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.
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.
Free-electron lasers generate high-brilliance coherent radiation at wavelengths spanning from the infrared to the X-ray domains. The recent development of short-wavelength seeded free-electron lasers now allows for unprecedented levels of control on longitudinal coherence, opening new scientific avenues such as ultra-fast dynamics on complex systems and X-ray nonlinear optics. Although those devices rely on state-of-the-art large-scale accelerators, advancements on laser-plasma accelerators, which harness gigavolt-per-centimetre accelerating fields, showcase a promising technology as compact drivers for free-electron lasers. Using such footprint-reduced accelerators, exponential amplification of a shot-noise type of radiation in a self-amplified spontaneous emission configuration was recently achieved. However, employing this compact approach for the delivery of temporally coherent pulses in a controlled manner has remained a major challenge. Here we present the experimental demonstration of a laser-plasma accelerator-driven free-electron laser in a seeded configuration, where control over the radiation wavelength is accomplished. Furthermore, the appearance of interference fringes, resulting from the interaction between the phase-locked emitted radiation and the seed, confirms longitudinal coherence. Building on our scientific achievements, we anticipate a navigable pathway to extreme-ultraviolet wavelengths, paving the way towards smaller-scale free-electron lasers, unique tools for a multitude of applications in industry, laboratories and universities.
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.
Spontaneous formation of spatial structures (patterns) occurs in various contexts, ranging from sand dunes [1] and rogue wave formation [2,3], to traffic jams [4]. These last decades, very practical reasons also led to studies of pattern formation in relativistic electron bunches used in synchrotron radiation light sources. As the main motivation, the patterns which spontaneously appear during an instability increase the terahertz radiation power by factors exceeding 10000 [5,6]. However the irregularity of these patterns [5][6][7][8][9][10][11] largely prevented applications of this powerful source. Here we show how to make the spatiotemporal patterns regular (and thus the emitted THz power) using a point of view borrowed from chaos control theory [12][13][14]. Mathematically, 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.Synchrotron light sources are used worldwide to produce brilliant light from THz to hard X-rays, allowing to investigate a very large range of matter properties. In these sources where electron bunches travel at relativistic velocities, an ubiquitous phenomenon occurs when the bunch charge density exceeds a threshold value. Due to the interaction between the electron bunch and its own emitted electric field, micro-structures spontaneously appear in the longitudinal profile (and phase-space) of the bunch [7, 15-17] (see Fig. 1(a) and (b) for the SOLEIL storage ring which will be considered here). In storage rings, these structures are responsible for a huge emission of coherent light in the terahertz range, typically 10 3 − 10 5 times normal synchrotron radiation power density. However, as the micro-structures appear mostly in the form of bursts [ Fig. 1(d)] this type of source is barely usable in user applications. Hence, research in this domain has naturally attempted to find regions for which coherent emission (CSR) occurs while bursting dynamics * Corresponding author : clement.evain@univ-lille.fr is absent. Such "parameter search" methods succeeded in identifying parameter regions with stable coherent emission. However, this corresponds to special configurations (with short and low charge electron bunches, in the so-called low-alpha operation [5,6,10,11,[18][19][20][21][22][23]) which are not compatible with most of the user experiments. Therefore, this type of THz source is used in relatively few synchrotron radiation facilities (SOLEIL, DIAMOND, BESSY-II), and only during a small part of the year."Parameter search approaches" are however not the only possibilities for avoiding instabilities. The point of view that we will use here is directly borrowed from the so-called chaos control theory, introduced by Ott, Grebogi and Yorke (OGY) [12,13]. Mathematically, when an undesi...
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