Buttons and Bellows for PEP-II
The Accelerator Systems Division has buttoned up the PEP-II rings. Now the accelerator is ready once again to deliver beams to BaBar after a maintenance shut down, and the accelerator sports 650 new buttons. The buttons are part of the beam position monitors that keep track of the beam's exact location.
Early this year, in the final days before closing the doors to the BaBar detector, the accelerator division squeezed in one more upgrade taskadding a new bellows unit on the forward side of the detector. The bellows unit provides a flexible place where the two separate pipes carrying electrons and positrons join into a single beam pipe just before reaching the detector.
Both bellows and buttons caused trouble during the 2006 run, when higher currents coursing through the accelerator generated "higher order mode" radio frequency power. The excess power overheated buttons; six fell off and disturbed the beam, requiring down time to remove them from the beam pipe. The extra power also triggered electrical arcing in the front bellows unit, forcing the machine to run at lower currents, and thus lower than desired luminosity, for months. The BaBar and PEP-II crews successfully solved the originally mysterious problems, and have just finished putting the long-term solutions into place.
"Higher order modes (HOM) can be quite potent. It hit home as we raised the currents last year," said Mike Sullivan, who co-leads PEP-II operations in the interaction region. "We're pretty excited about the new designs to absorb HOM power."
The new bellows unit features a new design and more ceramic tiles to absorb as much as 30 to 40 kilowatts of power. Alexander Novokhatski designed it, and Scott DeBarger oversaw its assembly. Sullivan expects that the second unit, still under construction, will be placed on the back end of the detector during a scheduled down time, probably this summer.
Most of the button popping occurred in the interaction region by the detector. There, the button units couldn't be replaced, so the team built a special device and pushed it into the otherwise inaccessible pipes. Like surgeons, crews used an optical fiber connected to a TV screen to see and effectively move the button puller. The device's arms pulled the buttons off the pins they normally adhere to. The pins can still deliver a beam position signal, albeit much weaker.
Crews also installed new beam position monitor buttons—with button and pin made from just one piece of metal—in the arcs of the low-energy ring where currents are higher.
The upgrades will allow PEP-II to ramp currents even higher than in the last run and shatter BaBar's pervious luminosity records.
—Heather Rock Woods, January 17, 2007
Above image: t the center of the photo is the new bellows unit recently installed on the forward side of the BaBar detector.