A Beam - Based Alignment Technique for Correction of Accelerator Structure Misalignments

This paper describes a method of reducing the transverse emittance dilution in linear colliders due to transverse wakefields arising from misaligned accelerator structures. The technique is a generalization of the Wake-Free [5] correction algorithm. The structure alignment errors are measured locally by varying the bunch charge and/or bunch length and measuring the change in the beam trajectory. The misalignments can then be corrected by varying the beam trajectory or moving structures. The results of simulations are presented demonstrating the viability of the technique. Introduction A number of e+/elinear colliders are being designed with center-of-mass energies from 0.5 to 1.5 TeV. One of the major problems facing these designs is the preservation of the tra-nsverse emittance through the multi-kilometer linear accelerators. [1] In the linacs, the magnets, accelerating structures, and beam position monitors (BPMs) are all misaligned with respect to the ideal centerline and thus the beam is offset in the magnets and the structures. This can lead to both dispersive errors and tra~verse wakefields which dilute the projected transverse emittance and thereby reduce the collider’s luminosity. In this paper, we will discuss a new approach to aligning the accelerator structures. In most designs, the magnets and structures must be aligned with an accuracy the order of microns. [2] This would be extremely difficult to achieve and maintain with a mechanical alignment system [3] and thus a number of beam-based alignment procedures have” been proposed to align the BPMs and the quadruple magnets. [4-8] These beam-bmed procedures utilize information from the response of the beam to changes in the strength of the quadruple magnets; the resulting alignment accuracy depends upon the BPM resolution (reading-t~reading memurement jitter) and is insensitive to the initial alignment. [9] Unfortunately, these techniques cannot be used to align the accelerating structures. At this time, there are four approaches to the structure alignment: (1) extremely accurate mechanical alignment, (2) direct measurement of the dipole mode (transverse wakefield) excited by the beam in the structure, (3) alignment by mechanically attaching a very accurate BPM to the structure, and (4) trajectory bumps, tuned by emittance memurements, which correct the effect of the emittance dilutions, * Work supported by Department & Energy, contract DE-