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Relativistic electron bunches used in synchrotron light sources are complex media, in which patterns might form spontaneously. These spatial structures were studied over the past decades for very practical reasons. The patterns, which spontaneously appear during an instability, increase the terahertz radiation power by factors exceeding 10,0001,2. However, their irregularity1–7 largely prevented applications of this powerful source. Here we show that principles from chaos control theory8–10 allow us to generate regular spatio-temporal patterns, stabilizing the emitted terahertz power. Regular unstable solutions are expected to coexist with the undesired irregular solutions, and may thus be controllable using feedback control. We demonstrate the stabilization of such regular solutions in the Synchrotron SOLEIL storage ring. Operation of these controlled unstable solutions enables new designs of high-charge and stable synchrotron radiation sources. A feedback loop based on chaos control theory permits the generation of stable and coherent terahertz radiation from relativistic electron bunches in synchrotron light sources.

Undulator based synchrotron light sources and Free Electron Lasers (FELs) are valuable modern probes of matter with high temporal and spatial resolution. Laser Plasma Accelerators (LPAs), delivering GeV electron beams in few centimeters, are good candidates for future compact light sources. However the barriers set by the large energy spread, divergence and shot-to-shot fluctuations require a specific transport line, to shape the electron beam phase space for achieving ultrashort undulator synchrotron radiation suitable for users and even for achieving FEL amplification. Proof-of-principle LPA based undulator emission, with strong electron focusing or transport, does not yet exhibit the full specific radiation properties. We report on the generation of undulator radiation with an LPA beam based manipulation in a dedicated transport line with versatile properties. After evidencing the specific spatio-spectral signature, we tune the resonant wavelength within 200–300 nm by modification of the electron beam energy and the undulator field. We achieve a wavelength stability of 2.6%. We demonstrate that we can control the spatio-spectral purity and spectral brightness by reducing the energy range inside the chicane. We have also observed the second harmonic emission of the undulator.

We present recent results on the characterization and control of the coherent terahertz pulses emitted by synchrotron radiation facilities. We first show novel ultrafast single-shot measurements techniques now allow these sources to be fully characterized in a pulse-by-pulse basis [1]. Then we present a novel feedback method that enables the dynamics of these sources to be fully controlled [2]. This opens the way to novel sources of stable THz radiation, and to high repetition-rate spectroscopic applications.

International audience; Abstract:Optical probes are among the most important techniques for characterizing new physical states induced by changes of temperature and/or pressure. In link with its high brightness and wide spectral coverage, infrared synchrotron radiation is ideally suited for such studies when the quantity of material to investigate is limited or when the system needs to be probed down to the sub mm domain.

25th International Conference on the Application of Accelerators in Research and Industry, CAARI 2018, Texas, USA, 12 Aug 2018 - 17 Aug 2018; AIP conference proceedings 2160(1), 040012-1 - 040012-9 (2019). doi:https://doi.org/10.1063/1.5127692, Plasma accelerators present one of the most suitable candidates for the development of more compact particle acceleration technologies, yet they still lag behind radiofrequency (RF)-based devices when it comes to beam quality, control, stability and power efficiency. The Horizon 2020-funded project EuPRAXIA (“European Plasma Research Accelerator with eXcellence In Applications”) aims to overcome the first three of these hurdles by developing a conceptual design for a first international user facility based on plasma acceleration. In this paper we report on the main features, simulation studies and potential applications of this future research infrastructure., Published by AIP Publishing, Melville, NY

Since the laser invention [1, 2], the advent of Free Electron Lasers (FEL) [3, 4] in the X-ray domain half a century later, has opened new areas for matter investigation with higher temporal resolution. In parallel, the development of laser plasma acceleration (LPA) [5] that delivers today several GeV beam acceleration [6] in a short distance appears also very promising. The qualification of the LPA for FEL application can thus be viewed as an important challenge [7]. Nevertheless, present LPA electron beam parameters (e. g. energy spread and beam divergence) do not meet conventional accelerator state-of-the-art performance and FEL application requires a specific beam handling. In such a prospect, the COXINEL beam manipulation line [8, 9] using variable permanent magnet quadrupoles for emittance growth mitigation and de-mixing chicane for the energy spread handling developed at SOLEIL has been installed for using electrons produced with an intense laser of Laboratoire d’Optique Appliquee (LOA). Strategies for controlling electron beam position and dispersion have been elaborated and demonstrated [10]. Finally, undulator spontaneous emission has been measured at the end of the line.

With gigaelectron-volts per centimetre energy gains and femtosecond electron beams, laser wakefield acceleration (LWFA) is a promising candidate for applications, such as ultrafast electron diffraction, multistaged colliders and radiation sources (betatron, compton, undulator, free electron laser). However, for some of these applications, the beam performance, for example, energy spread, divergence and shot-to-shot fluctuations, need a drastic improvement. Here, we show that, using a dedicated transport line, we can mitigate these initial weaknesses. We demonstrate that we can manipulate the beam longitudinal and transverse phase-space of the presently available LWFA beams. Indeed, we separately correct orbit mis-steerings and minimise dispersion thanks to specially designed variable strength quadrupoles, and select the useful energy range passing through a slit in a magnetic chicane. Therefore, this matched electron beam leads to the successful observation of undulator synchrotron radiation after an 8 m transport path. These results pave the way to applications demanding in terms of beam quality., Electron beam quality in accelerators is crucial for light source application. Here the authors demonstrate beam conditioning of laser plasma electrons thanks to a specific transport line enabling the control of divergence, energy, steering and dispersion and the application to observe undulator radiation.

More than 50 years after the laser discovery, X-ray free electron lasers (FEL), the first powerful tuneable, short pulse lasers in the X-ray spectral range, are now blooming in the world, enabling new discoveries on the ultra-fast dynamics of excited systems and imaging. LUNEX5 demonstrator project aims at investigating paths towards advanced and compact FELs. Two strategies are adopted. The first one concerns the FEL line where seeding and echo harmonic generation are implemented together with compact cryogenic in-vacuum undulators. In the second one, the electron beam is no longer provided by a conventional linear accelerator but by a laser plasma process, while a necessary particular electron beam manipulation is required to handle the electron properties to enable FEL amplification.

The behavior of submicrometer-sized thin-film YBa 2 Cu 3 O 7-x (YBCO) detectors under illumination with picosecond terahertz (THz) pulses was investigated. Real-time measurements with a temporal resolution of 15 ps full width at half maximum were performed at ANKA, the synchrotron facility of Karlsruhe Institute of Technology, and the UVSOR-III facility at the Institute for Molecular Science in Okazaki, Japan. The capability of YBCO detectors to reproduce the shape of a several picosecond long THz pulse was demonstrated. Single-shot measurements adhering to a reversal of the direction of the electrical field of the THz radiation were carried out. They provided evidence for the electrical field sensitivity of the YBCO detector. Exploiting the electrical field sensitivity of the YBCO detector, the effect of microbunching was observed at UVSOR-III.

The photonic time-stretch technique allows electric field pulse shapes to be recorded with picosecond resolution, at megahertz acquisition rates. Using this strategy, we could directly record spatial patterns that spontaneously appear in relativistic electron bunches, and follow their dynamical evolution over time. We present recent results obtained using two strategies. At SOLEIL, we present the shapes of the THz pulses which are emitted by the structures, and detected far from the emission point, at the end of a beamline. At ANKA, we present how it has been possible to monitor directly the electron bunch near-field. These new types of single-shot recordings allow direct and stringent tests to be performed on electron bunch dynamical models in synchrotron radiation facilities.

A high-speed YBa2Cu3O7-δ direct detection system was developed to monitor terahertz picosecond pulses in the time domain. High-TC superconducting thin-film YBa2Cu3O7-δ microbridges with critical temperatures of TC = 85 K were embedded into a planar log-spiral antenna to couple the broadband terahertz radiation (0.1 -2 THz) of several picosecond pulsed sources. The YBa2Cu3O7-δ detectors were installed in a liquid nitrogen cryostat equipped with 18 GHz effective bandwidth readout electronics. THz pulses generated at the electron storage rings ANKA and UVSOR-II have been resolved with a temporal resolution of 30 ps (full width at half maximum) limited by the readout electronics bandwidth. Beam dynamic effects of bursting coherent synchrotron radiation were successfully monitored.

Photonic time-stretch enables to record electric field pulse shapes with picosecond resolution, at megahertz acquisition rates. Using this strategy, we could record the complex spatiotemporal evolution of relativistic electron bunches circulating in storage ring accelerators. We tested two complementary approaches. At SOLEIL, we monitored the THz pulses of coherent synchrotron that are emitted by the electrons. At ANKA, we monitored directly the electron bunch near-field.

Spectral interferometry and dispersive Fourier transformation are combined to retrieve spectral phase information at ~80 MHz in a fiber ring-cavity pumped with ultrashort pulses. Shot-to-shot spectral phase reorganization is observed.

The original version of this Article contained an error in the last sentence of the first paragraph of the Introduction and incorrectly read ‘A proper electron beam control is one of the main challenges towards the Graal of developing a compact alternative of X-ray free-electron lasers by coupling LWFA gigaelectron-volts per centimetre acceleration gradient with undulators in the amplification regime in equation 11, nx(n-β) x β: n the two times and beta the two times should be bold since they are vectorsin Eq. 12, β should be bold as well.’ The correct version is ‘A proper electron beam control is one of the main challenges towards the Graal of developing a compact alternative of X-ray free-electron lasers by coupling LWFA gigaelectron-volts per centimetre acceleration gradient with undulators in the amplification regime.’This has been corrected in both the PDF and HTML versions of the Article.

We consider the problem of shot-by-shot acquisition of pulse shapes at high repetition rate in accelerator-basedsystems. More speci cally, we examine the two-step strategy consisting in (i) encoding the pulse information ontoa laser pulse, and (ii) use the so-called time-stretch strategy to \slow-down" the information before recording.We thus show that the repetition rate of already existing electro-optic sampling setups can be straightforwardlyincreased up to the 100 10 6 pulses/s range, and make a demonstration for the detection of coherent THz pulses.The strategy is however not limited to electro-optic sampling of THz pulses or electron bunches. It can be appliedto other types of wavelengths, provided the desired information (as e.g., FEL pulses or electron bunches shapes)can be imprinted onto a laser pulse.Keywords: Coherent Synchrotron Radiation, Electro-optic sampling, Photonic time-stretch, Terahertz optics 1. INTRODUCTION In accelerator-based coherent sources, there is a growing need for pulse shape characterization at high repetitionrates. High repetition rates is encountered in existing Free-Electron Lasers (FEL), when the emission is composedof bursts of pulses at multi-MHz repetition rates.

In this article we address the problem of tunability of seeded free-electron lasers (FELs), working typically in the domain of the vacuum ultraviolet. The seeding of FELs with an external laser permits us to obtain FEL pulses with both good longitudinal coherence and good stability properties (contrary to an amplification starting from noise). However, with a fixed external laser wavelength, only amplification at harmonics of this wavelength is possible. If full tunability is wanted, it is necessary to have a tunable external source; but this type of source has much less power, hence it is much harder to reach high harmonics numbers. Here we propose the so-called frequency mixing scheme (from its analogy with the similar process in nonlinear optics), based on two laser/electron interactions. Numerical and analytic studies show that it permits to increase significantly the tunability at short wavelengths using a low-energy tunable source. Numerical simulations are made with the parameters of FERMI-FEL 1, the first seeded FEL for user operation.

Emission of light by a single electron moving on a curved trajectory (synchrotron radiation) is one of the most well-known fundamental radiation phenomena. However experimental situations are more complex as they involve many electrons, each being exposed to the radiation of its neighbors. This interaction has dramatic consequences, one of the most spectacular being the spontaneous formation of spatial structures inside electrons bunches. This fundamental effect is actively studied as it represents one of the most fundamental limitations in electron accelerators, and at the same time a source of intense terahertz radiation (Coherent Synchrotron Radiation, or CSR). Here we demonstrate the possibility to directly observe the electron bunch microstructures with subpicosecond resolution, in a storage ring accelerator. The principle is to monitor the terahertz pulses emitted by the structures, using a strategy from photonics, time-stretch, consisting in slowing-down the phenomena before recording. This opens the way to unpreceeded possibilities for analyzing and mastering new generation high power coherent synchrotron sources., Comment: 7 pages, 6 figures

Nous montrons que les nonuniformites spatiales d'un parametres de controle peuvent destabiliser une onde optique. Nous presentons ici l'exemple d'ondes excitees parametriquement dans un laser, mais cette instabilite est a priori attendue dans d'autres systemes optiques. Il s'agit de l'instabilite d'Eckhaus induite par nonuniformites, deja observee en hydrodynamique: on observe une disparition de cellules (dislocations spatiotemporelles) dans l'onde stationnaire, de maniere periodique ou chaotique.

Nous etudions la formation de stuctures spatiotemporelles dans un cristal liquide nematique soumis a une contre-reaction optique. Dans cette etude preliminaire, nous avons restreint notre systeme a une seule dimension d'espace, afin de beneficier des theories existantes dans ce cas. Nous avons ainsi pu mettre en evidence que les non uniformites spatiales engendrent des instabilites spatiotemporelles.

We present the first high resolution (10-3 cm(-1)) interferometric measurements in the 200-750 GHz range using coherent synchrotron radiation, achieved with a low momentum compaction factor. The effect of microbunching on spectra is shown, depending on the bunch current. A high signal-tonoise ratio is reached thanks to an artifact correction system based on a double detection scheme. Combined to the broad emitted spectral range and high flux (up to 10(5) times the incoherent radiation), this study demonstrates that coherent synchrotron radiation can now be used for stability-demanding applications, such as gas-phase studies of unstable molecules. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793558]

We study the efficiency limitation affecting laser-induced coherent synchrotron radiation (CSR) at high laser power. Experiments are made on the UVSOR-II storage ring in conditions of narrowband terahertz CSR emission. While, at moderate power, CSR power increases quadratically with laser power, a noticeable decrease in efficiency and eventually a decrease in CSR power is observed experimentally at high power. Details of the underlying process are analyzed numerically. As the saturation effect depends almost instantaneously on the laser intensity, the saturation occurs locally in longitudinal space. This has important consequences on the modulation pattern induced on the electron bunch.

The laser–plasma accelerator (LPA) presently provides electron beams with a typical current of a few kA, a bunch length of a few fs, energy in the few hundred MeV to several GeV range, a divergence of typically 1 mrad, an energy spread of the order of 1%, and a normalized emittance of the order of π.mm.mrad. One of the first applications could be to use these beams for the production of radiation: undulator emission has been observed but the rather large energy spread (1%) and divergence (1 mrad) prevent straightforward free-electron laser (FEL) amplification. An adequate beam manipulation through the transport to the undulator is then required. The key concept proposed here relies on an innovative electron beam longitudinal and transverse manipulation in the transport towards an undulator: a 'demixing' chicane sorts the electrons according to their energy and reduces the spread from 1% to one slice of a few ‰ and the effective transverse size is maintained constant along the undulator (supermatching) by a proper synchronization of the electron beam focusing with the progress of the optical wave. A test experiment for the demonstration of FEL amplification with an LPA is under preparation. Electron beam transport follows different steps with strong focusing with permanent magnet quadrupoles of variable strength, a demixing chicane with conventional dipoles, and a second set of quadrupoles for further focusing in the undulator. The FEL simulations and the progress of the preparation of the experiment are presented.

In storage rings, short electron bunches can produce an intense THz radiation called Coherent Synchrotron Radiation (CSR). The flux of this emission between 250 and 750 GHz is very advantageous for spectroscopy, but intensity fluctuations lead to artifacts in the FTIR spectra and, until now, prevented the use of CSR for high-resolution measurements. At SOLEIL, we found stable CSR conditions for which the signal-to-noise ratio (S/N) allows for measurements at high resolution. Moreover, we developed an artifact correction system, based on a simultaneous detection of the input and the output signals of the interferometer, which allows improving further the signal-to-noise ratio. The stable CSR combined with this ingenious technique allowed us to record for the first time high-resolution FTIR spectra in the sub-THz range, with an exceptional S/N of 100 in a few hours.

We consider the suppression of the so-called "turbulent regimes" in optical systems with advection. It is well known that spatiotemporal systems submitted to advection can undergo a convective instability above a critical value of the drift parameter. The result is a hypersensivity to noise leading to spatiotemporal erratic regimes called noise sustained structures or "optical turbulence" (Morgner and Mitscke, 1999). This phenomenon appears with slight difference either in system with local saturation (e.g. liquid crystal with optical feedback) or with global saturation (e.g., actively mode-locked lasers). We demonstrate that it is possible to suppress those kind of instability with a rather simple method. To do this, we revisit the "coherent photon seeding" method applied empirically in the nineties to stabilized actively mode-locked laser (Beaud et al., 1990). We study numerically and experimentally the process of stabilization on a elementary advection-diffusion equation (Ginzburg-Landau equation with advection). In addition to determine the adequate parameters, this study allows to interpret the stabilization process: the feedback creates a new deterministic solution toward which the system evolves, the convective threshold has been shifted. This new solution is no more sensitive to noise. Although this process is very different from "Ott-Grebogy-Yorke" method, it requires extremely small perturbation to be achieved (just above noise level). Finally, we check experimentally this method on a free electron laser (UVSORTI FEL, Japan). We suppress the "turbulent" regimes occurring in this laser with a simple optical feedback. The ratio of the re-injected power is in the order of 10-8. This control scheme should be applicable in other systems (optical or not).

Quantum images, that is inhomogeneous field distributions purely generated by quantum fluctuations, persist when passing from the degenerate to the non-degenerate case of optical parametric oscillators (OPO). Below the threshold for parametric oscillation where the near-field distributions are homogeneous both in intensity and phase, appropriate spatial correlation functions anticipate the transverse spatial pattern that appears above threshold. In particular, the angular dependence of the far field spatial correlation function is able to reveal the travelling-wave nature of the phase pattern above threshold typical of nondegenerate OPOs. Cross-correlation functions between signal and idler intensities show clear evidence of the non-classical nature of the output light.

Many spatio-temporal systems can undergo instabilities, leading to the spontaneous formation of spatial structures (patterns). However, a range of cases exist for which the pattern itself is not directly visible because of technical or fundamental reasons. This is the case for the spontaneous formation of millimeter-scale patterns appearing inside relativistic electron bunches of accelerators. We demonstrate in this case how the study of responses to sine external perturbations can be used as a ‘probe’ to deduce the characteristic wavenumber of the pattern formation process. Experiments are performed in the UVSOR-II electron storage ring when the electron bunch is subjected to so-called microbunching instability, and the sine perturbations are provided by an external laser. The response is constituted of pulses of coherent synchrotron radiation, whose amplitude depends on the perturbation wavenumber. Experimental results on the dynamics are compared to numerical calculations obtained using a Vlasov–Fokker–Planck model.

Laboratoire de Physique des Lasers, Atomes et Mole ´cules, Universite des Sciences et Techniques de Lille,F-59655 Villeneuve d’Ascq Cedex, France~Received 3 April 2001; published 4 October 2001!We analyze analytically and experimentally a cascade of parametric instabilities in an inhomogeneouslybroadened multimode laser under pump modulation. From the physical model we use a multiple-scale analysisto derive slow time and slow space amplitude equations, taking nonlocal coupling into account. Each of thetwo waves appearing at the primary instability acts as a pump for the parametric excitation of two newtraveling waves with higher wave numbers. These predictions compare well with numerical simulations of theoriginal laser equations and experimental results obtained with a modulated Yb-doped fiber laser.DOI: 10.1103/PhysRevA.64.053808 PACS number~s!: 42.65.Sf, 47.54.1rINTRODUCTION

591-Mb/s noncoherent FSK 2/sup 23/-1 PRBS transmission experiments using two different commercial standard DFB (distributed feedback) lasers and a phase-tunable DFB laser have been carried out. The commercial DFB lasers are modulated with an AMI (alternated mark inversion) code and the phase-tunable DFB laser with a NRZ (nonreturn to zero) code. With an optical filter bandwidth of 2.7 GHz, receiver sensitivities (for a 10/sup -10/ BER) of -38 dBm for the phase-tunable DFB laser and -36 dBm for both standard DFB lasers are achieved. The influence of optical filter bandwidth and modulation index on the BER is also evaluated. >

Les Lasers a electrons libres (LEL) sont des lasers particuliers bases sur des accelerateurs. Leurs principaux atouts par rapport aux lasers dit conventionnels resident dans leur accordabilite et leur large gamme spectrale (IR-UV/X). Cependant le temps restreint d'acces aux grands instruments tels que les LEL limite leur etude dynamique. Par consequent, une maquette de LEL permettrait de preparer les experiences LEL et d'anticiper d'eventuels problemes. Nous nous efforcons de mettre en evidence les similitudes dynamiques tant au niveau theorique qu'experimental entre le LEL sur anneau de stockage et le laser a blocage de modes actif.

The LUNEX5 (free electron Laser Using a New accelerator for the Exploitation of X-ray radiation of 5th generation) in France aims at investigating the generation of short, intense, and coherent pulses in the soft x-ray region (with two particular targeted wavelengths of 20 and 13 nm). It consists in a single Free Electron Laser (FEL) line with cryo-ready in-vacuum undulators using a Conventional Linear Accelerator (CLA) using the superconducting technology of 400 MeV or a Laser Wake Field Accelerator (LWFA) ranging from 0.4 to 1 GeV with multi-TW or PW lasers. The FEL line can be operated in the seeded (High order Harmonic in Gas seeding) and Echo Enable Harmonic Generation configurations, which performances will be compared. Two pilot user experiments for time-resolved studies of isolated species and magnetization dynamics will take benefit of LUNEX5 FEL radiation.

Stable spatial structure such as stationary waves are expected to appear in a range of optical systems. However, the study of such waves in hydrodynamics revealed that spatial variations of control parameters can lead to instabilities that cannot, be predicted using the uniform medium approximation. Since nonuniformities are very common in optics (e.g., Gaussian pump profile, line shapes, curvature of mirrors...), it is important to know if this type of instability can also affect optical systems. Here we show how this instability affects parametric waves with one spatial dimension. The experimental orientations are performed on a modulated multimode fiber laser (displaying spectral waves), that is described by classical amplitude equations for parametric interaction.

Quantum images, that is inhomogeneous field distributions purely generated by quantum fluctuations, persist when passing from the degenerate to the non-degenerate case of optical parametric oscillators (OPO). Below the threshold for parametric oscillation where the near-field distributions are homogeneous both in intensity and phase, appropriate spatial correlation functions anticipate the transverse spatial pattern that appears above threshold. In particular, the angular dependence of the far field spatial correlation function is able to reveal the travelling-wave nature of the phase pattern above threshold typical of nondegenerate OPOs. Cross-correlation functions between signal and idler intensities show clear evidence of the non-classical nature of the output light.

Summary form only given. In this paper, a coherent narrowband tunable terahertz emission from an electron bunch is reported, transposing a technique well-known in nonlinear optics: interaction with laser pulses containing an internal modulation. The modulated pulses are produced from a titanium-sapphire laser using chirped pulse beating, and are focused on the electron beam, inside an undulator. The interaction leads to a modification of the energy distribution, which transforms into a charge density modulation when the electron bunch is deviated by the next bending magnet, and terahertz emission occurs.