Ir Upgrade Plans for the Pep-ii B-factory*

PEP-II, the SLAC, LBNL, LLNL B-factory has achieved a peak luminosity of over 9×10 cms, more than 3 times the design luminosity, and plans to obtain a luminosity of over 1×10 cm sec in the next year. In order to push the luminosity performance of PEP-II to even higher levels an upgrade to the interaction region (IR) is being designed. In the present design, the interaction point (IP) is a head-on collision with two strong horizontal dipole magnets located between 21-70 cm from the IP that bring the beams together and separate the beams after the collision. The first parasitic crossing (PC) is 63 cm from the IP in the present by2 bunch spacing. Future improvements to PEP-II performance include lowering the βy values of both rings. This will increase the βy value at the PCs which increases the beambeam effect at these non-colliding crossings. Introducing a horizontal crossing angle at the IP quickly increases the beam separation at the PCs but recent beam-beam studies indicate that a significant luminosity reduction occurs when a crossing angle is introduced at the IP. We discuss these issues and describe the present interaction region upgrade design. THE INTERACTION REGION The PEP-II asymmetric-energy electron and positron beams are brought into a head-on collision by strong (~0.8T) horizontal dipole magnets located ±21-70 cm from the IP. The strong dipoles (called B1) are made of permanent magnet material (Sm2Co17) and are tapered for the first 22.5 cm in order to accommodate the Silicon Vertex Tracker (SVT) of the BaBar detector. Figure 1 is a photograph of the B1 magnets with half of the SVT installed. The beam separation continues in the next magnet, a shared vertically focusing quadrupole (QD1) with a magnetic axis very close to the High-Energy Beam (HEB) trajectory. This maximizes the horizontal displacement for the Low-Energy Beam (LEB) thereby maximizing the beam separation. The beams are then separated enough to be able to enter individual beam pipes at about 2.5 m from the IP. Figure 2 shows a layout of the PEP-II interaction region. Note the expanded x scale on left of the drawing as compared to the z scale at the bottom. Figure 1. Picture of the tapered sections of the B1 magnets with half of the Silicon Vertex Tracker in place. Figure 2. Layout of the PEP-II Interaction Region. The B1 magnets are shown closest to the IP. The 1 parasitic crossing when every other RF bucket is filled with charge is located at the outboard end of the B1 magnets. PARASITIC CROSSINGS AND BEAMBEAM EFFECTS ___________________________________________ *work supported by the Department of Energy under contract number DE-AC03-76SF00515. [email protected] Proceedings of EPAC 2004, Lucerne, Switzerland