5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec Requirements for Tune, Coupling and Chromaticity Feedbacks for Light Sources Micha Dehler, Paul Scherrer Institut Switzerland Introduction: Motivation, requirements, typical challenges Case studies: Situation, measurements, feed forwards and feedbacks Outlook

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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 Tune resonances at SLS (Court. A. Streun) 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

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec On coupling: Global coupling: Photon flux/brilliance (~Luminosity) Local coupling: Low gap apertures y Q y x QxQx QxQx QyQy Horizontal tune shift

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec Source of optics variation: APPLE II undulator magnetselectron beam Lower half

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec sideview frontview movable fixed energy, polarization = f(gap,shift) 2 gap, energy 2 shift, polarization Bz, Bx horizontal circular left or right linear o vertical Shifting magnets to change polarization of synchrotron light influence the beam optics Very flexible for the user, but also having lots of side effects (T. Schmidt)

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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!

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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.

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec ELETTRA – 2.4 GeV light source Still using ramping: in early stage had feedback system to correct global tune during ramping from 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)

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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 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 R. Bartolini

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec Tune feedback at SLS

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec Our swiss (swedish) knife for looking at coupling...

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec

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

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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

5 th CARE workshop on Novel Methods for Accelerator Beam Instrumentation, Chamonix, Dec 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....