The PEP-II accelerator at the Stanford Linear Accelerator Center (SLAC) reached a new milestone on 11 March by phasing in “trickle injection”, a mode of operation that increases the production of B and Bbar particles by up to 50%. With the new technique, the BaBar detector can keep taking data virtually uninterrupted while the linear accelerator injects electrons and positrons into the PEP-II storage rings.
Up to 1400 bunches of particles circle around PEP-II’s two storage rings at any given time – electrons in the 9 GeV ring and positrons in the 3.1 GeV ring. The linear accelerator periodically injects more bunches to replace those that are used up in collisions inside 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, in other words, the injection of new bunches.
With trickle feed, new bunches are injected continuously, at a rate of up to 10 per second. Initially, the highly energetic newcomers push the other bunches around, thus increasing the amount of background in the detector. However, the researchers have learnt how to teach the unruly bunches to get quickly into line, so that within just one millisecond BaBar can once again take reliable data.
After more than a year of testing, trickle injection was introduced at the low-energy ring last December, bringing a 30% increase in the B-factory’s output. Then in March came what was meant to be a two-day test of trickling into the high-energy ring, to provide another 15% increase. This went so well that the experimenters on BaBar decided to keep it going and the experiment has since been running at its peak luminosity.
The advantages of trickle injection go beyond the numbers, as continuous injection makes the storage of particles more stable so that the PEP-II rings are easier to operate. The success of the whole process is the result of a close collaboration between the BaBar and PEP-II teams.
