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3rd International Workshop on Beam Orbit Stabilization - IWBS2004

Announcement First / Second Accommodation Registration Participants Program Overview Details Orals by Session Orals with Abstracts Download PDF (gs) Download for PDAs Presentation   Upload / Download Summary Support Information No. #1 / #2 / #3 Pictures Gallery #1 / #2 / #3 Upload / Download SLS & Sponsors WWW psi.ch Updated: 25.01.2005		ORBIT MEASUREMENT/CORRECTION The first part of the ``Orbit Measurement/Correction'' session was dedicated to technological advances in diagnostics and feedback hardware. It was opened by a stimulating rapporteur talk on the "SLS Workshop on Beam Stability", held at PSI on Sep 6, 2004. The intention of the internal meeting was to review the performance and limitations of the actual stability, relevant hardware installations and the applied correction schemes at the SLS from the perspective of present and future user and operational requirements [6]. The digital electron beam position processor ``Libera'' from ``Instrumentation Technologies'', a commercial descendant of the digital BPM system developed for the SLS, allows simultaneous position readouts with MHz, kHz and Hz BWs providing sub-micron resolution (BPM geometry dependent) at sampling rates of a few kHz. Applying multiplexing techniques on the 4-channel system allows to reduce systematic effects (self-calibrating system, e.g., beam current dependence). Connectivity options like fiber link ports and extra processing power make the processor suitable for building FOFBs. SOLEIL and DIAMOND have made ``Libera'' their digital BPM system of choice. At DIAMOND all 204 BPMs in the accelerator chain use ``Libera'' electronics. In the storage ring the primary BPMs at the start/end of the straight sections have increased sensitivity through smaller aperture. They are decoupled through bellows and monitored with respect to a reference pillar as in the SOLEIL case. Their RMS noise with 1 kHz BW in the current range 60-300 mA has been measured to be $\approx$0.3 $\mu$m. The SOFB (10 Hz sampling rate, 0.5 Hz feedback BW) will use EPICS with the IOCs running inside ``Libera''. A TRACY-2 based ``virtual accelerator'' implemented on a Soft-IOC allows to test applications before commissioning. The FOFB (aimed at feedback BW $>$100 Hz) running at sampling rates of up to 10 kHz will employ dedicated feedback CPU boards to run the correction algorithm (different to SOLEIL case where exclusively ``Libera'' is used). At PSI a generic VME PMC carrier (VPC) board is currently under development and is likely to become the common digital platform for diagnostics and feedbacks at PSI (e.g., new proton BPMs, FOFB integration of X-BPMs, PROSCAN, LEG, Femto, DESY-RF collaboration) [7]. The analog/digital front-ends will be customized depending on the analog diagnostics hardware, whereas the digital back-end hardware will be identical with some common firmware/software. The VPC board is also a possible candidate for the replacement of the present SLS FOFB hardware and the digital BPM system. A future SLS BPM system might have single bunch resolution, allowing for beam-based calibration of BPM bunch charge dependence, as well as for a ``fusion'' of BPM and bunch charge monitor hardware for FOFB, multi-bunch feedback and bunch-pattern feedback. The not yet commissioned FOFB at SPEAR-3 utilizes an Echotek digital receiver as part of the 4-channel BPM system consisting of 24 BPMs (54 Bergoz BPMs for SOFB). BPM samples are streamed over dedicated point-to-point Ethernet links (no TCP/IP) to a central FOFB controller at a rate of 4 kHz. The corrections are fed into power supply controllers with 24-Bit DACs. The copper chamber induces a correction cut-off frequency at $\approx$120 Hz. The SOFB for the LHC foresees only one central SOFB processing unit utilizing the SVD algorithm and a PID controller extended by a ``Smith Predictor'' which compensates for constant propagation delays induced by e.g., computation time, task switching or network transfers. More than 1000 BPMs and correctors distributed over 27 km of circumference make it a delicate task to handle, in particular network delays on the switched GBit Ethernet (not dedicated to SOFB) and hardware failures. Special routers with prioritization (``Quality of Service'', QoS) on the hardware level guarantee a response time of $\approx$320 $\mu$s to all involved nodes. ``Bergoz Instrumentation'' is promoting VIAQS, a data acquisition and control server package to simplify the operation of accelerator diagnostic devices. It's aim is to ease the task of implementing an EPICS based SOFB using Bergoz BPMs. In the second part of the session the latest advances in the refinement of orbit correction schemes were presented. Since DELTA is suffering from large alignment/magnetic errors of machine constituents and improper placements of BPMs/correctors, global orbit corrections can easily saturate correctors. Therefore, their SVD based correction scheme has been extended to include constraints imposed by the maximum available corrector strength. The proposed Super-SOR light source, to be built near Tokyo, includes a global FOFB in its design (sampling rate $>$2 kHz, feedback BW $>$100 Hz) aiming for sub-micron stability at the location of the IDs. An eigenvector method with constraints (EVC) has been developed which allows an ``hard'' correction on crucial BPMs or X-BPMs adjacent to IDs to be performed. This is achieved by introducing position references as constraints to the algorithm while simultanously relaxing conditions elsewhere, all within one global correction scheme. The algorithm has been successfully applied to the PF(PF-AR) ring at KEK where 4(6) out of 65(83) BPMs were constrained to zero. The PLS in Korea operates a SOFB at a correction rate of $\approx$0.2 Hz. In 2004 the machine was equipped with new corrector power supplies (controllers of BESSY type) with 20-Bit resolution. The implementation of a global FOFB is under consideration. Since PLS is in decaying beam operation, a large fraction ($\approx$80 %) of the observed changes in the BPM readings is caused directly or indirectly by variations in the beam current (180-120 mA in $\approx$7 h). Main contributions are the BPM electronics dependence on beam current and vacuum chamber movements due to the change of the synchrotron radiation heat load. A dependence of the BPM electronics on ambient temperature has also been observed. In order to compensate for the BPM electronics dependence on beam current, look-up tables have been successfully implemented. It turned out to be difficult to compensate for the chamber movement. ``Top-up'' operation, however, would ultimately solve the problem. At the SLS a global FOFB with a sampling rate of 4 kHz is in operation since one year providing sub-micron stability at the IDs in the range 0.1-100 Hz [8]. The FOFB has a decentralized structure consisting of 12 intercommunicating BPM sectors (digital BPM system with FOFB capability developed at PSI) exchanging BPM readings through dedicated unidirectional fiber optical links. Each sector performs the necessary sub-matrix multiplications for the SVD based orbit correction on a dedicated DSP and applies the proposed corrections to the correctors within the sector. The orbit stability on the time scale $\approx$6 ms - 1 s is mainly limited by the BPM/corrector resolution, the FOFB system latency and the induced eddy currents in the vacuum chamber, whereas limits on longer time scales are mainly imposed by the reliability of hardware components, systematic errors of the BPMs and the thermal stability of the machine (``top-up'' operation). A filling pattern dependence of the BPM readings was observed which became manifest in a significant change ($\approx$2 $\mu$m peak-to-peak) of the reference orbit while X-BPM readings at 2 IDs were kept constant through a change of the reference settings of the two adjacent BPMs (X-BPM feedback). Meanwhile a filling pattern feedback has been implemented [9] which eliminates these systematic variations. A 30-40 ms long transient orbit change of 300 $\mu$m at dispersive BPMs is observed whenever the RF frequency is altered. A use of the frequency modulation input of the RF generator instead of the IEEE interface would most probably resolve the problem. Soon the FOFB will be upgraded/extended for the integration of additional BPMs (Femto) and X-BPMs. At ANKA the beam energy was calibrated by means of resonant spin depolarization, as previously done at other ring based light sources (e.g., BESSY, ALS, PLS, SLS [10]). One of the main aims of the presentation was to encourage the community to make use of this precise calibration technique ($\Delta$E/E$\approx$10$^{-5}$) since all prerequisites are already fulfilled in most low and medium energy light sources: the existence of a finite transverse electron polarization of a few 10 %, a Touschek dominated beam which leads to a significant decrease in lifetime (typically $>\approx$10 %) whenever the polarization level is dropping to zero since the Touschek scattering process is polarization dependent, a vertical kicker being capable of exciting and thus depolarizing the beam at the spin precession frequency $\nu$ which is directly related to the energy factor $\gamma$ ( $\nu=a\gamma$, $a$=anomalous part of the $g$-factor). By depolarizing and measuring $\gamma$ at different RF frequencies the linear and even the quadratic momentum compaction term can be determined.