09/02/99 Update

PEP-II Update September 2, 1999 

Over the past three weeks PEP-II has been colliding beams about 70% of the time and engaged in accelerator studies 30%. The main studies concentrated on increasing the currents, raising the specific luminosity, and reducing the BaBar beam aborts during colliding beams. 

To increase the specific luminosity, systematic scans of interaction region beam sizes were made adjusting, primarily, local and global skew quadrupoles in the Low Energy Ring (LER) and betatron tunes. Considerable success was achieved at low currents where the vertical cap-sigma was reduced from about 16 microns to 7.1 microns. This value corresponds to an rms vertical beam size at the interaction point of 5.1 microns. The design is 4.7 microns. The rms horizontal beam size has remained at near its design value of 160 microns. However, at higher currents the specific luminosity drops due to beam-beam effects with an rms vertical beam size of approximately 8 microns. 

Several bunch spacings have been tried. During routine collisions a bunch spacing of every fourth bucket filled is used (830 bunches). A spacing of every second bucket (1660 bunches) and a "two-four" pattern (buckets: 2,4,8,10,14,16,Š or 1100 bunches) have been tried with success. The peak beam currents have to be increased to use these new patterns during routine collisions. 

The vacuum multipacting in the LER at high currents has had further study. The pressure in the straight section increases rapidly above a threshold of about 600 to 800 mA. This pressure rise is definitely seen by the beam lifetime and detector backgrounds but part of the pressure rise as seen by the ion pump currents are electrons generated by the beam related multipacting which get into the pump currents and mask the true pressure rise. The threshold changes with bunch spacing; although the threshold and vacuum rate of rise with current seems more related to the charge per bunch than the spacing. A solenoid has been wrapped around 150 m of vacuum chamber. This solenoid can make up to 50 gauss. Beam tests show that the solenoid can increase the threshold by 50 to 200 mA and decrease the pressure rise slope a factor of two. Finally, beam scrubbing seems to reduce this pressure rise giving about a factor of two decrease after an ampere-hour or so of integrated current. 

With the higher luminosity and currents during recent collisions, the beam coasts are plagued by frequent beam aborts. A "design" coast lasts about 40 minutes with an injection of about 10 minutes. About 20% of the recent coasts end normally with a top-off. 10% of the coasts end with an RF trip. Finally, about 70% of the coasts are aborted caused by a rapid rise background signal in the BaBar silicon vertex tracker (SVT). These SVT aborts occur uniformly during the coasts and are associated with a very rapid rise in backgrounds lasting under 1 second from only the high energy ring (HER) beam. The frequency varies from once every 30 minutes to once every 5 hours. The cause is not yet determined although the aborts appear not to be caused by changes of RF parameters, beam orbit, feedbacks, or power supplies. The HER lifetime sometimes drops from 500 to 300 minutes just before these aborts. This may be caused by short lived dust particles or tune changes at high luminosity. There have been several four to six hour periods where there have been no trips with four to five fills with top-offs, suggesting dust particles are not the cause. Brief local vacuum increases around the ring are sometimes associated with these events suggesting dust particles. More study is ongoing. These trips cut our integrated luminosity by about 30%. 

The luminosity has been steadily rising. On August 10 the luminosity reached 6x10**32/cm*2/s. By August 17, 1999, the luminosity reached 8.2x10**32 with 830 bunches, 950 mA in the LER and 475 mA in the HER. We achieved our present peak of 8.3x10**32 on Aug. 28. BaBar has taken data at 8.1x10**32. A typical fill starts at about 6 to 7 x10**32 and ends at 4 to 5 x10**32. The best integrated luminosity per 8 hour shift has been 13.2 pb-1 delivered to BaBar. BaBar logged 12.7 pb-1. The best day has been 28 pb-1. The total recorded data by BaBar is about 470 pb-1 on the 4S resonance plus 55 pb-1 off the resonance. 

The backgrounds are becoming harder to handle. The silicon strip in the BaBar Silicon Vertex Tracker which receives the most radiation has accumulated a dose of about 40 kRad since May. This strip now receives about 1 kRad per day at the present luminosity. 

The peak HER current, so far, is 651 mA with a lifetime of 500 minutes. The HER has an integrated current of 220 A-hr since May. The peak LER current is 1551 mA with a lifetime of about 50 minutes. The LER has integrated 560 A-hr since May. 

The near term schedule is to collide beams continuously until October 4, dispersed with short machine development periods. After a two week down to install BaBar DIRC bars, collisions will resume October 18 to stop at the year-end holidays in December. 

Congratulations to everyone on getting a luminosity above 8x10**32/cm**2/s.

John Seeman for the PEP-II staff 9/2/99

Suggestions to: John Seeman

[PEP-II Commissioning Updates] [PEP-II Home Page] [BABAR Detector Home Page] [SLAC Home Page]

Page owner: achan