A FAST ORBIT FEEDBACK FOR THE ELETTRA STORAGE RING

A fast global orbit feedback using digital Beam Position Monitor (BPM) detectors has been installed and commissioned at Elettra. The system uses 96 BPMs and 82 steerer magnets to correct closed orbit errors at a 10 kHz repetition rate. The feedback processing is performed by twelve VME stations equipped with commercial CPU boards running the Linux operating system with real-time extension and connected to each other by a low-latency fiber optic network. The system is fully controlled by a Tango based control system. The operational experience and the achieved results are presented. Plans for further improvements of the orbit stability are also discussed. FAST GLOBAL ORBIT FEEDBACK A project for the development of a fast orbit feedback started in 2005 with the aim of stabilizing the electron beam orbit of the Elettra storage ring to sub-micron levels. The demanding need of stability of the photon beam delivered to the beam lines requires fast digital loops with many BPMs and corrector magnets to counteract the different sources of closed orbit distortion like thermal drifts, vibrations, power supplies ripple, insertion devices gap changes, etc. The choice was to use all of the existing storage ring BPMs and corrector magnets already adopted for slow orbit correction. The BPM detectors have been upgraded with state of the art electronics (Libera Electron, by Instrumentation Technologies d.o.o.) to fulfil the requirements of precise beam position measurements and fast acquisition rate [1]. The analog input of the existing corrector power supplies specifically implemented for feedback purposes was available to drive the corrector magnets. The choice of the processing and communication framework was driven by the following requirements: • 10 kHz repetition rate with minimized latency • adoption of standard commercial components • processing performed by software • implementation of a number of diagnostic tools • effective integration into the Tango control system. The implemented architecture is a distributed processing system made of twelve local stations each interfaced to a number of BPM detectors and corrector power supplies [2]. Crate (VME), CPU board (Motorola PowerPC), operating system (Linux+RTAI) and control software (Tango) of the local stations are the same used in the accelerator control system. Gigabit Ethernet links connecting the Libera Electron detectors to the local stations via an Ethernet switch are used to acquire beam position data at 10 kHz. This low cost standard technology normally employed in local area networks is also suitable for real-time data transmission if UDP packets are used over a dedicated network. Another advantage is that standard instrumentation and widely available software tools can be used for debugging and troubleshooting of possible communication problems. The particular correction algorithm based on a global approach where the correction values are calculated by multiplying the position errors of all the BPMs by the inverted response matrix, requires a fast low-latency communication infrastructure to share position data among the local stations. A widely used commercial product (GE-Fanuc PMC-5565) implementing the reflective memory technology through fibre optics has been chosen for this purpose because of its performance and ease of use. A master station included in the reflective memory network is in charge of feedback supervision and data acquisition. The master station can simultaneously transmit through the reflective memory the following data to the local stations: • feedback set point (reference orbit) • inverted response matrix • feedback configuration parameters (PID, digital filter coefficients, etc.) • feedback control commands: close/open loop, freeze/unfreeze, etc.