The Sensitivity of the PEP Beam Transport Line to Perturbations

The sensitivity of a beam-transport line to various perturbations determines the extent to which one can simplify component design and relax tolerances. For the PEP injection lines we have studied the effects of various fabrication errors, magnet misalignments, and residual gas scattering. Using the TURTLE ray-tracing program1), we find that magnetic-field errors corresponding to a relative sextupole strength in the dipoles of 0.5% and/or a relative sextupole or octupole strength in the quadrupoles of 5% are permissible. This allows relatively loose tolerances in magnet fabrication. Transverse misalignment of a quadrupole by a distance × causes the beam centroid to be displaced downstream by as much as 5×. This requires a quadrupole alignment accuracy of ± 0.5 mm or better. No compensation for the earth's field is necessary because an integral number of optical wavelengths and a short wavelength were used for the design. Analysis shows that beam broadening from multiple coulomb scattering is insignificant for pressures of less than 1/10 torr.