Beam - Based Alignment and Tuning Procedures for e + - Collider Final Focus Systems

For future linear colliders, with very small emittances and beam sizes and demanding tolerances on final focus system alignment and magnet errors, it becomes incress-ingly important to use the beam as a diagnostic tool. We report here procedures we have identified and will be implemented in the Final Focus Test Beam at SLAC incorporating i) quadrupole strength changes, ii) central orbit modifications, iii) spot size measurements, and iv) beam stability monitoring. ITANALYSIS OF BEAM CENTROID A. Quadrupoie Alignment Each quadrupole will be mounted on a magnet mover with vertical and horizontal range of &3mm and setting accuracy of 5 1~. Alignment will be monitored by a wire alignment system, in which sensors indexed to each quad-rupole will detect position relative to a system of stretched .~-wires. The absolute positioning of the wires is expected to be m ilOOp and the location of quadrupole magnetic-centers relative to the wires is expected to be-110~. Relative motions in the 1 p range should be detectable. [l] If a quadrupole of inverse focal length ki is varied by an amount ZIL?&, the trajectory difference at a downstream position monitor is Ax' = 2Rj'Ak.x 3 12 'a (1) where x,i is the offset of the quadrupole relative to the beam and R{', is the x, x' matrix element from i .to j, assumed known from the optics model. The value of I,;-. may be found by a least-squares fit to measurements of Axj at several monitors. Figure 1 shows the layout of the FFTB. Beamline segments may be rotated about hinge points (HP), located at _ the beginning of the system or at vertices of the bends; base quadrupoles (BQ) are quadrupoles near the hinge points-which will be kept fixed relative to the wire alignment system ; an alignment segment is the straight line defined by a hinge point and the next base quadrupole. Figure 2 depicts the alignment procedure. The quadrupoles in a given segment are varied one at a time and each with the exception of the base quadrupole is moved onto the beam line according to Eq. (1). Then the segment is " hinged " into alignment with the base quad-rupole by steering at the hinge point and moving each quadrupole except the base quadrupole in proportion to its distance from the hinge point. The procedure is then repeated for each successive segment. If we assume that position measurement errors are random …