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Register a new userLattice and Compensation Schemes for the PEP-II Interaction Region
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The PEP-II interaction region is designed to accommodate asymmetric beam energies, head-on collisions, small bunch spacing and provide low beta* for high luminosity. Local correction schemes are implemented to compensate non-linear chromaticity from the IP doublets as well as coupling, orbit and focusing effects from the 6 Tm asymmetric detector solenoid. The main IR optics features and local correction schemes are presented. MAD code is used for the optics calculations.
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At present, the PEP-II bunch length and vertical beta function at the Interaction Point (IP) are about of the same size. To increase luminosity, it is planned to gradually reduce the IP beta function. For the maximum effect, bunch length has to be also reduced to minimize luminosity loss caused by the hourglass effect at IP. One of the methods to achieve a smaller bunch length is to reduce momentum compaction factor. This paper discusses a lattice option for the High Energy Ring, where the nominal 60 degree cells in four arcs are replaced by 90 degree cells to reduce momentum compaction factor by 30% and bunch length by 16%. The increased focusing in 90 degree cells results in 40% stronger arc quadrupoles and 150% stronger arc sextupoles due to reduced dispersion and larger chromaticity. Tracking simulations predict that dynamic aperture for this lattice will be more than 10 times the rms size of a fully coupled beam for a horizontal emittance of 30 nm and IP beta function of 1cm. The lattice modification and results of simulations are presented.
We present a novel method to characterize the e+/- phase space at the IP of the SLAC B-factory, that combines single-beam measurements with a detailed mapping of luminous-region observables. Transverse spot sizes are determined in the two rings with synchrotron-light monitors and extrapolated to the IP using measured lattice functions. The specific luminosity, which is proportional to the inverse product of the overlap IP beam sizes, is continuously monitored using radiative-Bhabha events. The spatial variation of the luminosity and of the transverse-boost distribution of the colliding e+/-, are measured using e+ e- --> mu+ mu- events reconstructed in the BaBar detector. The combination of these measurements provide constraints on the emittances, horizontal and vertical spot sizes, angular divergences and beta functions of both beams at the IP during physics data-taking. Preliminary results of this combined spot-size analysis are confronted with independent measurements of IP beta-functions and overlap IP beam sizes at low beam current.
The successful commissioning and operation of the PEP-II asymmetric e+e- collider motivated further studies to increase luminosity. In this paper, we discuss a modification of the PEP-II lattice to reduce the vertical beta function at the Interaction Point (IP) from the design value of 1.5cm to 1.0cm. This could potentially reduce the colliding beam size, increase particle density at the IP and the probability of beam-beam interactions. In this paper, we outline the optics modifications, discuss tracking simulations, and overview machine implementation.
Beam-beam simulations predict that PEP-II luminosity can be increased by operating the horizontal betatron tune near and above a half-integer resonance. However, effects of the resonance and its synchrotron sidebands significantly enhance betatron and chromatic perturbations which tend to reduce dynamic aperture. In the study, chromatic variation of horizontal tune near the resonance was minimized by optimizing local sextupoles in the Interaction Region. Dynamic aperture was calculated using tracking simulations in LEGO code. Dependence of dynamic aperture on the residual orbit, dispersion and distortion of beta function after correction was investigated.
We describe the construction of a prototype hybrid permanent magnet dipole and quadruple. The magnet consists of two concentric rings of SmCo magnetic material 5 cm in length. The outer ring is made of 16 uniformly magnetized blocks assembled as a Halbach dipole and the inner ring has 32 blocks oriented in a similar fashion so as to generate a quadruple field. The resultant superimposed field is an offset quadruple field which allows us to center the field on the high-energy beam in the interaction region of the PEPII B factory. The dipole blocks are glued to the inside surface of an outer support collar and the quadruple blocks are held in a fixture that allows radial adjustment of the blocks prior to potting the entire assembly with epoxy. An extensive computer model of the magnet has been made and from this model we developed a tuning algorithm that allowed us to greatly reduce the n=3-17 harmonics of the magnet.
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