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Trickle Treat
By Davide Castelvecchi
PEP-II reached a new milestone on March 11 by phasing in trickle
injection, a mode of operation which increases the production of B and
B-bar particles by up to 50 percent. With the new technique, the BABAR
detector can keep taking data virtually uninterrupted while the linear
accelerator (linac) injects electrons and positrons into the two PEP-II
storage rings.
“I think it’s a great improvement, and it should lead to much better
performance,” said John Seeman (AD).
Up to 1,400 bunches of particles circle around PEP-II’s storage rings at
any given time—electrons in the high energy (9 GeV) ring and positrons
in the low energy ring (3.1 GeV). The linac periodically injects more
bunches to replace those that are used up in collisions in the BABAR
detector.
In the old mode, BABAR
had to be switched off every 45 to 90 minutes to allow for a five-minute
‘top-off’ procedure to inject new bunches. With trickle feed, new
bunches are injected continuously at a rate of up to 10 bunches per
second. Initially the highly energetic newcomers push the other bunches
around, thus increasing the amount of experimental error, or background.
Researchers learned how to quickly teach the exuberant bunches to stay
in line, so that within just one millisecond BABAR
can get reliable data again.
After more than a year of testing, trickle injection was introduced in
the low energy ring last December, bringing the B Factory a 30 percent
increase in output. With the highenergy ring joining in this month, an
additional 15 percent increase is expected.
“This was meant to be a two-day test,” says BABAR
Run Coordinator Steve Playfer (Edinburgh U), “but it went well, so we
decided to keep it going. The experiment has been running at its peak
luminosity.”
The advantages go beyond the numbers according to Playfer. Continuous
injection makes the storage of particles more stable, so the PEP-II
rings are easier to operate. “Once you’ve done this mode,” says Playfer,
“you don’t want to go back.”
The success of trickle injection is the result of a close collaboration
between the BABAR
and PEP-II teams. Chris O’Grady, Amedeo Perazzo and Matt Weaver (all REG)
modified BABAR’s
data acquisition system to send feedback to the accelerator and the
storage rings. Thanks to better detection along the particles’ path,
Franz-Joseph Decker, Richard Iverson and Jim Turner (all AD) finely
tuned the timing of the injection system.
“It’s a true collaboration between the [PEP-II] machine and the [BABAR]
detector,” says Seeman. “We had to learn how to improve detection, while
[BABAR]
had to figure out how to deal with a modest amount of background.” |
