1. Low vertical emittance at the SLS (update December 2011 – October 2012)
A. Saa Hernandez on behalf of the TIARA WP6 collaboration
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

2. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
Outline
Summary of the project status
Motivation, Equipment, Procedures and Results
This year activities
Design and expected performance of the “New” Beam Size Monitor
Maintenance of the “Old” Beam Size Monitor
Low energy IBS measurements at SLS
Outlook
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

3. SLS Vertical Emittance Tuning (SVET)
Test Infrastructure and Accelerator Research Area Work package 6 “SVET”
Partially funded by the European Commission under the FP7-INFRASTRUCTURES /INFRA project TIARA (CNI-PP). Grant agreement no
“The main objective of SVET is to upgrade the Swiss Light Source (SLS) at PSI to enable R&D on ultra-low emittances. [...] SLS will – after this upgrade – be an R&D infrastructure suitable to investigate ultra-low vertical emittance tuning and control, in particular also in the regime of strong IBS. This is relevant for damping rings of future linear colliders and for next generation light sources.”
Activities:
1. minimization of vertical emittance: (PSI and INFN/LNF)
storage ring alignment, optics correction & tuning methods and automation
2. measurement of low vertical emittance: (PSI and Max-Lab)
design & construction of a high resolution beam size monitor
3. intra beam scattering (IBS) studies: (PSI and CERN)
simulations and measurements of emittance and energy spread increase at high currents
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

4. Pre-Requisites and Tools for SLS Vertical Emittance Tuning
1. high beam stability as a pre-requisite
top-up operation → high thermal (long term) stability
precise BPMs: ~ 100 nm rms (< 100 Hz)
fast orbit feedback
orbit control & short term stability
2. procedures & equipment for vertical emittance tuning
re-alignment (beam-assisted girder alignment) of storage ring
→ remote positioning of 48 girders
skew quadrupoles for coupling control (36 in case of SLS)
→ sextupoles with additional coils
high resolution beam size monitor
→ π-polarization method
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

5. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
Principle of the Beam Size Monitor – the π-polarization method
 Å. Andersson, et al., Determination of Small Vertical Electron Beam Profile and Emittance at the Swiss Light Source, NIM-A 592 (2008)
imaging of vertically polarized SR in the visible/UV
phase shift of π between two radiation lobes
→ destructive interference in the mid plane
→ Iy=0 = 0 for point-like beam
→ Iy=0 > 0 for beam with finite vertical beam size
look-up table for beam size determination from SRW
 O. Chubar & P. Elleaume, Accurate and Efficient Computation of Synchrotron Radiation in the Near Field Region, EPAC 1998
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

6. Procedure for SLS Vertical Emittance Tuning
1. measurement and correction of BPM roll error
→ avoid “fake” vertical dispersion readings (from 48 dispersive BPMs with hhor ≠ 0)
2. realignment of magnet girder to remove main sources of vertical dispersion
→ reduction of rms vertical correction kick from ~ 130 μrad to ~ 50 μrad
3. measurement & correction of vertical dispersion and betatron coupling
→ model-based skew quadrupole corrections (12 dispersive and 24 non-dispersive skew quads)
4. “random walk” optimization of vertical beam size (NEW!)
→ skew quadrupole corrections using beam size measurements from profile monitor
works in the background (small steps), overcomes measurement limitations and model deficiencies
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

7. Random walk optimization of vertical beam size
Model-independent correction
Target function of the optimization is the measured vertical beam size
The optimum solution is within easy “walking-distance” after systematic correction
Potential of online optimization, i.e. keeping small emittance during the operation
 M. Aiba, et al., Ultra Low Vertical Emittance at SLS Through Systematic and Random Optimization, NIM-A 694 (2012)
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

8. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
SLS Vertical Emittance Optimization – Results
Iterative Minimization Procedure
BPM roll error corrections
beam-based girder alignment
dispersion & coupling corrections
beam size monitor tuning
random walk optimization
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

9. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
SLS Vertical Emittance Optimization – Results
ESLS 2011
(results March 2011)
ESLS 2012
(results December 2011)
vertical beam size: 5.0 ± 0.5 µm ± 0.6 µm
vertical emittance: 1.9 ± 0.4 pm ± 0.4 pm
error estimate from beam size and β-function at monitor
dispersion not subtracted
Limited by
beam size monitor
resolution!
Horizontal and Vertical Emittances of Storage Rings
Beam Size display from SLS π-polarization monitor
existing () and planned ()
Figure taken from:
R. Bartolini, Low Emittance Ring Design, ICFA Beam Dynamics Newsletter, No. 57, Chapter 3.1, 2012 – and updated.
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

10. Main Features of the “New” SLS Beam Size Monitor
calibration & alignment
X08-DA allows for longer beam line → optics table fully accessible outside of accelerator tunnel
higher magnification ratio (M = -1.45) → increase of measurement precision
toroidal mirror as focusing element → free selection of SR wavelength without shift of image plane
→ shorter wavelength increases resolution
π-polarization & interferometric method → matched operating ranges (nominal and high resolution)
→ cross-checking of results
alignment & calibration set-up → online inspection of monitor at 266 nm and 532 nm
Main Features of the “New” SLS Beam Size Monitor
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

11. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
Expected performance
π-Polarization Method – Emittance Resolution
Interferometer Method – Emittance Resolution
25 mm obstacle width
π -Polarization Method – Resolution vs Wavelength
Interferometer Method – Resolution vs Obstacle Width
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

12. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
Expected performance
π-Polarization Method vs. Interferometer Method
improved resolution → ~ 4 μm (old monitor) to < 2 μm (new monitor)
two measurement methods → π-polarization for nominal SLS operation
→ interferometer for low emittance studies
calibration & alignment path → online check of monitor performance
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

13. Critical issues: misalignments
Toroidal mirror used for imaging → peak/valley ratio extremely sensitive to misalignments
SRW simulations used to define constraints on mechanical alignment
and for guidance during commissioning
Offsets
horizontal offset Dx
vertical offset Dy
→ not critical within ± 50 μm
Tilts and Axis Rotation
horizontal tilt Tx
symmetric broadening and
and washing out of peak-to-valley pattern
rotation around mirror axis R
and vertical tilt Ty
asymmetric washing out of
peak-to-valley pattern for R & Ty
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

14. Critical issues: surface quality Toroidal mirror specifications
Surface errors modifies path length → modeled to set specifications of optical elements
Model principle
 M. Sanchez del Rio and A. Marcelli, Waviness effects in
ray-tracing of real optical surfaces, NIM-A 319 (1992) + L
Toroidal mirror specifications
Slope error
0.2 arcsec
Roughness
21 nm p-v (λ/30), <4 nm RMS
Waviness pattern
vertical (also radial)
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

15. Modeling of interference of partially coherent light
Calibration of measurements with numerical simulations from SRW + analytical model
Interference of partially-coherent light is modulated, Ipeak/Ivalley varies as a function of the source size (e- beam) b θ
~size of partially-coherent light source w a
~aperture size and shape
~interference of light from 2 apertures s
 B. J. Thompson and E. Wolf, Two-Beam interference with partially coherent light, Journal of the optical society of America 47 (1957)
Preliminar!
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

16. Maintenance of the old monitor: X-ray branch
Replace exit window: aluminium - very low transmittance at low energy (1.6 GeV)
diamond + kapton (100 μm) - kapton burns → vacuum problems
diamond + diamond
Good measurements for beam sizes ≥ 10 μm
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

17. Maintenance of the old monitor: Visible/UV branch
Replace second mirror and gimbal mount - broken
Replace exit window
fused silica - UV damaged
sapphire - birrefringent = polarization changes
fused silica
Replace lens - UV damaged
30 %
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

18. Low energy IBS measurements at SLS
SLS top-up mode at 1.6 GeV, for measurements of intra-beam scattering
Not trivial procedure, scaling down parameters not enough → highly non-linear magnet behavior
Setting booster with flat top at 1.6 GeV → still some non-reproducible problems
Results from intra-beam scattering measurements:
Beam size measurements (blue squares) and IBS expectations (based on CIMP formalism) for different zero vertical emittances and bunch lengths (solid lines)
 F. Antoniou et al, Optics design optimization for IBS dominated beams, HB 2012
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

19. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
Summary and Outlook
methods for emittance tuning established at SLS
→ lowest vertical emittance of 0.9 ± 0.4 pmrad
design of “new” high resolution beam size monitor at SLS
→ application of π-polarization and interferometric methods with overlapping sensitivity ranges (for nominal SLS operation and low emittance studies)
→ mirror optics (toroidal focusing mirror) provides free selection of SR wavelength
→ sensitivity study using SRW provides specifications for optical elements
→ expected measurements resolution for vertical beam height < 2 mm
maintenance of old monitor; some difficulties most probably solved
promising results on IBS measurements; new data to be analyzed
next steps…:
→ installation of “new” monitor in SLS X08DA beamline in January 2013
→ further emittance minimization until mid of 2013 (SLS quantum emittance limit at 0.2 pmrad)
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012

20. ESLS Workshop, Helmholtz Zentrum Berlin, 19 - 20 November 2012
Thank you!
ESLS Workshop, Helmholtz Zentrum Berlin, November 2012