this first test the electrons were only stored, or sent around the ring, for about 20 milliseconds before they ran out of energy, but that corresponds to traveling around the ring many thousands of times.

Work on the ring is continuing and will include installing equipment that will put back the energy the electrons lose in moving around it. "This will lead to much longer storage times in the ring, perhaps up to seconds," Professor Kowalski said.

The South Hall Ring has been under construction for about five years. It complements the existing accelerator at Bates, which shoots electrons down a straight tube over 640 feet long then diverts them into a target at the end. Scientists can then analyze the particles resulting from the collision to get at fundamental questions of physics like the structure of atomic nuclei.

The electron beam from the accelerator, however, comes out in bursts. "And for some kinds of experiments, you don't want that. You want a continuous stream of electrons," Professor Kowalski said. The South Hall Ring provides such a continuous stream after the beam is channeled into it from the accelerator.

Electrons in the ring can then either be diverted to the target mentioned above (an external target), or sent through a target within the ring itself.

This internal target will allow several other kinds of experiments-for example, those exploring collisions between electrons and target nuclei that are polarized, or oriented in a certain direction. According to a Bates brochure, "many important physics questions can be addressed only by organizing the electrons in the beam, the target nuclei or both in such a polarized condition instead of the usual randomly oriented condition."

A polarized gas target, however, has so few nuclei that it's difficult to get enough collisions between the electrons of the beam and the nuclei of the target using conventional methods. The South Hall Ring solves that problem. "The idea is to put a beam of particles into the ring, keep them circulating around the ring for as long as we can, and have them go through the internal target in the ring as many times as we can," Professor Kowalski said. Such repetition increases the chance of collisions.

Further, experiments using the internal target will take advantage of the fact that the current inside the ring is 1,000 times higher than the current available if the beam is sent to the external target.

Within the next two months two more critical parts of the South Hall Ring should be completed, according to Professor Kowalski. The researchers will install the equipment to put back the energy the electrons lose in moving around the ring, and the equipment necessary to divert the continuous beam in the ring to the external target.

In addition, Bates has sent a proposal to the Department of Energy for the construction of a detector "that will catch the scattered electrons and other particles resulting from the collision of the electrons with the internal target," Professor Kowalski said. The detector, known as BLAST (Bates Large Acceptance Spectrometer Toroid) is being developed in collaboration with scientists from eight other institutions.

Bates, which is located in Middleton, Mass., is MIT's only particle accelerator. It is funded by the Department of Energy as a national user facility, which means that scientists around the country and the world can use the facility after their proposals pass review by a Program Advisory Committee.

In addition, Professor Kowalski stresses the Center's educational mission. "Our primary goal is to do nuclear physics, but connected to that is training young scientists. So almost every experiment here is associated with postdoctoral associates or graduate students.

"This lab averages five physics PhD experiments per year."

A version of this article appeared in the April 28, 1993 issue of MIT Tech Talk (Volume 37, Number 30).

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