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. Due to lattice compactness
Linear optics modelling with LOCO: SOLEIL case Linear 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
Model emittance
Measured emittance
-beating (rms)
Coupling*
(y/ x)
Vertical emittance
ALS
6.7 nm
0.5 %
0.1%
4-7 pm
APS
2.5 nm
1 %
0.8%
20 pm
ASP
10 nm
0.01%
1 pm
CLS
18 nm
17-19 nm
4.2%
0.2%
36 pm
Diamond
2.74 nm nm
0.4 %
0.08%
2.2 pm
ESRF
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 nm
0.3 %
4 pm
SPEAR3
9.8 nm
< 1%
5 pm
SPring8
3.4 nm nm
1.9% H; 1.5% V
6.4 pm
* 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. Sources of Perturbations Stability 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
Thermal effects
Insertion Devices
Cycling Booster
operation
Experimental hall activities
Ground vibrations
mains +harmonics
Frequency (Hz) 1 10
102
103
10-1
10-2
Time Period (s)
10-3 N O I S E U R C
1/Ti
Noise
Emittance growth only
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. Orbit stability Beam based aligned on quadrupole centers
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?
Frequency (Hz) 1 10
10-1
10-2 DC
SOFB
FOFB D E A B N

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
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
< 1.5 μm in H (40 μm)
 0.7 μm in V (0.8 μm)
ELETTRA
< 1.1 μm in H (24 μm)
< 0.7 μm in V (1.5 μm)
SLS
< 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)
Trends on Orbit Feedback
restriction of tolerances w.r.t. to beam size and divergence
higher frequencies ranges
* up to 500 Hz
** up to 200 Hz
Courtesy R. Bartolini 12

13. FOFB Efficiency (1-350 Hz) FOFB efficiency 100 Hz 50 Hz 3 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 distortion injection 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. Loss process SOLEIL case by varying chromaticity value xix = xiz = 0
2003, SPRing8: vacuum leakage at injection section during low emittance optics
EPAC 2006, Tanaka et al., pp
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,
2nd Non-linear Beam Dynamics Workshop, Diamond 2009,
Top up workshop, 2009, Melbourne, Australia.
ESLS 2009, 2009, DESY, Germany
THANK you