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Abstract
| An e+e− collider designed to serve as a B factory requires a luminosity of 3 x 1033 cm−2 s−1—a factor of 15 beyond that of the best present collider (the CESR ring)—and thus presents a considerable challenge to the accelerator builder. To optimize the experiment, it is necessary that the BB¯ system have a moving center-of-mass, which implies different energies for the two beams (hence an “asymmetric” collider). This feature dictates that a two-ring configuration be used. Accelerator physics issues that arise in such a design are related to the need to tightly focus the beams to a vertical beta function on the order of 1 cm, to bring the beams from two different rings into collision and then cleanly separate them again, and to mask the detector region sufficiently to permit measurements with very large beam currents passing through the interaction region. In addition, the process of optimizing the luminosity for asymmetric collisions breaks new ground. Because the luminosity is limited by the beam-beam interaction, any large improvement must come from considerably increasing both the beam current and the number of bunches in the ring. These choices place many demands on accelerator technology as well as accelerator physics. Vacuum systems must be designed to handle the thermal load from a multi-ampere beam of 8–9 GeV and to maintain an adequate running pressure (below 10 nTorr) in the face of a large gas load from synchrotron radiation induced photodesorption. An RF system capable of supporting the high beam currents must be developed. To reduce the growth of potentially strong multibunch instabilities, the cavity higher-order modes (HOMs) must be highly damped to Q ≤ 70. Even with a well-optimized RF system, the high beam currents typically mean that wideband multibunch feedback systems (both longitudinal and transverse) are needed to maintain beam stability.Effective approaches to deal with these issues have been identified by the various B factory design groups, and representative examples will be mentioned. |