Since they have been proposed, laser-plasma accelerators have interested the scientific community for their ability to generate electric fields exceeding the ones of Linacs and RF cavities. Several efforts have been made in order to produce monochromatic electron beams and to increase their maximum energy, often at the expense of the charge. However, some applications like femtosecond chemistry, radio-biology and industrial radiography do not need monochromatic beams, but rather highly charged ones (i.e., > 1 nC). For some of these applications it is also necessary to reduce the amount of high energy electrons (i.e., > 10 MeV), in order to avoid the activation of materials. Such beams can be produced using high Z gases like Nitrogen and Argon, exploiting the ionization injection of several plasma period.
Here we numerically and experimentally investigate this little-known regime, employing different laser energies, f-numbers and plasma densities. This allowed us to find the conditions to produce electron beams with charges up to tens of nC and exceeding 100 mrad in divergence. We will also show and explain the dependencies of these beams (e.g., their charges and energy spectra) as functions of the aforementioned laser and plasma parameters.
An electron beam generated by a laser-plasma accelerator is converted into an X-ray source by means of bremsstrahlung radiation in a dense material. This radiation source can be used to perform non-destructive testing (NDT) of dense objects.
To perform this type of X-ray imaging, it is necessary to work with a point-like source of high energy X-rays. For this purpose, a numerical optimization of the whole experiment is essential, we have to deal with two main parts, the laser-matter interaction for which we use a Particle-In-Cell code, and the X-ray emission part for which we use a Monte Carlo code.
In this talk we will discuss our recent approaches for tackling the limitations of NDT for dense objects : increasing the energy of the source and reducing its size.
While synchrotron light facilities and Free Electron Lasers (FELs) are widely used for matter investigation, Laser Plasma Acceleration (LPA), delivering nowadays GeV electron beams in few centimeter accelerating distance, can be considered to drive undulator radiation and FEL. We report on the generation of undulator radiation on the COXINEL dedicated manipulation line designed for an FEL application. The LPA large divergence is handled with variable gradient permanent magnet quadrupoles and the high energy spread is reduced via a magnetic chicane. We evidence the undulator spatio-spectral signature on the first and second harmonics while measuring the radiation focused onto the entrance slit of a spectrometer equipped with a CDD camera. A good agreement is found between measurements and SRW simulations, using electron beam parameters in the undulator deduced from the measured initial electron beam parameters transported along the beamline. In addition, ray optics approach is compared to Fourier optics for the radiation propagation through optical elements.
Laser-Plasma Accelerators (LPAs) produce electric fields exceeding 100 GV/m, that is 3 orders of magnitude larger than those obtained in metallic-cavity accelerators. They could thus allow for a drastic decrease of the size of accelerators for scientific, medical and industrial applications. A high field-gradient is however not sufficient for reaching high-energies; the electron beam has also to experience the accelerating field on long distances, which is challenging in a LPA because of 3 phenomenons: diffraction, pump depletion and dephasing. Diffraction and pump depletion leads to a decrease of the laser intensity during the acceleration, down to a level from which the laser can no more drive a wakefield. Dephasing corresponds to electrons reaching a decelerating phase of the electric field. It occurs because the phase velocity of the accelerating field is smaller than the velocity of the electron beam. To date, the highest beam energies have been obtained by guiding the laser in a capillary discharge, thus overcoming diffraction.
Here we propose a new acceleration concept, based on the use of high-intensity quasi-Bessel beams and spatio-temporal couplings, which allows to overcome not only diffraction but also pump depletion and dephasing. The velocity of the quasi-Bessel beam is superluminal in vacuum and dephasing is suppressed by using spatio-temporal couplings to phase lock the electron beam on the accelerating field. In this scheme, the electron energy is proportional to the laser energy and inversely proportional to the laser pulse length (the shorter the laser, the higher the beam energy).
We will first present Particle-In-Cell simulations demonstrating this concept. We will then show preliminary experimental results illustrating the generation of high-intensity quasi-Bessel beams as well as the generation of a 1 cm plasma-waveguide.
Betatron radiation from laser-plasma accelerators reproduces the principle of a synchrotron on a millimeter scale, but featuring femtosecond duration. Here we present the outcome of our latest developments, which now allow us to produce stable and polarized X-ray bursts. Moreover, the X-ray polarization can simply be adjusted by tuning the polarization of the laser driving the process. The excellent stability of the source is expressed in terms of pointing, flux, transverse distribution and critical energy of the spectrum. These combined features make our betatron source particularly suitable for applications in ultrafast X-ray science.
In this presentation we will describe the generation process, relying on the ionization injection scheme for laser-plasma acceleration. We will show experimental measurements, numerical results and first applications in time-resolved spectroscopy.
A. Loulergue, M. Labat, C. Evain, N. Hubert, F. Briquez, F. Marteau, C. Benabderrahmane, P. Berteaud, C. Bourassin-Bouchet, F. Bouvet, L. Cassinari, L. Chapuis, M. El Ajjouri, C. Herbeaux, M. Khojoyan, D. Dennetiere, N. Leclercq, JP. Duval, A. Lestrade, O. Marcouillé, P. Rommeluère, J.-L. Marlats, P. Morin, F. Polack, K. Tavakoli, M. Valleau, D. Zerbib, W. Yang, X. Davoine, I. Andriyash, G. Lambert, V. Malka, C. Thaury, S. Bielawski, C. Szwaj, M.-E. Couprie
KEYWORDS: Free electron lasers, Electron beams, Diagnostics, Plasma, Magnetism, Colorimetry, Synchrotrons, Spectroscopy, Optical testing, Electron transport
One direction towards compact Free Electron Laser is to replace the conventional linac by a laser plasma driven beam, provided proper electron beam manipulation to handle the large values of the energy spread and of the divergence. Applying seeding techniques enable also to reduce the required undulator length. The rapidly developing LWFA are already able to generate synchrotron radiation. With an electron divergence of typically 1 mrad and an energy spread of the order of 1 % (or few), an adequate beam manipulation through the transport to the undulator is needed for FEL amplification. Electron beam transfer follows different steps with strong focusing variable strength permanent magnet quadrupoles, an energy demixing chicane with conventional dipoles, a second set of quadrupoles for further dedicated focusing in the undulator. A test experiment for the demonstration of FEL amplification with a LWFA is under preparation and progress on the equipment preparation and expected performance are described.
One of the key ingredients of laser-plasma accelerators is their injector, which defines how electrons are trapped into the laser-driven plasma wave. The stability and control of laser-plasma electron bunches strongly depends on this injection stage. Self-injection is a convenient way to achieve the electron trapping and is the most widely used injector. Here we demonstrate, by using a variable length gas cell, that injection can be achieved by either longitudinal or transverse self-injection, giving rise to very different electron beam features. The results are supported by 3 dimensional particle-in-cell simulations.
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