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
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
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
storage ring synchrotron radiation sources (I)
Particle Physics Seminar
Tuesday 19 February 2008
storage ring synchrotron radiation sources (II)
Particle Physics Seminar
Tuesday 19 February 2008
Courtesy Z. Zhao
storage ring synchrotron radiation sources (III)
Particle Physics Seminar
Tuesday 19 February 2008
Courtesy Z. Zhao
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
diamond
1.E+20
1.E+18
1.E+16
2
2
Brightness (Photons/sec/mm /mrad /0.1%)
Brightness
1.E+14
1.E+12
X-rays from Diamond
will be 1012 times
brighter than from
an X-ray tube,
105 times brighter
than the SRS !
1.E+10
1.E+08
1.E+06
X-ray
tube
60W bulb
Candle
1.E+04
1.E+02
Particle Physics Seminar
Tuesday 19 February 2008
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
Photograph 51
Franklin-Gosling
Nowadays pump probe experiment in life science are
performed using 100 ps pulses from storage ring light
sources: e.g. ESRF myoglobin in action
DNA (form B)
1952
Particle Physics Seminar
Tuesday 19 February 2008
Brightness with IDs
Particle Physics Seminar
Tuesday 19 February 2008
2/3 filling pattern
1.2
1
Time structure
a.u.
Time resolved science requires operating modes
with single bunch or hybrid fills to exploit the short
radiation pulses of a single isolated bunch
0.8
0.6
312
ns
0.4
312
ns
0.2
0
0
rms bunch length (ps )
200
400
bunch number
45
40
35
25
20
15
10
5
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rms (ps)
30
Particle Physics Seminar
Tuesday 19 February 2008
600
800
Accelerator Physics challenges
Small Emittance
Insertion Devices (low gaps)
Brightness, Flux
Stability
Time structure
High Current; Control Impedance; Feedbacks
Control Vibrations; Orbit Feedbacks; Top-Up
Short Pulses; Short Bunches
Particle Physics Seminar
Tuesday 19 February 2008
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
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
Small emittance lattices
The horizontal emittance is determined by the dispersion generated by the main
bending magnets.
2
x 
 H  dipole
Jx
H   x2 Dx2  2 x Dx  x D' x   x2 D'2x
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
 x ,min
0
2
1
   3
Jx
Nb
3
b
Particle Physics Seminar
Tuesday 19 February 2008
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
 x 
x
   R model  
 
measured  y
 y
 
V
Using the Singular Value
Decomposition (SVD) of the Response
Matrix R we can invert R and correct
the closed orbit distortion
Particle Physics Seminar
Tuesday 19 February 2008
V
H
H
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
 2 (Q , GBPMs , BPMs ,...)   Rijmeasured  Rijmodel (Q , GBPMs , BPMs ,...) 
2
i, j
Particle Physics Seminar
Tuesday 19 February 2008
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
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
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
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
required
Particle are
Physics
Seminarto correct nonlinear aberrations
Tuesday 19 February 2008
[Consider the effect of realistic errors (and define magnetic error tolerances)]
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
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
Nonlinear beam dynamics optimisation (II)
The Dynamic Aperture problem
Vacuum chamber
Frequency Map Analysis
allows the identification of dangerous non
linear resonances during design and
operation
ALS measured
Strongly excited resonances can
destroy the Dynamic Aperture
Particle Physics Seminar
Tuesday 19 February 2008
ALS model
Touschek lifetime
Electrons performing betatron oscillations may scatter and be lost outside the
momentum aperture available from RF voltage and the 6D dynamic aperture
2
1
 Touschek
re cN b 1

8 3 s C ring
C  
Cring

0
 x ( s) y ( s) x ( s) acc ( s)
2
ds
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
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
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
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
Particle Physics Seminar
Tuesday 19 February 2008
Courtesy C. Bocchetta
Stability requirements in 3rd generation light
sources
Beam stability should be better than
10% of the beam size and divergence
x  0.1  x
y  0.1   y
x'  0.1  x '
y '  0.1   y '
but IR beamlines will have tighter
requirements
Courtesy L. Farvacque
For Diamond nominal optics (at the centre of the short straight sections)
x  0.1123 m  12.3m
x'  0.1 24 rad  2.4 rad
y  0.1 6.4 m  0.6m
y '  0.1 4 rad  0.4 rad
Particle Physics Seminar
Tuesday 19 February 2008
Beam vibrations induced by
ground and girder vibrations
Integrated H Girder 1 PSD 0.090 um
Integrated H Ground PSD 0.018 um Integrated H Girder 2 PSD 0.088 um
Integrated H Girder 3 PSD 0.072 um
Particle Physics Seminar
Tuesday 19 February 2008
Integrated H Beam PSD 2.41 um
Beam stability at the source points (1-100Hz bandwidth)
Horizontal
At ID source point
Position
(μm)
Angle
(μrad)
Vertical
Long Straight
Standard
Straight
Long Straight
Standard
Straight
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%)
Target
1.65
2.42
0.22
0.42
No
FOFB
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
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
Beam stability at the source points (1-100Hz bandwidth)
Horizontal
At ID source point
Position
(μm)
Angle
(μrad)
Vertical
Long Straight
Standard
Straight
Long Straight
Standard
Straight
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%)
Target
1.65
2.42
0.22
0.42
No
FOFB
0.49 (2.9%)
0.61 (2.5%)
0.14 (6.2%)
0.24 (5.8%)
FOFB
On
0.10 (0.6%)
0.13 (0.5%)
0.04 (1.7%)
0.07 (1.7%)
Particle Physics Seminar
Tuesday 19 February 2008
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
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
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
Particle Physics Seminar
Tuesday
February 2008
at APS
and 19
SLS
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
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
Particle Physics Seminar
adequate for Diamond
Tuesday 19 February 2008
AP challenges: Time structure
Diamond present layout:
2/3 filling pattern
1.2
1
0.8
a.u.
Injector and timing allow a very flexible fill
pattern control (single bunch – camshaft, etc)
0.6
0.4
312 ns 312 ns
0.2
but bunch length limited to 10 ps
0
0
200
400
600
800
bunch number
 z /  z0 and   /   0
Rep rate higher than 533 kHz
Current
Bunch length
Lifetime
3
0.1 mA
10 ps
92.3 h
2.5
0.5 mA
12 ps
22.7 h
2
0.8 mA
13 ps
15.5 h
4 mA
18 ps
4.3 h
10 mA
25 ps
2.4 h
1.5
1
0.5
0
0
2
4
6
8
10
I (mA)
Particle Physics Seminar
Tuesday 19 February 2008
The bunch length and energy
spread, increase with current due to
the "microwave instability":
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
Crab Cavities
Femto – slicing
Higher Harmonic Cavities
Synchro-betatron kicks
RF voltage modulation
Particle Physics Seminar
Tuesday 19 February 2008
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
c
3
z 
 
2 f s
d VRF / dz
 = 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
Particle Physics Seminar
Tuesday 19 February 2008

1 Dx
6
ds

10
L 
z  0.3 mm (1 ps)
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
Crab Cavities for optical pulse shortening
Courtesy M. Borland (APS)
Particle Physics Seminar
Tuesday 19 February 2008
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
Femto-second slicing
fs pulse
“dark”
pulse
electron
bunch
femtosecond
laser pulse
femtosecond
electron bunch
electron-laser interaction
in the modulator
(a)
spatial or angular separation
in a dispersive section
(b)
fs pulse
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 Xray pulses with few 100 fs pulse length
Particle Physics Seminar
Tuesday 19 February 2008
Energy modulation generated by a short laser pulse
Natural energy spread 0.1%
Particle Physics Seminar
Tuesday 19 February 2008
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
Comparison of options for short radiation pulses
Crab C.
Particle Physics Seminar
Tuesday 19 February 2008
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
Particle Physics Seminar
bunches (long range wakes)
the tail2008
of the same bunch (short range wakes)
Tuesdayor19inFebruary
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)
Particle
Physics
Seminar
Tuesday 19 February 2008
better design of vacuum chamber elements (SCRF, HOM damping, …)
Multi-bunch modes
V instability visible at 17 mA for
zero chromaticity
60 mA
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
Particle Physics Seminar
Tuesday 19 February 2008
2/3 fill
AP challenges: High current
Measurements at 160 mA
(Chromaticity +2/+2)
160 mA
2/3 fill
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%)
Particle Physics Seminar
Tuesday 19 February 2008
TMBF is required to damp
these instabilities
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
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
Single Bunch Longitudinal collective effects:
the microwave instability
When the current per bunch is larger than
the instability threshold:
2C E0  E E0 

e Z // n
2
I peak
the single particles get excited by the
wakes on exponentially growing
longitudinal oscillations. Because of nonlinearities, the oscillation frequency
changes with amplitude limiting the
maximum amplitude and in most of the
cases no particle loss happens.
The net effect on the bunch is an
increase of the energy spread above
threshold with a consequent increase of
Particle Physics Seminar
the bunch length
Tuesday 19 February 2008
Bunch length at zero current 17 ps
(with 1.9 MV;  = 1.410–4)
Z||/n ~ 0.4 
Transverse mode coupling instability
2
1.5
Re (  - w s)/w s
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)
1
0.5
0
-0.5
-1
-1.5
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
Particle Physics Seminar
Tuesday 19 February 2008
-2
0
0.5
1
1.5
current (mA)
Vertical beam blow-up at diamond
2
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
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
Conclusions (III)
3rd generation light source are still fashionable…
Particle Physics Seminar
Tuesday 19 February 2008