05/12/00 Update

PEP-II January Commissioning

During March 2000 the PEP-II operators made steady adjustments to skew quadrupoles, orbits, and bunch charges during collisions for BaBar. The luminosity in PEP-II by early April 2000 reached 1.95 x 10**33/cm**2/s with 829 bunches, 860 mA of electrons, and 975 mA of positrons.

On April 6 with a HER current of 900 mA a vacuum leak developed in the high power dump downstream of the interaction region where 50 kW radiation fans from the permanent magnet B1 dipoles strike. An external guard vacuum was successfully mounted over the leaky chamber and machine operation resumed. To avoid a potentially larger problem and to allow BaBar to collect data, the HER current has been administratively limited to 600 mA. A plan is being prepared to fix or replace the leaky chamber during the September-October down this fall. With the limited current the peak luminosity was about 1.6E33.  

Using the synchrotron light monitor, we have observed for several months that the positron beam size increases with beam current above about 800 mA. This enlargement is consistent with the Electron Cloud Instability. The likely source of electrons in the PEP-II LER is multipacting electrons in the LER straight sections. During our repair days, we have been winding a single layer solenoid on the beam pipes in the straight sections. About 260 m of solenoid is now in place. After energizing these solenoids from 5 to 10 gauss, the positron beam size enlargement with current is now smaller and the specific luminosity is higher. The solenoids were empirically adjusted to maximize the luminosity. The optimum luminosity depends somewhat at the polarity and amplitude of the solenoids. Most of the solenoids are not operating at their maximum excitation. More studies are planned and we will be winding more solenoids over the remaining 160 m of straight section.

After turning on the solenoids the luminosity jumped to about 2E33 in a few hours. A clear correlation. The present peak PEP-II operating parameters are:  The maximum luminosity is 2.02x10**33/cm**2/s with 554 bunches, 613 mA of e-, and 1090 mA of e+. The peak luminosity per bunch is now twice the design. The best integrated luminosity in one eight hour shift is 47.9 pb-1 and in three shifts (24 hours) 125.9 pb-1.

Thanks to everyone for helping with the solenoids and raising the luminosity. In our last 16 hour repair day the winding and power supply crews added 80 m of solenoid and energized 180 m.

John Seeman for the PEP-II Staff 5/12/00

Suggestions to: John Seeman

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