1. R. Bartolini John Adams Institute and Diamond Light Source Ltd
Accelerator Physics Challenges in 3rd Generation Synchrotron Light Sources
R. Bartolini
John Adams Institute and Diamond Light Source Ltd
Particle Physics Seminar
Tuesday 19 February 2008

2. Particle Physics Seminar
Summary
Introduction:
synchrotron radiation
storage ring synchrotron radiation sources
Accelerator Physics challenges:
brightness, flux, stability, time structure
Conclusion:
future trends
3rd generation vs 4th generation
Particle Physics Seminar
Tuesday 19 February 2008

3. What is synchrotron radiation
Electromagnetic radiation is emitted by charged particles when accelerated
The electromagnetic radiation emitted when the charged particles accelerated radially (v  a) is called synchrotron radiation
. It is produced in the synchrotron radiation sources using bending magnets undulators and wigglers
Particle Physics Seminar
Tuesday 19 February 2008

4. storage ring synchrotron radiation sources (I)
Particle Physics Seminar
Tuesday 19 February 2008

5. storage ring synchrotron radiation sources (II)
Particle Physics Seminar
Tuesday 19 February 2008
Courtesy Z. Zhao

6. storage ring synchrotron radiation sources (III)
Particle Physics Seminar
Tuesday 19 February 2008
Courtesy Z. Zhao

7. synchrotron radiation sources properties
Broad Spectrum which covers from microwaves to hard X-rays: the user can select the wavelength required for experiment
synchrotron light
High Flux and High Brightness: highly collimated photon beam generated by a small divergence and small size source (partial coherence)
High Stability: submicron source stability
Polarisation: both linear and circular (with IDs)
Pulsed Time Structure: pulsed length down to tens of picoseconds allows the resolution of processes on the same time scale
Flux = Photons / ( s  BW)
Brightness = Photons / ( s  mm2  mrad2  BW )
Particle Physics Seminar
Tuesday 19 February 2008

8. Brightness
diamond
X-rays from Diamond will be 1012 times brighter than from
an X-ray tube,
105 times brighter than the SRS !
X-ray tube
60W bulb
Candle
Particle Physics Seminar
Tuesday 19 February 2008

9. Life science examples: DNA and myoglobin
Franklin and Gosling used a X-ray tube:
Brightness was 108 (ph/sec/mm2/mrad2/0.1BW)
Exposure times of 1 day were typical (105 sec)
e.g. Diamond provides a brightness of 1020
100 ns exposure would be sufficient
Nowadays pump probe experiment in life science are performed using 100 ps pulses from storage ring light sources: e.g. ESRF myoglobin in action
Photograph 51 Franklin-Gosling
DNA (form B)
1952
Particle Physics Seminar
Tuesday 19 February 2008

10. Particle Physics Seminar
Brightness with IDs
Particle Physics Seminar
Tuesday 19 February 2008

11. Particle Physics Seminar
312 ns
Time structure
Time resolved science requires operating modes with single bunch or hybrid fills to exploit the short radiation pulses of a single isolated bunch
Particle Physics Seminar
Tuesday 19 February 2008

12. Accelerator Physics challenges
Small Emittance
Insertion Devices (low gaps)
High Current; Control Impedance; Feedbacks
Control Vibrations; Orbit Feedbacks; Top-Up
Short Pulses; Short Bunches
Brightness, Flux
Stability
Time structure
Particle Physics Seminar
Tuesday 19 February 2008

13. Brightness and emittance
The electron beam emittance is a parameter of the storage ring that defines the source size and divergence
brightness  1 / emittance
NSLS-II
Particle Physics Seminar
Tuesday 19 February 2008

14. Emittance in an electron storage ring
The quantum nature of the synchrotron radiation emission is responsible for the finite beam size, emittance and energy spread of the electron beam.
Transverse electron oscillations are excited by the emission of a photon and are damped on average when the electron travels through the RF cavities
Oscillation damping and excitation counterbalance and an equilibrium emittance is reached
Particle Physics Seminar
Tuesday 19 February 2008

15. Small emittance lattices
The horizontal emittance is determined by the dispersion generated by the main bending magnets.
Low emittance and adequate space in straight section to accommodate long Insertion Devices are obtained in the so called
DBA and TBA lattices
Theoretical Minimum Emittance
Particle Physics Seminar
Tuesday 19 February 2008

16. Commissioning of small emittance optics (I)
During commissioning the Accelerator Physicists have to ensure that storage ring operates successfully in the nominal linear optics.
Linear optics studies are based on the analysis of the closed orbit response matrix (LOCO-like approach)
The orbit response matrix R is the change in the orbit at the BPMs as a function of changes in the steering magnets strength V V
Using the Singular Value Decomposition (SVD) of the Response Matrix R we can invert R and correct the closed orbit distortion H H
Particle Physics Seminar
Tuesday 19 February 2008

17. Commissioning of small emittance optics (II)
The response matrix R is defined by the linear lattice of the machine, (dipoles and quadrupoles), therefore it can be used to calibrate the linear optics of the machine
The quadrupole gradients are used in a least square fit to minimize the distance 2
Particle Physics Seminar
Tuesday 19 February 2008

18. Quadrupole gradient correction
LOCO varies the quadurpoles individually to fit the measured RM;
Initially the quadrupole variations generated by LOCO could reach 4%;
Quads variation reduced with better closed orbit correction, BBA and SVD threshold for LOCO;
Within each family quads variations are less than 2 % with respect to the mean for each quad family. (Up to 5 % with respect to the nominal calibration)
Particle Physics Seminar
Tuesday 19 February 2008

19. Implementation of small emittance optics
The optic functions measured at the BPMs location (circles) agree very well with the measured one (crosses)
Residual beta-beating can be reduced to 1% or less
Particle Physics Seminar
Tuesday 19 February 2008

20. Emittance measurements with two pinhole camera
Measured emittance very close to the theoretical values confirms the optics
Emittance
2.78 (2.75) nm
Energy spread
1.1e-3 (1.0e-3)
Emittance coupling
0.5%
Particle Physics Seminar
Tuesday 19 February 2008

21. Small emittance and nonlinear beam dynamics
Small emittance  Strong quadrupoles  Large (natural) chromaticity
 strong sextupoles (sextupoles guarantee the focussing of off-energy particles)
Courtesy A. Streun
strong sextupoles reduce the dynamic aperture and the Touschek lifetime
 additional sextupoles are required to correct nonlinear aberrations
[Consider the effect of realistic errors (and define magnetic error tolerances)]
Particle Physics Seminar
Tuesday 19 February 2008

22. Chromatic (energy dependent) effect
Optics functions vary with relative energy offset
The betatron tunes crosses a wide range of resonances with relative energy offset
Particle Physics Seminar
Tuesday 19 February 2008

23. Nonlinear beam dynamics optimisation (I)
It is a complex process where the Accelerator Physicist is guided by
(semi-)analytical formulae for the computation of nonlinear maps, detuning with amplitude and off-momentum, resonance driving terms
numerical tracking: direct calculation of non linear tuneshifts with amplitude and off-momentum, 6D dynamics aperture and the frequency analysis of the betatron oscillations
Many iterations are required to achieve a good solution that guarantees a good dynamic aperture for injection and a good Touschek lifetime
Particle Physics Seminar
Tuesday 19 February 2008

24. Nonlinear beam dynamics optimisation (II)
The Dynamic Aperture problem
Frequency Map Analysis
allows the identification of dangerous non linear resonances during design and operation
Vacuum chamber
ALS measured
ALS model
Strongly excited resonances can destroy the Dynamic Aperture
Particle Physics Seminar
Tuesday 19 February 2008

25. Touschek lifetime
Electrons performing betatron oscillations may scatter and be lost outside the momentum aperture available from RF voltage and the 6D dynamic aperture
Synchrotron radiation light sources require a large off-momentum aperture
The full 6D dynamic aperture has to be optimised
Particle Physics Seminar
Tuesday 19 February 2008

26. How to achieve and even smaller emittance
Reduce the emission of radiation in bending magnets with either lower energy or weaker magnetic field → larger circumference (NSLS-II, Petra-III, PEP, Tristan). The radiated energy is proportional to E2B2
Damping wiggler in the storage ring (NSLS-II, PETRA-III): beam dynamics still manageable; sub-nm emittance looks feasible !
Tailor the magnetic field in the dipole – azimuthal dependence - in order to reduce the integral of the dispersion invariant in the dipole (studies ongoing at ESRF, SLS, Soleil): Dynamic Aperture correction quite complicated;
Particle Physics Seminar
Tuesday 19 February 2008

27. Closed Orbit correction and orbit stability
The beam orbit is corrected to the BPMs zeros by means of a set of dipole corrector magnets: the BPMs can achieve submicron precision and the orbit rms is corrected to below 1 m:
Particle Physics Seminar
Tuesday 19 February 2008

28. Closed orbit disturbances
ground settling
tidal motion
day/night (thermal variations)
re-injection
thermal drifts of the electronics
insertion device gap movements
ground vibrations
air conditioning units
refrigerators, compressor (cooling systems)
power supplies
cooling water flow
high current instabilities
Courtesy C. Bocchetta
Particle Physics Seminar
Tuesday 19 February 2008

29. Stability requirements in 3rd generation light sources
Beam stability should be better than 10% of the beam size and divergence
but IR beamlines will have tighter requirements
Courtesy L. Farvacque
For Diamond nominal optics (at the centre of the short straight sections)
Particle Physics Seminar
Tuesday 19 February 2008

30. Beam vibrations induced by ground and girder vibrations
Integrated H Girder 1 PSD um
Integrated H Girder 2 PSD um
Integrated H Girder 3 PSD um
Integrated H Ground PSD um
Integrated H Beam PSD 2.41 um
Particle Physics Seminar
Tuesday 19 February 2008

31. Particle Physics Seminar
Beam stability at the source points (1-100Hz bandwidth)
At ID source point
Horizontal
Vertical
Long Straight
Standard Straight
Position (μm)
Target
17.8
12.3
1.26
0.64
No FOFB
4.24 (2.4%)
3.08 (2.5%)
0.70 (5.5%)
0.36 (5.6%)
Angle (μrad)
1.65
2.42
0.22
0.42
0.49 (2.9%)
0.61 (2.5%)
0.14 (6.2%)
0.24 (5.8%)
We are within 10% of the beam size and divergence without FOFB
Particle Physics Seminar
Tuesday 19 February 2008

32. Performance of Diamond FOFB
Significant improvement up to 100 Hz; higher frequencies performance limited mainly by the correctors power supply bandwidth
Particle Physics Seminar
Tuesday 19 February 2008

33. Particle Physics Seminar
Beam stability at the source points (1-100Hz bandwidth)
At ID source point
Horizontal
Vertical
Long Straight
Standard Straight
Position (μm)
Target
17.8
12.3
1.26
0.64
No FOFB
4.24 (2.4%)
3.08 (2.5%)
0.70 (5.5%)
0.36 (5.6%)
FOFB On
0.89 (0.5%)
0.63 (0.5%)
0.19 (1.5%)
0.11 (1.7%)
Angle (μrad)
1.65
2.42
0.22
0.42
0.49 (2.9%)
0.61 (2.5%)
0.14 (6.2%)
0.24 (5.8%)
0.10 (0.6%)
0.13 (0.5%)
0.04 (1.7%)
0.07 (1.7%)
Particle Physics Seminar
Tuesday 19 February 2008

34. Long term drifts (30 minutes) SOFB OFF
3 m maximum drift over 30 minutes
H rms < 0.7 m
V rms < 0.4 m
Particle Physics Seminar
Tuesday 19 February 2008

35. Long term drifts (30 minutes) SOFB ON
SOFB running at 0.2 Hz
H rms < 0.5 m
V rms < 0.3 m
Particle Physics Seminar
Tuesday 19 February 2008

36. Improving stability: Top-Up operation (I)
Top-Up operation consists in the continuous (very frequent) injection to keep the stored current constant
Top-Up improves stability
constant photon flux
constant thermal load on components
provides more flexibility
Lifetime less important
Smaller ID gaps
Lower coupling
Already in operation at APS and SLS
Particle Physics Seminar
Tuesday 19 February 2008

37. Improving stability: Top-Up operation (II)
Total current stable at 128.4mA to 0.1%
Hybrid bunch stable at 0.43mA to 3.2%
Pk-pk ~ 0.2mA
σ ~ 0.06mA
Particle Physics Seminar
Tuesday 19 February 2008

38. Safety case for Top-Up operation
Beam-line safe for top-up if:
1. Electrons travelling forwards from straight section cannot pass down beam-line
2. Electrons travelling backwards from beam-line cannot pass through to straight section
3. Electrons travelling in either direction do not have same trajectory at any intermediate point
Machine Interlocks have to be defined to prevent a top-up accident under faulty conditions:
BTS energy ILK and stored beam ILK are adequate for Diamond
Particle Physics Seminar
Tuesday 19 February 2008

39. AP challenges: Time structure
Diamond present layout:
Injector and timing allow a very flexible fill pattern control (single bunch – camshaft, etc)
but bunch length limited to 10 ps
Rep rate higher than 533 kHz
312 ns
312 ns
Current
Bunch length
Lifetime
0.1 mA
10 ps
92.3 h
0.5 mA
12 ps
22.7 h
0.8 mA
13 ps
15.5 h
4 mA
18 ps
4.3 h
10 mA
25 ps
2.4 h
The bunch length and energy spread, increase with current due to the "microwave instability":
Particle Physics Seminar
Tuesday 19 February 2008

40. Generation of short radiation pulses in a storage ring
There are three main approaches to generate short radiation pulses in storage rings
e– bunch
1) shorten the e- bunch
2) chirp the e-bunch + slit or optical compression
3) local energy-density modulation
Low – alpha
Higher Harmonic Cavities
RF voltage modulation
Crab Cavities
Synchro-betatron kicks
Femto – slicing
Particle Physics Seminar
Tuesday 19 February 2008

41. Bunch length (low current)
The equilibrium bunch length is due to the quantum nature of the emission of synchrotron radiation and is the result of the competition between quantum excitation and radiation damping. If high current effects are negligible the bunch length is
 = 1.710–4; V = 3.3 MV;  = 9.6 10–4
z = 2.8 mm (9.4 ps)
z depends on the magnetic lattice (quadrupole magnets) via 
We can modify the electron optics to reduce 
 (low_alpha_optics)  10–6
z  0.3 mm (1 ps)
Particle Physics Seminar
Tuesday 19 February 2008

42. Low alpha optics
When the bunch is too short Coherent Synchrotron Radiation generates further instabilities
Microbunch instability (Stupakov-Heifets)
for Diamond the Microbunching threshold is about 10 A per bunch at 1 ps rms length
Single bunch: 10 A; 1 ps; rep. rate 530 kHz
Full fill: 10 A * 936 bunches; 1 ps; rep. rate 500 MHz
Bessy-II data
Courtesy P. Kuske
Bessy-II, ALS and SPEAR3 have successfully demonstrated low-alpha operation with few ps bunches for Coherent THz radiation
Particle Physics Seminar
Tuesday 19 February 2008

43. Crab Cavities for optical pulse shortening
Courtesy M. Borland (APS)
Particle Physics Seminar
Tuesday 19 February 2008

44. A possible implementation of crab cavities at Diamond
Crab cavities are located at 1.1 m from the centre of the long straight
4 in V
420 rad V kick
Looks feasible to get sub-ps x-ray pulses with very good transmission (80%)
Emittance degradation is modest
Impedance issues have still to be addressed (machine impedance, LOM and HOM in crab cavity)
This scheme is yet unproven
Particle Physics Seminar
Tuesday 19 February 2008

45. Femto-second slicing electron-laser interaction in the modulator (a)
fs pulse lW
“dark”
pulse
electron
bunch
femtosecond
laser pulse
wiggler
femtosecond
electron bunch
fs pulse
electron-laser interaction in the modulator
(a)
spatial or angular separation in a dispersive section
(b)
fs radiation pulses from a radiator
(c)
A.A. Zholents and M.S. Zolotorev, Phys. Rev. Lett. 76 (1996) 912.
BESSY-II, ALS and SLS have successfully demonstrated the generation of X-ray pulses with few 100 fs pulse length
Particle Physics Seminar
Tuesday 19 February 2008

46. Energy modulation generated by a short laser pulse
Natural energy spread 0.1%
Particle Physics Seminar
Tuesday 19 February 2008

47. Separation of the radiation from the two modulated bunchlets
max 1.5 % energy modulation
Pulse stretching at radiator 35 fs
separation x = 1.8 mm (w.r.t. 200 um beam size rms)
separation x’ = 0.6 mrad (w.r.t. 0.3 mrad opening angle of radiation)
Radiation pulses of 35 fs can be generated;
modulator
radiator
Particle Physics Seminar
Tuesday 19 February 2008

48. Comparison of options for short radiation pulses
Crab C.
Particle Physics Seminar
Tuesday 19 February 2008

49. AP challenges: high current operation
The beam and its electromagnetic field travel inside the vacuum chamber while the image charge travels on the chamber itself.
Any variation on the chamber profile, on the chamber material, breaks this configuration.
Negative
Charged Beam
The beam loses a (usually small) part of it is energy that feeds the electromagnetic fields that remain after the passage of the beam. Such fields are referred as wake fields
Wake fields generated by beam particles, mainly affect trailing particles in previous bunches (long range wakes) or in the tail of the same bunch (short range wakes)
Particle Physics Seminar
Tuesday 19 February 2008

50. AP challenges: High current
Collective effect are usually categorised as
Multi-bunch and Single-bunch;
Transverse and Longitudinal;
Main causes in synchrotron light sources are
Resistive Wall impedance (narrow gap chambers, SS vacuum chambers)
Ion related instabilities (Ion Trapping; Fast Ion Instability)
Poor design of vacuum chamber elements (tapers, bellows, BPMs, …)
RF cavities High Order Modes (HOMs)
Main cures are
Operation with high positive chromaticity
Bunch lengthening (low voltage RF voltage, Harmonic cavities)
Feedback systems (TMBF, LMBF)
better design of vacuum chamber elements (SCRF, HOM damping, …)
Particle Physics Seminar
Tuesday 19 February 2008

51. Particle Physics Seminar
Multi-bunch modes
V instability visible at 17 mA for zero chromaticity
Onset of sidebands not too far from predicted RW threshold (40 mA)
Increasing chromaticity counteract the instability
Beam is stable up to 110 mA with chromaticity +2 in both planes
60 mA
2/3 fill
Particle Physics Seminar
Tuesday 19 February 2008

52. AP challenges: High current
Measurements at 160 mA (Chromaticity +2/+2)
Vertical betatron lines appears at about 12 MhZ
Some evidence of ion related instabilities
Fill pattern and chromaticity dependence are under investigations.
Tracking shows that chromaticity > 5 impacts injection efficiency (95% to 65%)
TMBF is required to damp these instabilities
160 mA
2/3 fill
Particle Physics Seminar
Tuesday 19 February 2008

53. Particle Physics Seminar
Current thresholds
Ion related instabilities are clearly visible in the initial stage of commissioning. They become less important with vacuum improvement due to synchrotron radiation cleaning of the vacuum chamber, but a TMBF is required.
Diamond
Soleil
Particle Physics Seminar
Tuesday 19 February 2008

54. Transverse Multibunch Feedback at Diamond
The TMBF system detects coherent betatron oscillation bunch-by-bunch and damps them with a pair of stripline kickers
Particle Physics Seminar
Tuesday 19 February 2008

55. Single Bunch Longitudinal collective effects: the microwave instability
When the current per bunch is larger than the instability threshold:
the single particles get excited by the wakes on exponentially growing longitudinal oscillations. Because of non-linearities, the oscillation frequency changes with amplitude limiting the maximum amplitude and in most of the cases no particle loss happens.
Bunch length at zero current 17 ps
(with 1.9 MV;  = 1.410–4)
Z||/n ~ 0.4 
The net effect on the bunch is an increase of the energy spread above threshold with a consequent increase of the bunch length
Particle Physics Seminar
Tuesday 19 February 2008

56. Transverse mode coupling instability
The transverse impedance of the machine can generate an instability of internal modes of oscillation of a bunch (head-tail instability  real part of the impedance)
or shift the frequency of the modes until they coalesce
(transverse mode coupling instability  imaginary part of the impedance)
It can be cured with increasing chromaticity and the voltage
It cannot be cured simply by the TMBF system
Vertical beam blow-up at diamond
Particle Physics Seminar
Tuesday 19 February 2008

57. Particle Physics Seminar
Conclusions (I)
Third generation light sources provide a very reliable source of high brightness, very stable X-rays
Medium energy machines (~ 3 GeV) performances are now covering the needs of a wide user’s community and the number of beamlines per facility is increasing;
Future developments will target
even lower emittance < 1 nm
higher stability tens of nm over few hundreds Hz
short pulses < 1 ps
higher current > 500 mA
more undulator per straights (canted undulators)
Technological progress is expected to further improve brightness and stability (IDs, BPMs, …)
Particle Physics Seminar
Tuesday 19 February 2008

58. Particle Physics Seminar
Conclusions (II)
It is generally believed that 3rd generation light sources will not be replaced by SASE-FEL (4th generation light sources) but rather they can coexist.
3rd generation will remain unrivalled in terms of stability and cost effectiveness, and will still be competitive in terms of average brightness, tunability, reliability.
4-th generation light sources will be superior in their peak brightness and time structure, providing fs and sub-fs radiation pulses.
Although it is a mature technology and one cannot expect many order of magnitude improvements in the coming years, upgrades and new ideas are continuously proposed and new light sources are under commissioning (SSRF) or under constructions (ALBA) or planned (NSLS-II, …)
Particle Physics Seminar
Tuesday 19 February 2008

59. Particle Physics Seminar
Conclusions (III)
3rd generation light source are still fashionable…
Particle Physics Seminar
Tuesday 19 February 2008