Lessons learned from machine studies on existing rings

1. Lessons learned from machine studies on existing rings Laurent S. Nadolski Accelerator Group Synchrotron SOLEIL

2. Contents What performance have been reached in modern light sources? What are the main showstoppers and possible improvements? • Linear optics restoration • Orbit, tune stabilities • BBA techniques • Orbit(s) feedback, feedforward systems • Top-up and injection (See Also L. Emery’s talk) • Coupling control • Stable low coupling value, re-alignment, robustness • Non-linear dynamics (see Also R. Bartolini and T. Garvey’s talks) • On and off momentum transverse beam dynamics • Tuneshift control • ID characterization (see Also J. Bahrdt’s talk) • Physical aperture • Measurement • Beam loss location • Energy measurement • Impedance and instability (See last task of the session)

3. Linear optics modelling with LOCO: SOLEIL caseLinear Optics from Closed Orbit response matrix J. Safranek et al. Hor.  - beating Modified version of LOCO with constraints on gradient variations Due to lattice compactness (see ICFA Newsl, Dec’07)  - beating reduced to 0.3% rms Results compatible with mag. meas. and internal DCCT calibration of individual power supply Ver.  - beating Hor. dispersion Quadrupole gradient variation Quadrupole gradient variation

4. Comparison model/machine for linear optics * best achieved Courtesy R. Bartolini

5. Linear Orbit Restoration • Beta-beating, tune shift • Compensation • Static (e.g. LOCO) or Dynamics (e.g. feedforward)  Individual power supplies for quadrupole magnets • Local or global compensation for perturbations induced by insertion devices (IDs) • IDs are freely controlled by users, many different combinations: the storage ring is alive! • Small residual effects X many IDs = not so small perturbation • At SOLEIL, need of a global tune feedback • Impedance induced tune-shift with current variation  Improve injection efficiency  Necessary step for going to low coupling value and fine resonance correction • Is it possible to get an online measurement during user operation? • Tracking beta beating for all ID configurations? • How to get LOCO precision with turn by turn data? • How to get enough turns while a TFB is running? • For small emittance rings, multipole specs have to become more tighter? • At SOLEIL, focusing tolerance is 1/10 of the natural focusing of the IDs • More exotic insertion devices to come with linear and nonlinear perturbations (specific multipole correction patterns cf. J. Bahrdt)

6. Coupling • Achievement: close to 0.01% (almost 1 pm.rad) • Minimum emittance close to natural limit (factor 5 at SLS) • Diffracted limited in V-plane already. No push from users • Vertical emittance control • Betatron coupling suppression necessary for a fine correction of resonance widths. • Correct for low coupling and control with a vertical dispersion wave (ALS, SOLEIL, …) for few bunch filling pattern. • Local control of beam sizes (Upgrade project for very low emittance ring: local round beam with solenoid B-field, skew quads?) • Some beamlines want round beams, other flat beams: contradictory needs • Lessons • LOCO is an efficient tools. How to get a quick, on the fly method while keeping similar precision with turn by turn measurements? • Increase the number of individual skew quadrupoles • Difficulties to measure low coupling values (Vertically polarized SR by Anderson et al. SLS, …) • Difficult to maintain low coupling value over time  BBA on quadrupole and sextupole centers is the next step to reduce further more vertical emittance • ID skew quad have to be fully included in the shimming process

7. Low value: high sensitivity to tiny disturbance

8. Is it possible to maintain ultra-low coupling value other time with ID motions? L. Farvacque reporting of ESRF experiment, ESLS 2009

9. Experimental hall activities N O I S E S O U R C E S Ground vibrations Insertion Devices Cycling Booster operation mains +harmonics Thermal effects Frequency (Hz) 10-2 10-1 1 10 102 103 Time Period (s) 102 10 1 10-1 10-2 10-3 Emittance growth only Noise 1/Ti Sources of PerturbationsStability requirements Beamlines Integration time: • If FPERTURBATION > 1/TINTEGRATION→ Emittance growth, Lower photon flux in a stable way • If FPERTURBATION < 1/TINTEGRATION→ Noise on the measurement Beam stability should be better than 10% of the beam sizes 10% of the beam divergences An orbit feedback is needed to stabilize the beam position from DC up to ~100 Hz Identification of perturbation sources + passive techniques to damp oscillation amplitudes

10. Vertical position shift due to moving crane AT SOLEIL FB OFF. Vertical noise amplitude over one week: cultural noise 0.5 mm

11. D E A D B A N D SOFB FOFB DC 10-2 10-1 1 10 Frequency (Hz) Orbit stability Dead band approach not suitable • FOFB efficiency is suppressed at low frequencies (< 0.1 Hz) • ID motion frequency band • Beam based aligned on quadrupole centers • Precision reached is a few micro-meters (10µm) • Orbit feedback systems • Slow and/or Fast feedback system • How to cope with slow/fast correctors? • Feedback efficiency at 100 Hz reached: reduction factor 2 to 3 • Challenging part: • temperature stabilization < 0.1°C • Orbit stability sub-micrometric, a few tens of nm for ultra-low coupling values • Increase the correction bandwidth (new fast switching magnets) • Trends • XBPM in feedbacks • For Dipole: V-plane only • For IDs how to cope with gap motion dependency XBPM response?

12. Overview of fast orbit feedback performance Summary of integrated rms beam motion (1-100 Hz) with FOFB and comparison with 10% beam stability target * up to 500 Hz ** up to 200 Hz • Trends on Orbit Feedback • restriction of tolerances w.r.t. to beam size and divergence • higher frequencies ranges Courtesy R. Bartolini

13. FOFB Efficiency (1-350 Hz) Measurement on a BPM outside the feedback loop HORIZONTAL VERTICAL FOFB efficiency 100 Hz 50 Hz 3 Hz

14. Top-up operation • Increases average current, brilliance, Beam-line resolution, resolving power • Allow us to accept lower lifetime but there is still a limitation from radiation shielding, activation of component inside tunnels • Requires to high injection efficiency, large enough beam lifetime • Mandatory for sub-micrometric stability both for machine and beam-lines components • Orbit distortion during injection time • With the standard injection scheme, it cannot be cancelled out completely: • Fine kicker tuning, stray field (septum magnet) • SPRing8: use of pulse corrector • Pretty large beta-function required at injection point • Other injection scheme are heavily pursued both for on and off axis injection (pulse multipoles, swapped beams, See L. Emery’s talk)

15. Residual closed orbit distortioninjection bump A. Loulergue, SOLEIL

16. Probing non-linear beam dynamics • Goals: • 100% injection efficiency • Large beam lifetime (off-momentum aperture) • Small footprint in the tune-space • A good non-linear model of the storage ring • Measurement of tune shift with amplitude • Frequency contents of turn by turn data • What to include in the model • Lessons: • Details multipole errors from magnetic measurements • Thick sextupoles • Quadrupole fringe fields • Details B-field map for insertion devices • Vertical chromaticity is not well modeled

17. Non-linear-beam dynamics • Means/tools • FMA on/off momentum • Touschek lifetime computation with local energy acceptance (6D tracking) • SLS resonance correction • Non-linear LOCO see R. Bartolini’s talk  Still difficult but very promising • Trends • Symmetry broken by insertion devices in very large number • Introduction of damping wigglers • Local modification of focusing with additional quadrupole (double low beta)  lattice may becomes very sensitive to tune, phase advance variations, etc … • BPM requirements for machine studies (analysis of fundamental lines, beta-functions, phase advances, driving terms analysis) • High resolution even at low current • Turn mixing, timing errors, non-linear response, sensor tilt, channel cross talk, decoherence  challenging

18. Frequency Map Analysis: ALS and BESSY-II ALS linear lattice corrected to 0.5% rms -beating FM computed including residual -beating and coupling errors BESSY-II with harmonic sextupole magnets, chromaticity, coupling ALS measured ALS model BESSY-II measured BESSY-II model • A very accurate description of machine model is mandatory • fringe fields: dipole, quadrupole (and sextupole) magnets • systematic octupole components in quadrupole magnets • decapoles, skew decapoles and octupoles in sextupole magnets Courtesy C. Steier (ALS) P. Kuske (BESSY-II)

19. Beam Loss • Top-up operation, low gap insertion devices, narrow chamber give more stringent limits for beam losses • Losses have to be located in shielded areas to keep radiation dose below the 0.5 µSv/h regulation limit. • Off momentum particles may be lost either in the H-plane in dispersion region or in V-plane through non-linear diffusion. • In machine like SPRing8 or SOLEIL, non-linear dispersion has to be taken into account. Moreover, changes of chromaticities modify the loss process. • Lessons • Keeping the same location of losses for all machine conditions is not always possible. • Slight misalignment of a vacuum chamber may change loss location. • Need a regular survey with beam position/angle bumps, … • Mechanical gap of in-vacuum insertion is often smaller than magnetic gap because of shim, copper shield protecting magnet blocks (liner) • Precision vertical centering with e-beam is mandatory • Save operation can allow gap reduced down to 3-4 mm

20. SOLEIL case by varying chromaticity value Loss process xix = xiz = 0 2003, SPRing8: vacuum leakage at injection section during low emittance optics EPAC 2006, Tanaka et al., pp3369-3361 Losses in short straight section xix = xiz = 2 Losses in medium straight section

21. Conclusion • Lots of progress have been done during the last two decades. • Linear optics in well understood • Beam energy measurement by spin depolarization is not a easy task: success in ALS, ANKA, BESSY II, SLS, … not for the last build light sources! • Still a lot of work for tuning complex lattices with many sextupole families, trends to introduce octupole magnets to control tune shift with amplitude • Non-linear optics correction et measurement based on turn by turn data is still challenging and requires improved BPM systems  individual sextupole PSs? • Other challenging parts • Maintaining performance with many insertion devices freely controlled by users. • Top-up operation means beam delivered over many day period of time: it required the development of on-line continuous tools to measure beta-beat evolutions, local coupling, … performance degradation • Going to low emittance lattices makes requirements tighter for multipole tolerances of insertion devices. • Fast switching devices, low beam sizes drive orbit feedback improvement • Local control of beam sizes

22. References • CERN LER2010 workshop, 2010, http://ler2010.web.cern.ch/ler2010/ • 2nd Non-linear Beam Dynamics Workshop, Diamond 2009, http://www.diamond.ac.uk/Home/Events/Past_events/NBD_workshop.html • Top up workshop, 2009, Melbourne, Australia.http://www.synchrotron.org.au/index.php/news/events/australian-events/event/4-accelerator-physics-top-up-workshop • ESLS 2009, 2009, DESY, Germany https://indico.desy.de/conferenceDisplay.py?confId=2325 THANK you