Abstract
In conventional beam-based alignment (BBA) procedures, the relative alignment of a quadrupole to a nearby beam-position monitor is determined by finding a beam position in the quadrupole at which the closed orbit does not change when the quadrupole field is varied. The final focus magnets of the interaction regions (IR) of circular colliders often have some specialized properties that make it difficult to perform conventional beam-based alignment procedures. At the HERA interaction points, for example, these properties are the following: (a) the quadrupoles are quite strong and long. Therefore a thin lens approximation is quite imprecise. (b) The effects of angular magnet offsets become significant. (c) The possibilities to steer the beam are limited as long as the alignment is not within specifications. (d) The beam orbit has design offsets and design angles with respect to the axis of the low-beta quadrupoles. (e) Often quadrupoles do not have a beam-position monitor in their vicinity. Here we present a beam-based alignment procedure that determines the relative offset of the closed orbit from a quadrupole center without requiring large orbit changes or monitors next to the quadrupole. Taking into account the alignment angle allows us to reduce the sensitivity to optical errors by 1 to 2 orders of magnitude. We also show how the BBA measurements of all IR quadrupoles can be used to determine the global position of the magnets. The sensitivity to errors of this method is evaluated and its applicability to HERA is shown.
Highlights
The new HERA interaction regions are designed to achieve a maximum possible luminosity by strongly focusing the proton beam. This results in -function values at the interaction point (IP) which are in the range of the bunch length
The vacuum chambers downstream of the IP have a keyhole shape to allow the synchrotron radiation fan to propagate through the low-beta quadrupoles
Because of the large vertical divergence of the beam in these quadrupoles, the synchrotron radiation fan will only fit inside the keyhole shape if the quadrupoles in the low-beta region are aligned to a precision of better than 0.5 mm
Summary
The new HERA interaction regions are designed to achieve a maximum possible luminosity by strongly focusing the proton beam. Because of the large vertical divergence of the beam in these quadrupoles, the synchrotron radiation fan will only fit inside the keyhole shape if the quadrupoles in the low-beta region are aligned to a precision of better than 0.5 mm. It has been successfully applied to calibrate the beam-position monitors in the HERA electron ring, where it was the basis for an orbit steering algorithm of minimizing the residual vertical kicks which yielded a record electron spin polarization [5,6,7] Future accelerators such as NLC will depend heavily on extensive beam-based steering algorithms [8,9]. (e) Often quadrupoles do not have a beam-position monitor in their vicinity Under these circumstances the results are very sensitive to errors and it turned out to be very difficult to achieve the desired precision of the beam-based alignment of 0.1 mm. Since the beam cannot be centered in all the magnets simultaneously, a global analysis of the magnet positions becomes necessary which uses the results of the beam-based alignment measurements in all the IR quadrupoles
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