1. Swiss Light Source The next 20 years
Andreas Streun Paul Scherrer Institut (PSI) Villigen, Switzerland
Future Research Infrastructures: Challenges and Opportunities
Varenna, Italy, July 8-11, 2015

2. Outline
Portrait of the SLS; history and achievements The new generation of light sources The challenge to upgrade the SLS A new type of lattice cell for lower emittance: longitudinal gradient bends and anti-bends SLS-2 design: performance, challenges, highlights

3. Paul Scherrer Institut (PSI)
1960 Eidgenössisches Institut für Reaktorforschung (EIR)
1968 Schweizer Institut für Nuklearphysik (SIN)
1988 EIR + SIN = PSI  research with photons, neutrons, muons
PSI Accelerators:
590 MeV proton cyloctron: 1.3 MW beam power  spallation neutron source SINQ & muon source SmS
5.8 GeV / 1 Å free electron laser SwissFEL: operation 2017
2.4 GeV synchrotron light source SLS

4. The SLS Electron beam cross section in comparison to human hair
transfer lines
90 keV pulsed (3 Hz) thermionic electron gun
100 MeV pulsed linac
Synchrotron (“booster”) 100 MeV  2.4 [2.7] GeV within 146 ms (~160’000 turns)
Current vs. time
2.4 GeV storage ring ex = nm, ey = pm 400±1 mA beam current top-up operation
1 mA
4 days
shielding walls

5. SLS: beam lines overview

6. SLS: history Dec. 2000 First Stored Beam July 2001 First experiments
1990 First ideas for a Swiss Light Source
1993 Conceptual Design Report
June Approval by Swiss Government
June Finalization of Building
Dec First Stored Beam
June Design current 400 mA reached Top up operation started
July First experiments
Jan Laser beam slicing “FEMTO”
May Tesla super bends
2010 ~completion: 18 beamlines

7. SLS achievements Rich scientific output Reliability
> 500 publications in refereed journals/year
four spin-off companies (e.g. DECTRIS)
Reliability
5000 hrs user beam time per year
97.3% availability ( average)
Top-up operation since 2001
constant beam current mA over many days
Photon beam stability < 1 mm rms (at frontends)
‏fast orbit feedback system ( < 100 Hz )‏
undulator feed forward tables, beam based alignment, dynamic girder realignment , photon BPM integration etc...
Ultra-low vertical emittance: 0.9 ± 0.4 pm
model based and model independent optics correction
high resolution beam size monitor developments
150 fs FWHM hard X-ray source FEMTO
laser-modulator-radiator insertion and beam line

8. The storage ring generational change
Riccardo Bartolini (Oxford University) 4th low emittance rings workshop, Frascati , Sep , 2014
Horizontal emittance normalized to beam energy
Storage rings in operation (•) and planned (•). The old (—) and the new (—) generation.

9. New storage rings and upgrade plans
Name
Energy [GeV]
Circumf. [m]
Emittance* [pm]
Status
PETRA-III
2304
4400  (round beam)
operational
MAX-IV
3.0
528
328  200
2015
SIRIUS
518
280
2016
ESRF upgrade
6.0
844
147
2020
DIAMOND upgrade
562
275
started
APS upgrade
1104 65
study
SPRING 8 upgrade
1436 68
PEP-X
4.5
2200
29  10
ALS upgrade
2.0
200
100
ELETTRA upgrade
260
250
SLS now
2.4
288
5020**
SLS-2
2.4 (?) ?
2024 ?
*Emittance without  with damping wigglers **without FEMTO insertion

10. The Multi-Bend Achromat (MBA)
Miniaturization
small vacuum chambers [NEG coated]
high magnet gradients
more cells in given circumference

11. SLS upgrade constraints and challenges
get factor lower emittance ( pm)
keep circumference & footprint: hall & tunnel.
re-use injector: booster & linac.
keep beam lines: avoid shift of source points.
“dark period” for upgrade months
Main challenge: small circumference (288 m)
Multi bend achromat: e  (number of bends)─3
Damping wigglers (DW): e  radiated power
Low emittance from MBA and/or DW requires space !
Scaling MAX IV to SLS size and energy gives e  1 nm 
New lattice concept  e  pm 
ring
ring + DW

12. Theoretical minium emittance (TME) cell dilemma
Conditions for minimum emittance (h = 1/r = eB/p curvature)
periodic/symmetric cell: b ’ = h’ = 0 at ends
 over-focusing of bx  phase advance m min =284.5°
2nd focus, useless
overstrained optics, huge chromaticity...
long cell
better have two relaxed cells of f/2
MBA concept...
bx by h
f, L, h

13. Conventional cells = relaxed TME cells
Deviations from TME conditions
Ellipse equations for emittance
Cell phase advance
 Real cells: m < 180°  F ~ MBA: F > 10

14. how to do better ? disentangle dispersion h and beta function bx
release constraint: focusing is done with quads only.
use “anti-bend” (AB) out of phase with main bend
suppress dispersion (ho  0) in main bend center.
allow modest bxo for low cell phase advance.
optimize bending field for minimum emittance
release constraint: bend field is homogeneous.
use “longitudinal gradient bend” (LGB)
highest field at bend center (ho = (e/p) Bo)
reduce field h(s) as dispersion h(s) grows
sub-TME cell (F < 1) at moderate phase advance

15. step 1: the anti-bend (AB)
General problem of dispersion matching:
dispersion is a horizontal trajectory
dispersion production in dipoles  “defocusing”: h’’ > 0
Quadrupoles in conventional cell:
over-focusing of beta function bx
insufficient focusing of dispersion h
 disentangle h and bx
use negative dipole: anti-bend
kick Dh’ =  , angle  < 0
out of phase with main dipole
negligible effect on bx , by
dispersion: anti-bend off / on
bx by
relaxed TME cell, 5°, 2.4 GeV, Jx  2
Emittance: 500 pm / 200 pm

16. AB emittance effects AB emittance contribution
h is large and constant at AB
 low field, long magnet
Cell emittance (2AB +main bend)
main bend angle to be increased by 2| |
 in total, still lower emittance
AB as combined function magnet
Increase of damping partition Jx
vertical focusing in normal bend
horizontal focusing in anti-bend.
horizontal focusing required anyway at AB
 AB = off-centered quadrupole  half quadrupole 
Disp. h
bx by

17. AB impact on chromaticity
Anti-bend  negative momentum compaction a
 Head-tail stability for negative chromaticity!
small
large
< 0
negative
side note: AB history
1980’s/90’s: proposed for isochronous rings and to increase damping - but 
PAC 1989

18. step 2: the longitudinal gradient bend (LGB)
Dispersion’s betatron amplitude
Orbit curvature
Longitudinal field variation h(s) to compensate H (s) variation
Beam dynamics in bending magnet
Curvature is source of dispersion:
Horizontal optics ~ like drift space:
Assumptions: no transverse gradient (k = 0); rectangular geometry
Variational problem: find extremal of h(s) for
numerical optimization
h(s) = B(s)/(p/e)

19. LGB numerical optimization
Half bend in N slices: curvature hi , length Dsi
Knobs for minimizer: {hi}, b0, h0
Objective: I5
Constraints:
length: SDsi = L/2
angle: ShiDsi = F/2
[ field: hi < hmax ]
[ optics: b0 , h0 ]
Results:
hyperbolic field variation (for symmetric bend, dispersion suppressor bend is different)
Trend: h0   , b0  0 , h0  0
Results for half symmetric bend
( L = 0.8 m, F = 8°, 2.4 GeV )
optimized
hyperbola fit
homogeneous
I5 contributions

20. LGB optimization with optics constraints
Numerical optimization of field profile for fixed b0, h0
Emittance (F) vs. b0, h0 normalized to data for TME of hom. bend
F = 1
F = 2
F = 3
F  0.3
small (~0) dispersion at centre required, but tolerant to large beta function

21. The LGB/AB cell } at R = 13 mm
Conventional cell vs. longitudinal-gradient bend/anti-bend cell
both: angle 6.7°, E = 2.4 GeV, L = 2.36 m, Dmx = 160°, Dmy = 90°, Jx  1
conventional: e = 990 pm (F = 3.4) LGB/AB: e = 200 pm (F = 0.69)
Disp. h
Disp. h
bx by
bx by
dipole field
quad field
total |field|
longitudinal
gradient
bend
} at R = 13 mm
anti-bend

22. SLS-2 lattice layout TBA  7BA lattice: ½ + 5 + ½ cells of LGB/AB type
SLS-2 arc
SLS arc
TBA  7BA lattice: ½ ½ cells of LGB/AB type
periodicy 3: 12 arcs and 3 different straight types:
6  4 m  6  2.9 m 3  7 m  3  5.1 m
split long straights: 3  11.5 m  6  5.1 m
beam pipe: 64 mm x 32 mm   20 mm  magnet aperture  26 mm

23. SLS-2 lattice db02l (one superperiod = 1/3 of ring)
optics and magnetic field  (field at poletips for R = 13 mm)
Superbends in arcs 2/6/10
3  2.9 T  3  5.0 T

24. SLS-2 lattice parameters
Name
SLS*)
db02l
fa01f
status
operating
baseline
fallback
Emittance at 2.4 GeV [pm]
5022
137
262
Lattice type
TBA
7BA
5BA
Total absolute bending angle
360°
585°
488°
Working point Qx/y
20.42 / 8.74
38.38 / 11.28
28.29 / 10.17
Natural chromaticities Cx/y
-67.0 / -19.8
-67.5 / -36.0
-64.1 / -39.9
Optics strain1)
7.9
5.6
8.9
Momentum compaction factor [10-4 ]
6.56
-1.39
-1.86
Dynamic acceptance [mm.mrad] 2) 46 10 17
Radiated Power [kW] 3)
205
228
271
rms energy spread [10-3 ]
0.86
1.05
1.15
damping times x/y/E [ms]
9.0 / 9.0 / 4.5
4.5 / 8.0 / 6.4
5.0 / 6.8 / 4.1
product of horiz. and vert. normalized chromaticities C/Q
max. horizontal betatron amplitude at stability limit for ideal lattice
assuming 400 mA stored current, bare lattice without IDs
*) SLS lattice d2r55, before FEMTO installation (<2005)

25. Non-linear optimization
13 sextupole & 10 octupole families
step 1: perturbation theory: insufficient
1st & 2nd order sextupole terms
1st order octupole terms
up to 3rd order chromaticities
step 2: multi-objective genetic optimizer
objectives: dynamic aperture at Dp/p = 0, 3%
contraints: tune fooprint within ½ integer box 
 Lattice acceptance results (ideal lattice)
horizontal acceptance 10 mm·mrad sufficient for off-axis multipole injection from existing booster synchrotron
Touschek lifetime 3.2 hrs 1 mA/bunch, 10 pm vertical emittance, 1.43 MV overvoltage further increase to 7-9 hrs by harmonic RF system.

26. More challenges... work just started
Collective effects
large resistive wall impedance ( aperture-3)
low momentum compaction factor |a|, and a < 0
close thresholds for turbulent bunch lengthening 
head-tail stability for chromaticity  0 
intrabeam scattering  15-30% emittance increase 
Alignment tolerances
common magnet yoke = girder
initial mechanical alignment will be insufficient
extensive use of beam based alignment methods
longer commissioning than 3rd gen. light source

27. Advanced options Round beam scheme A new on-axis injection scheme
Wish from users (round samples...)
Maximum brightness & coherence
Mitigation of intrabeam scattering blow-up
“Möbius accelerator”: beam rotation on each turn to exchange transverse planes
SLS
SLS-2
SLS-2 RB
A new on-axis injection scheme
cope with reduced aperture (physical or dynamic)
interplay of radiation damping and synchrotron oscillation forms attractive channel in longitudinal phase space for off-energy off-phase on-axis injection.
Figure taken from R. Hettel, JSR 21 (2014) p.843

28. Longitudinal gradient superbend
Photon energy range
YBCO1) HTS2) tape in canted-cos-theta configuration on hyperbolic mandrel
 hyperbolic field profile
open for radiation fan
> 5T peak field
1) Yttrium-Barium-Copper-Oxide
2) High Temperature Superconductor
Courtesy Ciro Calzolaio, PSI

29. Time schedule
Jan Letter of Intent submitted to SERI (SERI = State secretariat for Education, Research and Innovation)
schedule and budget
studies & prototypes 2 MCHF
new storage ring 63 MCHF beamline upgrades 20 MCHF
Oct positive evaluation by SERI: SLS-2 is on the “roadmap”.
Concept decisions fall 2015.
Conceptual design report end

30. Summary
The Swiss Light Source is successfully in operation since 15 years...
...but progress in storage ring design enforces an upgrade.
Upgrade of the Swiss Light Source SLS has to cope with a rather compact lattice footprint...
... but the new LGB/AB cell provides five times lower emittance than a conventional lattice cell:
Anti bends (AB) disentangle horizontal beta and dispersion functions.
Longitudinal gradient bends (LGB) provide minimum emittance by adjusting the field to the dispersion.
The baseline design for SLS-2 is a 12  7BA lattice providing times lower emittance.
The design is challenged by non-linear optics optimization, beam instabilities and correction of lattice imperfections.
A conceptual design report is scheduled for end 2016.

31. Acknowledgements
Beam Dynamics: Michael Ehrlichman, Ángela Saá Hernández, Masamitsu Aiba, Michael Böge Instabilities and impedances: Haisheng Xu, Eirini Koukovini-Platia (CERN), Lukas Stingelin, Micha Dehler, Paolo Craievich Magnets: Ciro Calzolaio, Stephane Sanphilippo, Vjeran Vrankovic, Alexander Anghel Vacuum system: Andreas Müller, Lothar Schulz General concept and project organisation: Albin Wrulich, Lenny Rivkin, Terry Garvey, Uwe Barth