The DELTA Beam-Based BPM Calibration System

A third-generation synchrotron light source like DELTA (1) requires a measuring system to determine the beam position with high resolution and great accuracy with respect to the center of the quadrupole magnets. This paper presents the beam-based BPM calibration system developed for DELTA, providing a calibration accuracy of about 150 μm. We describe the basic idea of the measurements, the installed hardware, and present the results of an initial calibration of the closed-orbit measuring system (2). INTRODUCTION The conventional approach for calibrating a closed-orbit measuring system (CO system) requires several steps carried out one after the other. The first step is to measure each BPM on a test bench to obtain a calibration with respect to the center of the BPM. Possible offsets of the BPM electronics and the influence of varying damping factors of the measuring cables have then to be taken into account. This procedure is very time consuming and has to be performed with great accuracy. It is also very difficult to repeat this calibration after the final installation of the BPMs, cables, and electronics. Since most of the BPMs are normally mounted in or close to quadrupole magnets, the last step of calibration is to determine the position of the BPMs with respect to the axis of the quadrupoles. The DELTA BPMs are assembled to the quadrupoles in two different ways. The heads for one half of the BPMs fit to the aperture of the quadrupole magnets with an accuracy of 70 μm (“fixed” BPMs). The other half of the BPMs have no mechanical connection to the quadrupole and can move to avoid any stress to the quadrupoles resulting from thermal movements of the vacuum chamber (“floating” BPMs). There is 1 Dortmund ELectron Test Accelerator an uncertainty of the position of the floating BPMs with reference to the quadrupole axis of approximately 1 mm. For these reasons, it was decided to install a system for beam-based calibration of the CO system. BASIC IDEA OF BEAM-BASED CALIBRATION MEASUREMENTS An electron beam passing through a quadrupole magnet (length l) displaced by ∆xQ with respect to the magnetic axis gains an orbit kick with angle θQ Q l k x = ⋅ ⋅ ∆ ∆ if the focusing strength is changed by ∆k (3). In linear approximation, this leads to a closedorbit distortion of ∆ ∆ ∆ x k x s s Q Q s s i Q Q i x x i Q Q ( , ) ( ) ( ) sin( ) cos( ( ) ( ) ) = − − ⋅ β β π π ψ ψ θ 2 (1) where ∆xi is the orbit displacement in the i-th BPM and the usual nomenclature has been used. For a beam passing along the axis, this orbit deviation vanishes and therefore the magnetic center of the quadrupole can be determined as follows.The strength of the quadrupole magnet whose BPM is to be calibrated is varied by ∆k and the resulting average quadratic orbit distortion ∆ ∆ ∆ ∆ ∆ x x N x k x x N Q i Q i N Q 2 2 1 2 1 ( ) ( , ) = ∝ = = ∑