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Requirements for Tune, Coupling and Chromaticity Feedbacks for Light Sources

Micha Dehler, Paul Scherrer Institut Switzerland. Requirements for Tune, Coupling and Chromaticity Feedbacks for Light Sources. Introduction: Motivation, requirements, typical challenges Case studies: Situation, measurements, feed forwards and feedbacks Outlook. General remarks:

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Requirements for Tune, Coupling and Chromaticity Feedbacks for Light Sources

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  1. Micha Dehler, Paul Scherrer Institut Switzerland Requirements for Tune, Coupling and Chromaticity Feedbacks for Light Sources • Introduction: Motivation, requirements, typical challenges • Case studies: Situation, measurements, feed forwards and feedbacks • Outlook

  2. General remarks: Due to synchrotron radiation damping electron machines as synchrotron light sources have quite different sensitivity to fluctuations in the optics compared to hadron collider: • Less critical with respect to magnet resonances, more leeway for tune • Chromaticity typically less important (energy acceptance, life time, tune foot print) • High interest in minimizing coupling → vertical emittance and beam size for low gap insertion devices Tune resonances at SLS (Court. A. Streun)

  3. On coupling: • Global coupling: • Photon flux/brilliance (~Luminosity) • Local coupling: • Low gap apertures Qy Qx Horizontal tune shift y Qy x Qx

  4. Fluctuation of beam optics: • Special case accelerators using energy ramping, (e.g ELETTRA, Australian Light Source): • Hysteresis effects, magnet heat up • Decaying current during storage → non constant heat load, drift • Tune tolerance → Feed Forward • Modern designs inject at upper energy and use Top Up mode • Main effects due to orbit fluctuations (should be taken care of by orbit feedback) • Current dependencies (Top up mode stabilizing current and fill pattern) • High field insertion devices with adjustable configuration as field strength, gap size etc. (wigglers, SC undulators, in vacuum devices). • Effects need to be transparent for ring optics and other users • Perturbation is predictable, so major candidates for feed forward. • Typical example Apple II undulator

  5. electron beam magnets Source of optics variation: APPLE II undulator Lower half

  6. frontview 2 gap, energy 2 shift, polarization Shifting magnets to change polarization of synchrotron light influence the beam optics movable fixed sideview horizontal Bz,Bx linear 0 - 90o circular left or right (T. Schmidt) energy, polarization = f(gap,shift) vertical Very flexible for the user, but also having lots of side effects

  7. Local compensation of coupling at PolLux beamline (Boege et al. EPAC06) Synchrotron light in storage rings is horizontally polarized in the plane of deflection. As we move up or down, polarization becomes circular (and photon flux decreases). Microspectroscopy beam line PolLux uses bending magnet: • Cannot move ‘insertion device’, create local bump resulting in up to 300 urad beam angle to give up to 80 % circularly polarized light. Attractive technique allowing relatively fast switching of polarization. • Global fast orbit feedback takes care of trajectory offsets • Challenge: got sextupoles within the bump, deterioration in coupling!

  8. Layout of vertical 'polarization bump: • Four dipole correctors (magenta) generate bump • Sextupoles within bump (green) introduce betatron coupling • Dedicated skew quads (red) are used in feed forward for coupling correction Twist of electron beam ellipse versus position in storage ring assuming 300 urad steering for PolLux with/without correction (Simulation with TRACY). Combined with global orbit feedback expect to be able to switch polarization at 10 Hz without affecting other beam lines.

  9. ELETTRA – 2.4 GeV light source Still using ramping: • in early stage had feedback system to correct global tune during ramping from 0.9-2.4 GeV (see EPAC94 paper by Nagaoka et al.) • Nowadays replaced by Feed Forwards using interpolation from set points at 0.9 – 1.5 – 1.8 – 2.0 – 2.4 GeV • Problems with hysteresis and heat load effects due to ramping cycles • From 2008 on, will do full energy injection with new booster → with more stable conditions, expect push towards higher performance • Currently no local coupling and chromaticity measurements • Storage ring FEL mode at 1 GeV: Special Feed Forward to control magnetic multipole effects of APPLE II undulator • Optional tune measurement using bunch by bunch feedbacks: • For individual bunch provide negative feedback to excite instability – problem: initiating instability and controlling amplitude • Excite individual bunch with pink noise and analyze motion with e.g. NAFF algorithm (Laskar)

  10. Diamond: • Tune: • On shorter time scale stable within few 1e-4 • Run by run variations of tune in the order of 0.03 • Coupling drifts in range 1-2.2% (due to orbit drifts in the sextupoles?) • Chromaticity rarely measured and touched • Did some experiments with feed forward to compensate effect of SC wiggler (Currently not required). H V R. Bartolini • On demand tune measurement via frequency sweep excitation of beam (as also e.g. ESRF), chromaticity via variation of RF frequency • Currently no feedbacks/feed forwards – will probably need systems in the future

  11. Soleil: • Tunes: Target is stabilization to within 1e-3. Main sources are imperfection of insertion devices (APPLE II/ In Vacuum undulators), current dependency due to machine impedance • Chromaticity: Need variations below 0.1 (dispersion in insertion devices). Sources as for tunes (ID) • Coupling Current value 0.3%, crucial for life time and emittance Final target < 0.1% using top up • Currently running without feedback/feedforward: • Real time tune measurement by exciting total beam via pink noise – challenges: • measurement should be transparent for users → very small levels • Bunch by bunch feedback widens tune resonance peak → resolution deteriorates from 1e-4 to >1e-2 • Coupling measurement via synchrotron light/pin hole • Future plans: • Tune feed forwards for insertion devices • Dedicated skew quad for compensating coupling for one ID • Chromaticity feed forward for one of the Ids

  12. SLS: • Tunes: In principle have leeway for tune (~1e-2). But high chromaticity settings enlarge footprint of beam in tune diagram → life time reacts sensitive to tune. Strongest effects from the wiggler. • Chromaticity: Relatively insensitive. • Coupling: ~1e-3 crucial for life time and emittance (see the following) • Global tune feedback, tune and coupling feed forward for selected ID (see the following) • Real time tune measurement by • residual orbit oscillations from injection during Top Up mode, approximately every 1-2 minutes (current standard mode) • Using data bunch by bunch feedbacks (similar to ELETTRA approach) • Emittance, coupling, beam size measurement via synchrotron light (see the following) • Future plans: Local measurement and compensation of coupling

  13. Tune feedback at SLS

  14. Our swiss (swedish) knife for looking at coupling ...

  15. Minimizing coupling to allow for flat beams with minimum vertical emittance for high brilliance synchrotron light (In principle detrimental for Touschek life time, but can be recovered by introducing vertical dispersion) Vert. emittance: 3.5 pm rad

  16. Outlook: • Wishlist: • Global parameters: Yeah, OK ...... • Local optics: Yes, of course!! • Special emphasis on tune, coupling control • Low bandwidth, high precision • Betatron (dispersion/chroma) feedback? • Measurements: • Classic: driving the beam (Noise, hmm...) • Synchrotron light monitors (pin hole, emittance) • Interesting: BTF, PLL

  17. ... still assembling .... Thanks for the invitation and to: A. Anderson, R. Bartolini, J. Bengtsson, M. Boege, E. Karantzoulis, M. Lonza, L. Nadolski, E. Plouviez, G. Rehm, A. Streun....

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