Digital Signal Processing for Beam Position Feedback

Stabilization of the particle beam position with respect to the focusing optics in the third generation synchrotron light sources is crucial to achieving low emittance and high brightness. For this purpose, global and local beam orbit correction feedbacks will be implemented in the APS storage ring. In this article, we will discuss application of digital signal processing to par-ticle/photon beam position feedback using the PID (proportional, integral, and derivative) control algorithm. 1. Introduction In the third generation synchrotron light sources, of which the Advanced Photon Source is one, it is very important to stabilize the beam position on the magnetic axis of the quadrupoles and the sextupoles in order to achieve low emittance and high brightness of the photon beams. A number of correction dipole magnets will be installed around the APS storage ring to stabilize the beam against the low frequency vibration, below 25 Hz, from various sources. The corrector magnet system will consist of 318 vertical/horizontal field magnets for horizontal and vertical corrections. The displacement of the quadrupole magnets due to vibration has the most significant effect on the stability of the positron closed orbit in the storage ring. A small displacement of the quadrupole magnet leads to a large distortion of the closed orbit, and hence, the growth of the emittance. The internal sources of vibration include the mechanical motion of the various components of the ring, such as rotating machinery, pumps, compressors and high vacuum equipment. This internal vibration can be reduced by balancing the equipment and isolating the sources. The primary external source of the low frequency vibration is the ground motion of maximum 20 µm amplitude, with frequency components concentrated below 10 Hz. These low frequency vibrations can be corrected using the corrector magnets, whose field strengths are controlled individually through the feedback loop comprising the beam position monitoring (BPM) system. 1 For the global beam position feedback, the beam motion is detected by the BPMs located around the storage ring, and is relayed in digital format from the BPMs to several feedback processors. The feedback processors compare the current positions and the desired positions set by the control system and compute the necessary kick strengths for the corrector magnets to restore the beam to the desired positions. The feedback processors then send commands to individual corrector magnet power supplies. The corrected beam positions will then be measured by the BPMs and the feedback …