Digital closed orbit feedback system for the Advanced Photon Source storage ring

The Advanced Photon Source (APS) is a dedicated third-generation synchrotron light source with a nominal energy of 7 GeV and a circumference of 1104 m. The closed-orbit feedback system for the APS storage ring employs unified global and local feedback systems for stabilization of particle and photon beams based on digital signal processing (DSP). Hardware and software aspects of the system will be described in this paper. In particular, we will discuss global and local orbit feedback algorithms, PID (proportional, integral, and derivative) control algorithm, application of digital signal processing to compensate for vacuum chamber eddy current effects, resolution of the interaction between global and local systems through decoupling, self-correction of the local bump closure error, user interface through the APS control system and system performance in the frequency and time domains. The system hardware including the DSPs is distributed in 20 VME crates around the ring and the entire feedback system runs synchronously at a 4-kHz sampling frequency in order to achieve a correction bandwidth exceeding 100 Hz. The required data sharing between the global and local feedback systems is facilitated by the use of fiber-optically-networked reflective memories. I. INTRODUCTION The Advanced Photon Source (APS) is one of the third-generation synchrotron light sources which are characterized by low emittance of the charged particle beams and high brightness of the photon beams radiated from insertion devices (IDs). In order to take full advantage of the intense synchrotron radiation, the incident intensity, position, and angle of the x-ray beam need to be tightly controlled (1-3). Even though every effort is made to stabilize the electrical and mechanical components of the ring, there will inevitably be residual beam motion primarily caused by the quadrupole vibration. The sources of vibration include ground vibration, mechanical vibration of the accelerator subcomponents, thermal effects, and so forth. These are manifested in the undesired particle and x-ray beam motion. This results in increased beam size and diluted beam emittance in the short term. An example of the long-term effect is the diurnal changes in the ring circumference and periodic shift of the x-ray beam at the user station (4). At the APS, the stringent transverse x-ray beam position stability required by the current user community will be achieved through extensive beam-position feedback systems with the correction bandwidth exceeding 100 Hz (5). The APS has 360 rf beam position monitors (BPMs) and 318 corrector magnets distributed around the storage ring, miniature BPMs for ID beamlines, and x-ray BPMs in the front end of x-ray beamlines for global and local orbit feedback. The real-time (AC) feedback systems, which are the main focus of this work, will use a subset of these. The feedback systems can be largely divided into the global and local feedback systems according to the scope of correction. The global feedback system uses up to 80 rf BPMs and 38 corrector magnets distributed in 40 sectors. The primary function is to stabilize the selected perturbation modes of the global orbit. The local feedback systems, on the other hand, stabilize the source regions of the x-ray beams locally for angle and displacement. An ideal local feedback system would not affect the rest of the closed orbit or other local feedback systems. In reality, the global and local feedback systems constantly interact with one another. The effect of global orbit feedback unavoidably interferes with the local feedback. On the other hand, the bump closure error in the local feedback due to bump coefficient error, magnet field error, eddy current effect, etc., causes global orbit perturbation and affects other local feedback systems. If this interaction is too strong, the feedback systems can become ineffective, oscillatory, or even unstable. In order to minimize such effects and maximize the feedback efficiency, it is necessary to decouple the global and local feedback systems and to compensate for the local bump closure error. The required data sharing between the global and local feedback systems is facilitated by the use of fiber-optically-networked reflective memories. In this work, we will discuss the feedback control algorithms and hardware and software configuration for the APS orbit feedback systems. The remainder of this paper will be an overview of the feedback algorithm and system description in Section II, system performance results in Section III and the current status of system integration in Section IV.