Fast Global Orbit Feedback System in Spear3*

The new fast orbit feedback system (FOFB) is being commissioned at the SPEAR3 light source. The system has a 4kHz sampling rate and 100Hz bandwidth. The correction algorithm is based on the Singular Value Decomposition (SVD) of the orbit response matrix. For performance tuning and additional flexibility when adding or removing Corrector Magnets (CM) and Beam Position Monitors (BPMs), we implement an independent control loop for each orbit eigenvector used. This paper discusses design and performance of the system. INTRODUCTION SPEAR3 [1] is a 3 generation light source. The orbit must be kept stable to within a few percent of the electron beam size. Sub-micron stability in a 100 Hz bandwidth is desirable and has been achieved in similar machines. Since the start of operations in 2004, SPEAR3 relied on a slow orbit feedback (SOFB) that corrected the orbit every 5 seconds. In addition, to compensate for the user controlled Insertion Device (ID) movements, we implemented a feed-forward compensation scheme running at 10Hz, using four CMs adjacent to the IDs. The new fast feedback (FOFB) replaces the SOFB. One of the requirements is that it be a sole orbit feedback system in the DC to 100Hz bandwidth using all available BPMs and CMs. It should correct orbit error caused by weak uncorrelated 'background' vibrations of magnets and supports as well as strong localized disturbances, such as undulator gap changes. SYSTEM DESIGN The main purpose of the transverse orbit feedback system is the reduction of beam motion due to mechanical vibrations which are dominant below 100Hz. The limiting factors to the achievable open-loop bandwidth are CM field penetration into the vacuum chamber (copper with CuNi inlays for enhanced bandwidth to ~100Hz), CM power supply bandwidth (~700Hz) and aliasing of the multiplexed beam position monitors [2]. A discrete-time ('digital') controller closing the feedback loop inevitably introduces a finite time delay ('dead-time') which must be kept short enough so that the reduction of phase margin remains acceptable [3].