Coherent Synchrotron Radiation Studies at the Advanced Light Source

1
Coherent Synchrotron Radiation Studies at the
Advanced Light Source and the IKNO proposal
Fernando Sannibale
2
AFRD (ALS Acc. Physics and CBP) J. M. Byrd, D.
S. Robin, F. Sannibale,
A. Zholents, M. Zolotorev.
ALS Z. Hao, M. C. Martin.
MSD R. W. Schoenlein.
Collaborations BESSY, DLR (Berlin), SLAC
Important contributions M. Abo-Bakr, J. Feikes,
E. Forest, S. Heifets, K. Holldack, H. W. Hubers,
P. Kuske, W. Leemans, A. Loftsdottir, B.
Marcelis, J. Murphy, T. Scarvie, C. Steier, G.
Stupakov, M. Venturini, R. Warnock, G. Wustefeld,
...
Present IKNO Collaboration P. Innocenzi, A.
Marcelli, F. Sannibale.
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Coherent Synchrotron Radiation (CSR) has been
matter of great interest and study in the last
years

• As something to carefully avoid or at least
  control in every short bunch high charge
  accelerator where CSR can jeopardize the
  performances (linear colliders, short pulses
  synchrotron radiation sources, damping rings,
  ...)

• As a powerful diagnostic for bunch compressors
  in free electron lasers (FEL) (FLASH, LCLS,
  FERMI, )

• But also as a dream for potential
  revolutionary synchrotron radiation (SR) source
  in the THz frequency range.

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Scarcity of broadband powerful source in such a
region of the spectrum
1 THz 4.1 meV 33 cm-1 300 mm
THz Science collective excitations, protein
motions dynamics, superconductor gaps,
magnetic resonances, terabit wireless, medical
imaging, security screening, detecting
explosives bio agents
DOE-NSF-NIH Workshop on Opportunities in THz
Science February 12-14, 2004 http//www.science.d
oe.gov/bes/reports/abstracts.htmlTHz
5

• High Stability

• Many users capability

• Multicolor experiments capability

• Capability of "exotic" experiments
  (femtoslicing, stacking, ...)

• Non interceptive radiation processes are
  required.
• Synchrotron and edge radiation most efficient

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2002 The microbunching instability (MBI) First
experimental proof. (J.M.Byrd et al., PRL 89,
224801, 2002.)
2003-2004 Stable CSR in storage rings
Development of a model accounting for
experimental observations. (F. Sannibale et al.,
PRL 93, 094801, 2004.)
2004-2005 CSR from femtoslicing experiment
First experimental data and characterization.
(J.M.Byrd et al., PRL 96, 164801, 2006.)
2004-2006 Laser seeding of the MBI First
experimental observation and model for the
phenomenon. (J.M.Byrd et al, PRL 97, 074802,
2006.)
7
The power spectrum of the radiation from a bunch
with N particles is given by
The CSR factor g(w) determines the high frequency
cutoff for CSR, while the vacuum chamber
(shielding) defines the low frequency one.
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To extend the CSR spectrum towards higher
frequencies the bunches must be shortened.
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To extend the CSR spectrum towards higher
frequencies the saw-tooth distribution is the
"best".
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Extend the vacuum chamber cutoff towards
wavelengths as long as possible.
Shorten the bunches as much as possible.
Find a mechanism for generating sharp edged
distributions at equilibrium (saw-tooth like
possibly).
And of course, put as much particles as possible
in the bunch.
Unfortunately, in this description a major player
is missing. We will see that for short bunches (
ps) the situation becomes a little bit more
complicate...
11
Because of the curved trajectory of the beam, the
photons radiated from particles in the tail of
the bunch catch up with the particles in the head.
The curved trajectory also allows for the
electric field of these photons to have a
component parallel to the direction of the motion
of the particles in the head, and therefore to
change their energy.
12
J.B. Murphy, S. Krinsky, R. Gluckstern , Particle
Accelerators 57, 9 (1997)
Typical region of interest
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The bunch distribution in a real ring is never
completely smooth and shows a modulated profile
that changes randomly with time (noise).
These micro-structures have characteristic length
ltlt than the bunch length and radiate CSR. The CSR
wakefield modulates the energy of the neighbor
particles that start to move inside the bunch due
to the accelerator longitudinal dispersion. Some
particles move in the direction that increases
the radiating micro-structure, and thus
increasing the CSR intensity and creating a gain
mechanism.
Above a current threshold, this gain becomes
large enough to sustain the micro-bunching
process against the decoherence effect due to the
energy spread, and to generate an exponential
growth of the micro-structure amplitude (up to
saturation in the non linear regime of the
instability).
Such an instability, often referred as the
micro-bunching instability (MBI), is nothing else
that a SASE process in the THz regime.
The MBI, for the case of storage rings, was
predicted by Sam Heifets and Gennady Stupakov
(PRST-AB 5, 054402, 2002) and simulated by Marco
Venturini and Bob Warnock (PRL 89, 224802, 2002)
14
According to what said before, the presence of
the micro-bunching instability should be
associated with the emission of random "burst" of
CSR.
In many electron storage rings around the world,
strong random pulses (bursts) of CSR in the THz
frequency range were observed for high single
bunch current.
15
Experiments at the ALS provided the experimental
confirmation that the THz CSR bursts were
associated with the MBI.
The instability thresholds predicted by the
Heifets-Stupakov model for the instability were
in agreement with the measured thresholds
The beam becomes unstable if the single bunch
current is larger than (SI Units)
16
Below the MBI threshold, the strongly nonlinear
SR wake generates a distortion of the parabolic
potential well due to the RF cavity, and the
bunch assumes non-Gaussian equilibrium
distributions.
The current distribution I(s) can be calculated
by the Haissinski Equation
where S(s) is the Step Function Wake and szo is
the natural bunch length.
The free space SR wake generates the saw-tooth
like distributions we were looking for! (Bane,
Krinsky and Murphy AIP Proc. 367, 1995)
17
The vacuum chamber shielding reduces the CSR
emitted power
In a CSR optimized source the shielding effects
must be minimized
J.B. Murphy, S. Krinsky, R. Gluckstern , Particle
Accel. 57, 9 (1997)
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Other wakes can be added to the CSR one in the
calculation of the equilibrium distribution by
the Haissinski equation. From the distribution
the CSR factor and spectrum are then readily
evaluated.
Long range resistive wall if not controlled it
can reduce the CSR intensity. In general larger
gap (once more) and high conductivity chambers
are preferred.
The wake fields due to the vacuum chamber of an
accelerator can be modeled by using the a generic
impedance model
In the short bunch regime ( 1 ps), one finds
that the effect of these wakes is usually
negligible.
19
In 2002, The BESSY-II group provided the first
demonstration of stable CSR in a storage
ring. Abo-Bakr et al., PRL 88, 254801 (2002),
and M. Abo-Bakr et al., Phys. Rev. Lett. 90,
094801 (2003)

• Very interesting characteristics of the BESSY
  results were
• a very stable CSR flux (no presence of bursts),
• an impressive power radiated in the THz region,
• and a spectrum significantly broader than the
  one expected for a Gaussian distribution their
  bunch length.

We started a collaboration with them
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(F. Sannibale et al., PRL 93, 094801,
2004.) (F.Sannibale et al., ICFA BD-Newsletter
35, 2004)
The understanding of the physics behind the BESSY
results showed the dominant role played in the
short bunch regime by the SR wake and allowed to
develop a model for optimizing a storage ring as
a stable source of THz CSR.
Such a model has been used for calculating the
CSR performance of a number of existing storage
rings (DAFNE, Bates, SPEAR, ) and also for
designing storage rings completely optimized for
the generation of stable CSR in the THz frequency
range (CIRCE, IKNO).
21
Beamline optimized for the generation of
femtosecond x-ray pulses

• A. A. Zholents, M. S. Zolotorev, Phys. Rev.
  Lett. 76, 912, (1996)

In operation at the ALS since 1999, and in the
last few years also at BESSY II, SLS and UVSOR,
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For a few GeV electron beam, laser pulses with
several mJ per pulse are required. This limits
the max rep-rate to the order of few KHz
23
Because of the ring longitudinal dispersion, when
the beam propagates the energy modulation induces
a density modulation.
The characteristic length of these modulations is
the laser pulse length (100 fs) after the
energy modulation, of the order of the ps after
a turn, and it is completely absorbed by the
bunch in the next few turns.
Such density modulation radiate intense CSR in
the THz frequency range
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Our group at the ALS did the first experimental
observation and characterization of the CSR from
the femtoslicing experiment
(J.M.Byrd et al., PRL 96, 164801, 2006.)
The CSR signal is now one of the main diagnostics
for the tune-up for the slicing experiment.
The femtoslicing scheme, if optimized, could be
used as a source of high energy per pulse THz
radiation ( 10 mJ) for pump and probe
experiments.
Similar results have been obtained also at BESSY
II
K. Holldack et al., PRL 96, 054801 (2006) and K.
Holldack et al., PRST-AB 8, 040704 (2005).
25
Qualitative agreement. Actual geometry difficult
to simulate.
Wavefront for photons with wavenumber of 50 cm-1
at the window position
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• Laser Modulation 6 times the energy
  spread
• Laser pulse width 50 fs FWHM
• Distance modulator-radiator 2.5 m
• Current per bunch 10 mA
• Horizontal Acceptance 100 mrad
  (single mode)

• Energy per pulse 8.5 mJ
• Electric field 1 MV/cm
• Rep. rate 10 - 100 kHz
• Pulse shaping capability

Physics Retreat, Sept. 22, 04
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• In the described femtoslicing experiment, several
  mutually synchronous photon beams with very
  different wavelengths are simultaneously
  available

• x-ray pulses with 100 fs length

• Near-IR or visible 100 fs pulse from the
  slicing laser

• THz CSR synchrotron radiation pulse with
  transform limited length

This opens the possibility for many interesting
combinations of "pump and probe" experiments
where one of the beams is used for exciting the
sample while another is used for measuring its
characteristics during the excitation
transient. By varying the delay between the
pulses, one can reconstruct the whole sample
response with resolution of the order of 100 fs.
Physics Retreat, Sept. 22, 04
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One additional interesting possibility of this
scheme is the ability of tailoring the electric
field of a terahertz pulse by an appropriate
shaping of the slicing laser pulse.
The example shows how by using a train of laser
pulses instead of one single pulse one can
concentrate the CSR power within a narrow
bandwidth. The number of pulses defines the
bandwidth while the distance between pulses
defines the central frequency of the peak.
In principle by this technique, arbitrary
spectrum shapes can be obtained
An example of application that could benefit from
this capability is the control of complex
chemical reactions where the shape of the
exciting radiation is dynamically adjusted for
optimizing the reaction.
(J.M.Byrd et al., PRL 96, 164801, 2006.)
29
A. Mochihashi et al., UVSOR Workshop on THz CSR
(September 2007)
30
During the experiments for characterizing the CSR
from femtoslicing, we discovered that if the beam
is sliced above the MBI threshold the instability
can be seeded.
The MBI CSR bursts become synchronous with the
slicing laser and the radiated THz power
increases exponentially with the current per
bunch.
31
In the framework of the Heifets-Stupakov MBI
theory the micro-bunching can be represented by
the combination of modes with shape
In the cold beam approximation, the authors
derived an analytical expression for the
dispersion function between w and k2p/l . From
that, the growth rate and the velocity for the
mode can be calculated
If one assumes, that the dynamic of the
micro-bunch is dominated by the mode with larger
growth rate then the for the CSR power radiated
by the micro-bunch will be
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J.M.Byrd et al, PRL 97, 074802, 2006
The comparison of this simple model predictions
with the experimental data showed a good
agreement up to a certain current (19 mA).
Above this value, the experimental points show a
saturation effect that we think is due to the
fact that at high currents, the MBI goes in the
nonlinear regime before the perturbation arrives
at the threshold point.
Heifets and Stupakov studied the case by
considering the combination of all unstable modes
(SLAC-PUB-11815, 2006).
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The graph shows how the 1 kHz power (synchronous
with the laser) loses the quadratic dependence
for currents above the MBI threshold at 13 mA.
At saturation, the average power of the seeded
CSR burst is about two orders of magnitude larger
than for the conventional slicing case, but
shows very large power fluctuations. Pump and
probe and other experiments not requiring shot to
shot intensity stability could benefit from this
several orders of magnitude increase in power.
In a speculative scenario, part of the THz signal
can be brought back into the ring to co-propagate
the bending magnet with a subsequent electron
bunch, modulating its energy and seeding the MBI
that generates a new burst that is then used in
the loop for seeding a new fresh bunch. By this
process, that continues involving all the
bunches, one can in principle bring the CSR
emission to a stable high power saturation regime
where all the bunches radiate coherently. Other
FEL-like schemes exploiting the MBI gain are
possible as well.
34
IKNO (Innovation and KNOwledge) is also the
ancient name of Sardinia and in our proposal to
the Italian Roadmap of the Research
Infrastructure, is the name of the Italian
infrastructure, proposed in Sardinia to host this
new high performance electron storage ring.

• IKNO is a multi-user facility completely
  optimized for the THz CSR production and exploits
  all the described techniques for generating
  extremely powerful radiation on a very broad
  spectrum.
• IKNO also produces an incoherent photon flux
  extending up to the VUV frequencies comparable
  with the one of existing 3rd generation light
  sources

Present IKNO Collaboration P. Innocenzi, A.
Marcelli, F. Sannibale.
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More CIRCE STORY
The IKNO storage ring is a more compact version
of CIRCE (the LBNL proposal).
It includes six periods (DBA cells) with six 3.1
m straight sections. Sextupole and octupole
magnets allow full control of the ring nonlinear
dynamics.
Physics Retreat, Sept. 22, 04
Fernando Sannibale
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In the ultra-stable mode, IKNO generates a photon
flux of CSR many order of magnitude higher than
in existing 3rd generation light sources.
The 3.1 m straight sections, allow for CSR for
femtoslicing systems, extending the spectrum to
few tens of THz, allowing for pulse shaping and
opening to multicolor pump and probe experiments
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More CIRCE STORY
The bending magnet ports and the 3.1 m straight
sections, can also be used for generating a
powerful flux of incoherent synchrotron radiation
in the UV-VUV frequency range.
Physics Retreat, Sept. 22, 04
Fernando Sannibale