Commissioning of the APS Real-Time Orbit Feedback System

A unified global and local closed-orbit feedback system has been implemented at the Advanced Photon Source in order to stabilize both particle and photon beams. Beam stability requirements in the band up to 50Hz are 17μm in the horizontal plane and 4.4μm vertically. Orbit feedback algorithms are implemented digitally using multiple digital signal processors, with computing power distributed in 20 VME crates around the storage ring. Each crate communicates with all others via a fast reflective memory network. The system has access to 320 rf beam position monitors together with x-ray beam position monitors in both insertion device and bending magnet beamlines. Up to 317 corrector magnets are available to the system. The global system reduces horizontal rms beam motion at the x-ray source points by more than a factor of two in the frequency band from 10mHz to 50Hz. 1 SYSTEM DESCRIPTION 1.1 Hardware Architecture The real-time orbit feedback system is implemented digitally using multiple digital signal processors (DSPs). A total of twenty VME crates are distributed around the circumference of the 40-sector APS storage ring to manage interfaces to beam position monitors (BPMs) and correctors, and to compute orbit corrections. Figure 1 shows an overview of a typical crate. Each crate accesses locally available BPMs and correctors. A 68040 processor runs standard EPICS software, and one or more TMS320 DSPs perform orbit correction calculations at a 1kHz system sampling rate. Data from each crate is shared with all other crates via a synchronous reflective memory network running at 29MBytes/second. A 21st VME crate serves as master and performs realtime analysis of data collected from the other crates. 1.2 Software Architecture The 68040 EPICS processor interfaces the DSP(s) to the control system and provides the means to monitor and control feedback algorithms running on the DSP. Data structures residing in dual-ported RAM on the DSP board are used to communicate commands and return status and data. The EPICS processor uses state programs to interface database variables with the shared data structures. The reflective memories contain a shared data structure that is replicated at each node and is accessible to all processors in the feedback system. At each sample tic the DSP fetches BPM data from the BPM interfaces, computes the error in position, and writes the error values to assigned locations in the reflective-memory shared data structure. When all other DSPs have written their BPM errors, the DSP reads in the complete BPM error vector from the reflective memory and computes and writes new corrector values. The master feedback crate provides global controls to the other 20 feedback crates by writing to specific locations in the reflective-memory shared data structure. In addition, it provides analysis tools such as ‘dspscope’ which functions like a digital scope, and ‘ac voltmeter’ which performs a sliding discrete Fourier transform in real time, both tools working on 40 channels of data. 1.3 RF Beam Position Monitors The APS storage ring contains 360 rf BPMs, of which 320 are available to the real-time feedback system. In order to improve spatial resolution and minimize measurement error, it is preferable to use as many of the available BPMs as possible in the global correction algorithm. However computational resources are limited, and there is a trade-off between the number of BPMs in the algorithm and the overall sampling rate. The present configuration uses 160 BPMs and updates at 1kHz [1]. 1.4 X-ray Beam Position Monitors In addition to the rf BPMs, the APS is implementing x-ray BPMs on all the insertion device (ID) and bending magnet (BM) beamlines. These are intended to provide an absolute measure of the x-ray beam position for control of the x-ray source points.