Complicated Bunch Pattern in Pep-ii*

PEP-II, the asymmetric B-Factory at SLAC, has delivered a luminosity of 3.1 · 10 cm s. This was achieved with 692 bunches in a basic "by-3" bunch pattern. Many gaps in the bunch train were necessary to suppress a blow-up of the positron beam due to an electron cloud instability (ECI). The actual pattern had 10 bunches in a row (every third bucket), then 6 bunches missing. This 48-bucket long pattern was repeated till the last bucket (3320) before the ion clearing gap. The Low Energy Ring (LER) required an additional modification of the first bunch after each of the gaps. With a smaller spot size due to less ECI blow-up, the charge in these bunches was reduced by 10-20% to avoid kicking out the corresponding High Energy Ring (HER) bunches. A readout error of the charge along the whole train was counteracted with a 20% charge variation in the input. The reasons and developments of these and more complicated bunch patterns are discussed. 1 PATTERN RECOGNITION The maximum number of RF buckets in PEP-II is 3492. With an abort (or ion clearing) gap of 171 buckets (or about 5%) 3321 buckets are left to be filled with bunches. Filling every other bucket in a train (or a “by-2” pattern) results close to the design number of 1658, namely 1661 filled buckets. Other straight patterns will have a certain maximum allowed number of bunches (Tab. 1). Pattern by-1 by-2 by-3 by-4 by-5 by-6 # nb 3321 1661 1106 831 665 553 Table 1: Maximum number of bunches per pattern. A basic pattern might need some adjustments to get the highest luminosity for a certain total current. By reducing the number of filled buckets each bunch gets more current till the beam-beam limit is reached. A typical example was a basic by-4 pattern with 9 bunches in a row followed by 3 empty spaces in a so-called micro-gap: |...|...|...|...|...|...|...|...|...|..._..._..._... and so on. The input for this pattern would look like this: 0:3320:4[=1];36:3320:48=0;40:3320:48=0;44:3320:48=0. This can be further varied by alternating bunch trains of 9 and 10; or by introducing mini-gaps which consist of a whole bunch train missing at 1/3 and 2/3 of the fill, which help to balance the effect of the ion clearing gap. The intensity of each bunch can also be adjusted. This was necessary for the LER ring (positrons), which sees some blow-up from the electron cloud around it. The first bunch of each train is reduced to 80 or 90% to avoid kicking out the first HER bunches (e.g.: 0:3320:48=0.8). The front of the whole fill needs a slow ramp up for the positrons, e.g.: 0=0.7; 0:160:4=R would give a ramp from 70% to 100% over 160 buckets with every 4 bucket filled. All these different patterns were derived from setups and measurements, which are discussed next.