Lessons learned from machine studies on existing rings · Lessons learned from machine studies on...

1March 1-5, 2010 – FLS workshop – Laurent S. Nadolski

Lessons learned from machine studies on existing rings

Laurent S. NadolskiAccelerator Group

Synchrotron SOLEIL


Contents

• 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)

What performance have been reached in modern light sources? What are the main showstoppers and possible improvements?


Linear optics modelling with LOCO: SOLEIL caseLinear Optics from Closed Orbit response matrix

J. Safranek et al.

Quadrupole gradient variation

Modified version of LOCO with constraints on gradient variations

Due to lattice compactness(see ICFA Newsl, Dec’07)

β - beating reduced to 0.3% rms

Results compatible with mag. meas. and internal DCCT calibration of

individual power supply

Hor. dispersion

Hor. β - beating

Ver. β - beating

Quadrupole gradient variation


Comparison model/machine for linear optics

Model emittance

Measured emittance

β-beating (rms) Coupling*(εy/ εx)

Vertical emittance

ALS 6.7 nm 6.7 nm 0.5 % 0.1% 4-7 pm

APS 2.5 nm 2.5 nm 1 % 0.8% 20 pm

ASP 10 nm 10 nm 1 % 0.01% 1 pm

CLS 18 nm 17-19 nm 4.2% 0.2% 36 pm

Diamond 2.74 nm 2.7-2.8 nm 0.4 % 0.08% 2.2 pm

ESRF 4 nm 4 nm 1% 0.25% 10 pm

SLS 5.6 nm 5.4-7 nm 4.5% H; 1.3% V 0.05% 2.8 pm

SOLEIL 3.73 nm 3.70-3.75 nm

0.3 % 0.1% 4 pm

SPEAR3 9.8 nm 9.8 nm < 1% 0.05% 5 pm

SPring8 3.4 nm 3.2-3.6 nm 1.9% H; 1.5% V 0.2% 6.4 pm

* best achievedCourtesy R. Bartolini


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)


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


Low value: high sensitivity to tiny disturbance


Is it possible to maintain ultra-low coupling value other time with ID motions?

L. Farvacque reporting of ESRF experiment, ESLS 2009


Sources of PerturbationsStability requirements

Thermal effects

Insertion DevicesCycling Booster

operation

Experimental hall activitiesGround vibrations

mains +harmonicsFrequency (Hz)

1 10 102 10310-110-2

Time Period (s)10-210-11102 10 10-3

NOISE

SOURCES

An orbit feedback is needed to stabilize the beam position from DC up to ~100 HzIdentification of perturbation sources + passive techniques to damp oscillation amplitudes

Beamlines Integration time:– If FPERTURBATION > 1/TINTEGRATION

→ Emittance growth, Lower photon flux in a stable way

– If FPERTURBATION < 1/TINTEGRATION→ Noise on the measurement

1/TiNoise Emittance

growth only

Beam stability should be better than

10% of the beam sizes

10% of the beam divergences


Vertical position shift due to moving crane AT SOLEILFB OFF.

Vertical noise amplitude over one week: cultural noise

0.5 µm


Orbit stabilityBeam 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 3Challenging part: – temperature stabilization < 0.1°C– Orbit stability sub-micrometric, a few tens of nm for ultra-low

coupling valuesIncrease 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?

Frequency (Hz)1 1010-110-2DC

SOFB FOFB

DEAD

BAND

Dead band approach not suitable• FOFB efficiency is suppressed

at low frequencies (< 0.1 Hz)• ID motion frequency band


Trends on Orbit Feedback

• restriction of tolerances w.r.t. to beam size and divergence • higher frequencies ranges

Overview of fast orbit feedback performance

FOFB BW Horizontal Vertical

ALS 40 Hz < 2 μm in H (30 μm)* < 1 μm in V (2.3 μm)*

APS 60 Hz < 3.2 μm in H (6 μm)** < 1.8 μm in V (0.8 μm)**

Diamond 100 Hz < 0.9 μm in H (12 μm) < 0.1 μm in V (0.6 μm)

ESRF 100 Hz < 1.5 μm in H (40 μm) ∼ 0.7 μm in V (0.8 μm)

ELETTRA 100 Hz < 1.1 μm in H (24 μm) < 0.7 μm in V (1.5 μm)

SLS 100 Hz < 0.5 μm in H (9.7 μm) < 0.25 μm in V (0.3 μm)

SPEAR3 60Hz ∼ 1 μm in H (30 μm) ∼ 1 μm in V (0.8 μm)

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

Courtesy R. Bartolini


FOFB Efficiency (1-350 Hz)Measurement on a BPM outside the feedback loopHORIZONTAL VERTICAL

100 Hz

3 Hz

50 Hz

FOFB efficiency


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)


Residual closed orbit distortioninjection bump

A. Loulergue, SOLEIL


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


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


ALS linear lattice corrected to 0.5% rms β-beating

FM computed including residual β-beating and coupling errors

ALS measured ALS model

Frequency Map Analysis: ALS and BESSY-II

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

BESSY-II with harmonic sextupole magnets, chromaticity, coupling

BESSY-II measured BESSY-II model

Courtesy C. Steier (ALS) P. Kuske (BESSY-II)


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


2003, SPRing8: vacuum leakage at injection section during low emittance opticsEPAC 2006, Tanaka et al., pp3369-3361

xix = xiz = 0

xix = xiz = 2Losses in short straight section

Losses in medium straight section

SOLEIL case by varying chromaticity valueLoss process


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


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

http://ler2010.web.cern.ch/ler2010/�

http://www.synchrotron.org.au/index.php/news/events/australian-events/event/4-accelerator-physics-top-up-workshop�

http://www.synchrotron.org.au/index.php/news/events/australian-events/event/4-accelerator-physics-top-up-workshop�

https://indico.desy.de/conferenceDisplay.py?confId=2325�

Lessons learned from machine studies on existing rings · Lessons learned from machine studies on...

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