07/12/99 Update

PEP-II Update July 12, 1999

Work on PEP-II over the past two weeks has concentrated on increasing the peak luminosity and integrating luminosity on the Upsilon 4S resonance. Most of the time has been with colliding beams with small machine development (MD) experiments during or between luminosity fills. There have been a few dedicated MD shifts.

Two weeks ago the horizontal beam sizes were about as expected but the vertical beam sizes at the IP were about three times too large. Considerable work has gone into reducing the vertical beam sizes. Work on decoupling, vertical dispersion reduction, and longitudinal beta waist adjustments was very interesting but made only small improvements. The large change came from the IR 'tilt' knob which rotates the LER beam about the longitudinal axis to align with the HER beam. An immediate luminosity gain of 20% was achieved with more following in subsequent shifts. The vertical spot size (cap-sigma) has been reduced from about 18 microns to about 11 microns with the design being 6.7. The IP horizontal cap-sigma is about 225 microns.

During the past week, the peak LER current has reached 798 mA and the HER 250 mA, both during colliding beams. We have accumulated 150 A-hr of positrons and 41 A-hr of electrons since May.

As of Monday 4 am July 12 the peak luminosity was 5.6e32/cm^2 s with 250 mA in the HER and 611 mA in the LER in 415 bunches including a 5% ion gap. The e- lifetime was 304 minutes and the e+ 133 minutes. The beams are most often left in collision during injection.

A typical colliding beam fill starts with a luminosity of 5.5e32 delivered to BaBar. After the luminosity drops to about 4e32 in 40 minutes or so the beams are topped off. A top-off cycle lasts about 6 minutes: a minute to ramp down BaBar's voltages, 4 minutes to inject, and one minute to ramp BaBar back up. The record is a little over 4 minutes. Injection into the LER at 30 Hz has been tested and was successful, giving an accumulation rate of 6 to 7 mA/sec. HER injection is at 10 Hz. About half the fills end normally and are topped off. The other half are lost to beam aborts divided among very short (~ microsecond) radiation losses in the SVT after about 10 minutes of stable collisions, various RF trips, and a few MD experiments.

PEP-II has delivered about 115 pb^-1 of integrated luminosity to BaBar. BaBar has logged about 89 pb^-1. The best 8 hour shift has been 7.7 pb^-1 and the best 24 hour day 16.3 pb^-1.

Today BaBar and PEP-II have started a 5 day repair down. Beam work will resume the evening of July 16.

Congratulations to all for the PEP-II luminosity record (5.60e32/cm^2 s) achieved this morning with 250 mA e- on 610 mA e+ in 415 bunches and BaBar taking data! This surpasses our previous record of 5.22e32/cm^2 s achieved February 8, 1999, without the BaBar solenoid.

John Seeman and Michiko Minty for the PEP-II Staff 7/12/99

Suggestions to: John Seeman

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