Fast Orbit Feedback and Beam Stability at the Swiss Light Source

1. Fast Orbit Feedback and Beam Stability at the Swiss Light Source V. Schlott, M. Böge, B. Keil, P. Pollet, T.Schilcher • Introducing the Swiss Light Source • Stability Requirements • The SLS “digital” BPM System • FOFB Architecture • FOFB Results • Medium Term Beam Stability at SLS • Conclusions and Outlook

2. The Swiss Light Source – Accelerator Complex Pre-Injector LINAC Parameters LINAC Storage Ring Storage Ring Parameters Booster Parameters Booster

3. The Swiss Light Source – Beamlines • 7 beamlines in operation (May 2004) • 2 beamlines under commissioning • 2 beamlines under construction • 3 beamlines planned

4. Noise Sources:medium term – diurnal temperature, filling pattern and machine refills • 100 mm up to 1 mm • short term – ID gap changes and polarization switching • – ground vibration, cooling water, crane motion, • – traffic, rf-drifts, injector operation • 10 mm up to 100 mm Stability Requirements for SLS – A 3rd Generation Light Source • Angular Stability: photon energy resolution of 10-4 to 10-5 in planar crystal monochromators • require: DQbeam < 1 mrad • Position Stability: S/N-ratios of 10-3 to 10-4 on samples require: Dxbeam and Dybeam < s/10 • corresponding to Dybeam~ 1 mm in SLS low b ID-straights at 1% coupling • User Requirements: autonomous andindependent ID gap changes transparent to other users • Bandwidth Requirements: are given by the integration times of experiments • >> 100 s …. inelastic x-ray scattering • 0.1 – 100 s …. protein crystallography (PX) • …. m-tomography (CMT) • < 0.1 s …. time resolved XAS and x-ray diffraction • …. dichroism spectroscopy

5. Instrumentation for Monitoring and Control of Beam Stability: Short Term Motion • Performance of “Digital” BPM System: resolution at 4 kS/s: 0.8 mm current dep.: < 5 mm (@ fixed gain) stability: < 2.5 mm (@ < ± 1°C) The DBPM system is operated under “top-up” conditions (I = 330 mA) at fixed gain levels providing highest reproducibility and stability while excluding current dependencies! • Corrector Power Supplies: The resolution of the SLS corrector power supplies was specified to 15 ppm to obtain residual (vertical) rms orbit deviations of < 0.25 mm (200 seeds have been simulated with TRACY) Achieved resolution: ~ 1 ppm Achieved short term stability (< 60 s): < 10 ppm Achieved long term stability (> 1000 h): < 30 ppm

6. Global Stability with SOFB Slow Orbit Feedback SOFB • data points from all 72 BPMs are averaged over 0.32 s • they correspond to rms deviation from “golden orbit” • centralized data processing • uses all 72 BPMs and all 72 correctors per plane • RF frequency correction for path length changes • corrector setting through controls network • alternate correction of vert. and horiz. planes • correction rate < 3 s (~ 0.4 Hz) • in operation during Aug. 2001 – Nov. 2003 orbit distortions due to ID-gap and RF frequency changes cannot be corrected by SOFB

7. DBPM system consists of RF front end, • Quad digital receiver and DSP • CS interface via VME-bus • FOFB is integral part of DBPM system • fiber optic links (40 MB/s) via DSP2 • system is expandable for additional • RF and/or Photon BPMs Fast Orbit Feedback FOFB Architecture • the use of all 72 BPMs and 72 correctors in • both transverse planes allows de-centralized • data processing with 6 BPMs and 6 corrector • magnets per station (sector) • off energy orbits are corrected by SOFB using • central RF frequency as additional control parameter • circular fiber optic network for data distribution • initializing and control via central BD server • sampling and correction rate at 4 kS/s • correction bandwidth up to 100 Hz DBPM System - the Core Piece of the FOFB

8. FOFB - Transfer Function (closed loop performance) Theoretical Bode plots as function of sampling rate 20 Hz - f = 1 kS/s 45 Hz - f = 2 kS/s 100 Hz - f = 4 kS/s • -10dB damping reached at: • assuming PID controller • bandwidth of vacuum chamber and corrector • magnets including eddy currents taken into account Measured Bode plot (both transverse planes) OverallTime Delays: • quad digital receiver settings ~ 600 ms • position calculation (DSP1) ~ 60 ms • global data exchange ~ 8 ms • feedback algorithm and • SVD matrix calculation (DSP) ~ 70 ms • data transfer via PS controller ~ 150 ms • asynchronous mode of QDR < 250 ms

9. vacuum pumps ? girder eigenmodes booster FOFB – Power Spectral Densities • measured at tune BPM outside of feedback loop (bx = 11 m, by = 18 m) • no ID-gap changes FOFB – Accumulated Power Densities (1 – 150 Hz) • Examples (1 – 150 Hz): • tune BPM (by = 18 m): sy = 1.2 mm • ID 6S (by = 0.9 m): sy = 0.28 mm

10. FOFB – “In Loop” Performance • data points from all 72 BPMs are averaged over • 0.32 s representing a relevant bandwidth for • most SLS users • data points correspond to RMS deviation from • “golden orbit” • orbit distortions due to ID gap changes are • corrected, which makes ID gap changes • transparent to all users • vertical RMS during “top-up” run ~ 0.06 mm • horizontal RMS during “top-up” run ~ 0.71 mm • “blurring” of global horizontal RMS distribution • due to drifts of dispersion orbit, which is corrected • by a high level SOFB on RF frequency • this does not affect users, since ID straight • sections are dispersion free vertical RMS beam offset [mm] horizontal RMS beam offset [mm]

11. FOFB – External Reference with Photon BPMs • still strong gap dependence of photon BPMs • which needs calibration !!! • but photon BPMs can be used as external • reference, whenever ID-gaps are unchanged X-BPM position [mm] • photon BPM signals show systematic drifts • of photon beam in the order of ± 1.5 mm at • ~ 10 m distance from source points X-BPM position [mm]

12. Systematic Effects of FOFB Slow FB on Photon BPM Readings Photon BPM signal (at 06S) at ~ 10 m from source point data points are integrated over period of 1 s Photon BPM signal (at 06S) at ~ 10 m from source point data points are integrated over period of 1 s • systematic oscillations have been suppressed by • implementing a slow, high level feedback on • photon BPM readings. • resulting photon beam stability at the location • of first optical elements of the material sciences • and protein crystallography beamlines < 1 mm ! • systematic oscillations of ± 1.5 mm at the location • of the photon BPMs (~ 10 m from source point) • are not caused by temperature drifts of DBPM • electronics but could be correlated with injection • clock cycle! • bunch pattern dependence of electronics • bunch pattern control (feedback) is under commissioning

13. 50°C 20°C Quadrupole BPM chamber BPM support girder alignment rail Instrumentation for Monitoring and Control of Beam Stability: Medium Term Motion Deformation of SLS Storage Ring Vacuum Chamber due to Thermal Loads FEA-simulations indicate movements of up to 2 mm/°K in the transverse plane at the positions of the BPM blocks in case of thermal transients in the machine. • Position Monitoring System (POMS) of BPM Stations: Dial gauges equipped with linear encoders (0.5 mm res.) as sensing devices attached to quadrupole magnets “top-up” operation avoids thermal transients in storage ring

14. Medium Term Beam Stability at SLS due to “Top-Up” Operation • “Top-up” operation stabilizes beam current • in storage ring within a pre-defined dead • band (here: 200 ± 0.5 mA) • Only thermal transients caused by • “beam dumps” and/or decaying beam induce • mechanical movements of BPM blocks • Mechanical movements of all BPM blocks • in reference to adjacent quadupole magnets • are monitored with POMS system with a • resolution of 0.5 mm • During “top-up” operation no active • stabilization of mechanical movements is • needed. • Apart from mechanical stability due thermal equilibrium, “top-up” operation allows to operate • BPM electronics at constant gain levels under most stable conditions!

15. Conclusions and Perspectives • The FOFB stabilizes the electron beam orbit in the SLS storage ring up to • 100 Hz bandwidth to a mm-level in reference to a pre-defined “golden orbit” • Integrated short term electron beam stability (~ 0.1 – 100 Hz) at the SLS low gap • ID-straights in the vertical direction are below 0.3 mm • ID gap changes are in user’s control and fully transparent to other users • Additional slow feedback on photon BPMs stabilize photon beams on a sub-mm • level at the location of the first optical elements of the beamlines • Medium term stability is constantly monitored by POMS system • “Top-Up” operation leads to thermal equilibrium in SLS storage ring and • leads to mm medium term (minutes to days) stability • FOFB will be expanded by integration of additional RF and photon BPMs