Keeping Antiprotons Cool

by Kurt Riesselmann

Scientists judge the performance of the Recycler by the lifetime of the antiprotons that circle the storage ring. If an antiproton doesn’t encounter any ordinary matter, it will exist forever.

In practical terms, however, it’s challenging to keep a beam of antiprotons from interacting with other particles. First, a beam constantly hits residual particles in the less-than-ideal vacuum inside the beam pipe. Second, small fluctuations in the motion increase the size of the antiproton beam despite the fact that the number of antiprotons remains constant. Like cars in a high-speed bumper car race, antiprotons get pushed out of the ideal path as the beam becomes more random, causing antiprotons to crash into the surrounding beam pipe.

Scientist use the concept of stochastic cooling, a Nobel Prize-winning idea, to reduce the random motion, minimizing the differences in momentum among beam particles and making the size of the beam physically smaller.

“Inside the Recycler beam pipe, we’ve installed microwave antennas,” said Ralph Pasquinelli. “They sense the size of the beam and the spread in momentum. Their signals are transferred across the ring, where a corrective signal is applied to the beam.”

Since the antiprotons travel close to the speed of light, it’s a challenge to get the signal across the ring before the antiprotons have passed by. Standard electrical cables are far too slow to transfer the signal. Fiberoptic cables also fail as they transmit signals at far less than the speed of light in vacuum. The only solution fast enough is a straight beam of light itself. At the Recycler, scientists use the antenna signals to modulate an infrared laser beam that shines through a 2,000-foot-long pipe across the Recycler ring. At the end of the pipe the signals are transformed back into electrical signals, which steer the equipment that nudges the beam.

“Without cooling, the lifetime of the antiprotons would be in the teens of hours compared to the forty-plus hours we’ve achieved so far,” Pasquinelli explained. “We have made great progress, but there is still a lot to learn.”