Coherent Synchrotron Radiation Studies at the Advanced Light Source and the IKNO proposal Fernando Sannibale INFN - LNF, Frascati, Italy, July 10, 2008

AFRD ( ALS Acc. Physics and CBP) : J. M. Byrd, D. S. Robin, F. Sannibale, A. Zholents, M. Zolotorev. F. Sannibale, A. Zholents, M. Zolotorev. 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,... INFN - LNF, Frascati, Italy, July 10, 2008 A Team Work CSR Studies at the ALS and IKNO. F.Sannibale ALS: Z. Hao, M. C. Martin. MSD: R. W. Schoenlein. Present IKNO Collaboration: P. Innocenzi, A. Marcelli, F. Sannibale.

Coherent Synchrotron Radiation (CSR) has been matter of great interest and study in the last years: As a powerful diagnostic for bunch compressors in free electron lasers (FEL) (FLASH, LCLS, FERMI, …); As a powerful diagnostic for bunch compressors in free electron lasers (FEL) (FLASH, LCLS, FERMI, …); 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 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,...); But also as a ‘dream’ for potential revolutionary synchrotron radiation (SR) source in the THz frequency range. But also as a ‘dream’ for potential revolutionary synchrotron radiation (SR) source in the THz frequency range. INFN - LNF, Frascati, Italy, July 10, 2008 Why Studying CSR? CSR Studies at the ALS and IKNO. F.Sannibale

Scarcity of broadband powerful source in such a region of the spectrum INFN - LNF, Frascati, Italy, July 10, 2008 The “Terahertz Gap” CSR Studies at the ALS and IKNO. F.Sannibale THz Science: collective excitations, protein motions & dynamics, superconductor gaps, magnetic resonances, terabit wireless, medical imaging, security screening, detecting explosives & bio agents … Hz 1 THz = 4.1 meV = 33 cm -1 = 300  m “DOE-NSF-NIH Workshop on Opportunities in THz Science” February 12-14,

High Stability High Stability Non interceptive radiation processes are required. Non interceptive radiation processes are required. Synchrotron and edge radiation most efficient Many users capability Many users capability Multicolor experiments capability Multicolor experiments capability Capability of "exotic" experiments (femtoslicing, stacking,...) Capability of "exotic" experiments (femtoslicing, stacking,...) INFN - LNF, Frascati, Italy, July 10, 2008 Why CSR from Storage Rings Why CSR from Storage Rings CSR Studies at the ALS and IKNO. F.Sannibale

INFN - LNF, Frascati, Italy, July 10, 2008 A Multi-Year Effort CSR Studies at the ALS and IKNO. F.Sannibale 2002: The microbunching instability (MBI): First experimental proof. (J.M.Byrd et al., PRL 89, , 2002.) : Stable CSR in storage rings: Development of a model accounting for experimental observations. (F. Sannibale et al., PRL 93, , 2004.) : CSR from “femtoslicing” experiment: First experimental data and characterization. (J.M.Byrd et al., PRL 96, , 2006.) : Laser seeding of the MBI: First experimental observation and model for the phenomenon. (J.M.Byrd et al, PRL 97, , 2006.)

Synchrotron radiation from a bend Normalized Bunch Longitudinal Distribution The CSR factor g(  ) determines the high frequency cutoff for CSR, while the vacuum chamber (shielding) defines the low frequency one. Single particle power spectrum for the radiating process under consideration (including shielding effects) CSR for: The power spectrum of the radiation from a bunch with N particles is given by: INFN - LNF, Frascati, Italy, July 10, 2008 CSR Basic CSR Studies at the ALS and IKNO. F.Sannibale

1.0 ps - Gaussian Distribution 1.5 ps - Gaussian Distribution 2.0 ps - Gaussian Distribution To extend the CSR spectrum towards higher frequencies the bunches must be shortened. Very sensitive knob! INFN - LNF, Frascati, Italy, July 10, 2008 CSR Form Factor vs. Bunch Length CSR Studies at the ALS and IKNO. F.Sannibale

To extend the CSR spectrum towards higher frequencies: the ‘saw-tooth’ distribution is the "best". INFN - LNF, Frascati, Italy, July 10, 2008 CSR Form Factor vs. Longitudinal Bunch Distribution CSR Studies at the ALS and IKNO. F.Sannibale

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. Extend the vacuum chamber cutoff towards wavelengths as long as possible. INFN - LNF, Frascati, Italy, July 10, 2008 The “Cocktail” for a Good THz Source CSR Studies at the ALS and IKNO. F.Sannibale 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...

FrontBack bunch distribution SR wake accelerates bunch front r e-e- EfEf SR Emission Cone 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. INFN - LNF, Frascati, Italy, July 10, 2008 The Synchrotron Radiation Wake CSR Studies at the ALS and IKNO. F.Sannibale 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.

J.B. Murphy, S. Krinsky, R. Gluckstern, Particle Accelerators 57, 9 (1997)  Coulomb Free Space Infinite parallel plate shield Tail Head Tail Head Typical region of interest INFN - LNF, Frascati, Italy, July 10, 2008 The Analytical Expression for the SR Wakefield CSR Studies at the ALS and IKNO. F.Sannibale

Simulated instability showing the microbunching. Venturini, WarnocK SLAC 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 << 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, , 2002) and simulated by Marco Venturini and Bob Warnock (PRL 89, , 2002) INFN - LNF, Frascati, Italy, July 10, 2008 Increasing the Current per Bunch. CSR Studies at the ALS and IKNO. F.Sannibale

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. 10 mA 29 mA 40 mA Time (msec) Bolometer signal (V) ALS Data G.L. Carr et al.:, NIMA 463, 387, (2001). A. Anderson et al.:, Opt. Eng. 39, 3099, (2000). M. Abo-Bakr et al.:, EPAC2000 Proceedings.... According to what said before, the presence of the micro-bunching instability should be associated with the emission of random "burst" of CSR. INFN - LNF, Frascati, Italy, July 10, 2008 Terahertz CSR Bursts CSR Studies at the ALS and IKNO. F.Sannibale

Model predictions Burst threshold (mA) Energy (GeV) J. Byrd, et. al. PRL 89, , (2002). wavelength The instability thresholds predicted by the Heifets-Stupakov model for the instability were in agreement with the measured thresholds Experiments at the ALS provided the experimental confirmation that the THz CSR bursts were associated with the MBI. The beam becomes unstable if the single bunch current is larger than (SI Units): The MBI dramatically limits the maximum stable single bunch current in the short bunch regime! INFN - LNF, Frascati, Italy, July 10, 2008 CSR Instability Experimental Verification CSR Studies at the ALS and IKNO. F.Sannibale

where S(s) is the Step Function Wake and  zo is the natural bunch length. The current distribution I(s) can be calculated by the Haissinski Equation: 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. Tail Head Free Space SR Wake The free space SR wake generates the saw-tooth like distributions we were looking for! (Bane, Krinsky and Murphy AIP Proc. 367, 1995) INFN - LNF, Frascati, Italy, July 10, 2008 Below the MBI Threshold CSR Studies at the ALS and IKNO. F.Sannibale

Tail Head Free Space SR Wake Tail Head Shielded SR Wake The vacuum chamber shielding reduces the CSR emitted power In a CSR optimized source the shielding effects must be minimized: INFN - LNF, Frascati, Italy, July 10, 2008 Vacuum Chamber Shielding CSR Studies at the ALS and IKNO. F.Sannibale J.B. Murphy, S. Krinsky, R. Gluckstern, Particle Accel. 57, 9 (1997)

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. INFN - LNF, Frascati, Italy, July 10, 2008 Other Wakefields CSR Studies at the ALS and IKNO. F.Sannibale 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.

In 2002, The BESSY-II group provided the first demonstration of stable CSR in a storage ring. In 2002, The BESSY-II group provided the first demonstration of stable CSR in a storage ring. Abo-Bakr et al., PRL 88, (2002), and M. Abo-Bakr et al., Phys. Rev. Lett. 90, (2003) INFN - LNF, Frascati, Italy, July 10, 2008 The BESSY II Results CSR Studies at the ALS and IKNO. F.Sannibale 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… We started a collaboration with them…

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. (F. Sannibale et al., PRL 93, , 2004.) (F.Sannibale et al., ICFA BD-Newsletter 35, 2004) INFN - LNF, Frascati, Italy, July 10, 2008 Understanding Understanding the BESSY Results CSR Studies at the ALS and IKNO. F.Sannibale Such a model has been used for calculating the CSR performance of a number of existing storage rings (DA  NE, Bates, SPEAR, …) and also for designing storage rings completely optimized for the generation of stable CSR in the THz frequency range (CIRCE, IKNO). G = C = [SI units] g( )

In operation at the ALS since 1999, and in the last few years also at BESSY II, SLS and UVSOR, … A. A. Zholents, M. S. Zolotorev, Phys. Rev. Lett. 76, 912, (1996) A. A. Zholents, M. S. Zolotorev, Phys. Rev. Lett. 76, 912, (1996) Beamline optimized for the generation of femtosecond x-ray pulses INFN - LNF, Frascati, Italy, July 10, 2008 The `Femtoslicing’ Experiment The `Femtoslicing’ Experiment CSR Studies at the ALS and IKNO. F.Sannibale

Right before modulation Right after modulation 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 INFN - LNF, Frascati, Italy, July 10, 2008 Energy Modulation CSR Studies at the ALS and IKNO. F.Sannibale

BL 5.3 BL 1.4 Such density modulation radiate intense CSR in the THz frequency range 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. INFN - LNF, Frascati, Italy, July 10, 2008 From Energy Modulation to CSR Emission CSR Studies at the ALS and IKNO. F.Sannibale

Our group at the ALS did the first experimental observation and characterization of the CSR from the femtoslicing experiment: INFN - LNF, Frascati, Italy, July 10, 2008 THz CSR From Femtoslicing THz CSR From Femtoslicing CSR Studies at the ALS and IKNO. F.Sannibale 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  J) for pump and probe experiments. (J.M.Byrd et al., PRL 96, , 2006.) K. Holldack et al., PRL 96, (2006) and K. Holldack et al., PRST-AB 8, (2005). Similar results have been obtained also at BESSY II:

Wavefront for photons with wavenumber of 50 cm -1 at the window position Window Position Qualitative agreement. Actual geometry difficult to simulate. INFN - LNF, Frascati, Italy, July 10, 2008 The ‘Waveguide’ Effect CSR Studies at the ALS and IKNO. F.Sannibale

Physics Retreat, Sept. 22, 04 Laser Modulation: 6 times the energy spread Laser Modulation: 6 times the energy spread Laser pulse width: 50 fs FWHM Laser pulse width: 50 fs FWHM Distance modulator-radiator: 2.5 m Distance modulator-radiator: 2.5 m Current per bunch: 10 mA Current per bunch: 10 mA Horizontal Acceptance 100 mrad (single mode) Horizontal Acceptance 100 mrad (single mode) [ps] Longitudinal Distribution Modulation at Radiator Energy per pulse: 8.5  J Energy per pulse: 8.5  J Electric field ~ 1 MV/cm Electric field ~ 1 MV/cm Rep. rate: kHz Rep. rate: kHz Pulse shaping capability Pulse shaping capability INFN - LNF, Frascati, Italy, July 10, 2008 Optimized CSR by Slicing (CIRCE) CSR Studies at the ALS and IKNO. F.Sannibale

Physics Retreat, Sept. 22, 04 In the described femtoslicing experiment, several mutually synchronous photon beams with very different wavelengths are simultaneously available: THz CSR synchrotron radiation pulse with transform limited length 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. x-ray pulses with ~ 100 fs length x-ray pulses with ~ 100 fs length Near-IR or visible ~ 100 fs pulse from the slicing laser Near-IR or visible ~ 100 fs pulse from the slicing laser INFN - LNF, Frascati, Italy, July 10, 2008 "Multicolor" Experiments Capability CSR Studies at the ALS and IKNO. F.Sannibale

Charge distribution at the “source” point Laser Intensity Spectrum at the “source” point 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. 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. (J.M.Byrd et al., PRL 96, , 2006.) 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. In principle by this technique, arbitrary spectrum shapes can be obtained INFN - LNF, Frascati, Italy, July 10, 2008 Tailoring the Shape of the THz Pulse CSR Studies at the ALS and IKNO. F.Sannibale

A. Mochihashi et al., UVSOR Workshop on THz CSR (September 2007) INFN - LNF, Frascati, Italy, July 10, 2008 Tailoring the THz Pulse is Really Possible! CSR Studies at the ALS and IKNO. F.Sannibale

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. INFN - LNF, Frascati, Italy, July 10, 2008 Laser Seeding of the MBI CSR Studies at the ALS and IKNO. F.Sannibale The MBI CSR bursts become synchronous with the slicing laser and the radiated THz power increases exponentially with the current per bunch.

In the framework of the Heifets-Stupakov MBI theory the micro-bunching can be represented by the combination of “modes” with shape: INFN - LNF, Frascati, Italy, July 10, 2008 A Model for the Seeded MBI CSR Studies at the ALS and IKNO. F.Sannibale In the “cold beam” approximation, the authors derived an analytical expression for the dispersion function between  and k=2  / . From that, the growth rate and the velocity for the mode can be calculated: I ITIT I ITIT I ITIT I ITIT I ITIT I ITIT I ITIT 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:

The comparison of this simple model predictions with the experimental data showed a good agreement up to a certain current (~19 mA). INFN - LNF, Frascati, Italy, July 10, 2008 Comparing with Data CSR Studies at the ALS and IKNO. F.Sannibale 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. J.M.Byrd et al, PRL 97, , 2006 Heifets and Stupakov studied the case by considering the combination of all unstable modes (SLAC-PUB-11815, 2006).

INFN - LNF, Frascati, Italy, July 10, 2008 Possible Applications CSR Studies at the ALS and IKNO. F.Sannibale 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. 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.

INFN - LNF, Frascati, Italy, July 10, 2008 The IKNO Infrastructure Proposal CSR Studies at the ALS and IKNO. F.Sannibale 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 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 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. 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.

Physics Retreat, Sept. 22, 04 Fernando Sannibale More CIRCE STORY INFN - LNF, Frascati, Italy, July 10, 2008 The IKNO Storage Ring CSR Studies at the ALS and IKNO. F.Sannibale 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.

INFN - LNF, Frascati, Italy, July 10, 2008 IKNO: The Optimized CSR Source CSR Studies at the ALS and IKNO. F.Sannibale 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

Physics Retreat, Sept. 22, 04 Fernando Sannibale More CIRCE STORY INFN - LNF, Frascati, Italy, July 10, 2008 The IKNO As a Source of Incoherent UV SR CSR Studies at the ALS and IKNO. F.Sannibale 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.