[Show abstract][Hide abstract] ABSTRACT: While large efforts have been devoted to improving the quality of electron beams from laser plasma accelerators, often to the detriment of the charge, many applications do not require very high quality but high-charge beams. Despite this need, the acceleration of largely charged beams has been barely studied. Here we explore both experimentally and numerically the physics of highly loaded wakefield acceleration. We find that the shape of the electron spectra is strikingly independent of the laser energy, due to the emergence of a saturation effect induced by beamloading. A transition from quasi-Maxwellian spectra at high plasma densities to flatter spectra at lower densities is also found, which is shown to be produced by the wakefield driven by the electron bunch itself after the laser depletion.
Physical Review Special Topics - Accelerators and Beams 06/2015; 18(6). DOI:10.1103/PhysRevSTAB.18.061301 · 1.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Laser-plasma technology promises a drastic reduction of the size of high
energy electron accelerators. It could make free electron lasers available to a
broad scientific community, and push further the limits of electron
accelerators for high energy physics. Furthermore the unique femtosecond nature
of the source makes it a promising tool for the study of ultra-fast phenomena.
However, applications are hindered by the lack of suitable lens to transport
this kind of high-current electron beams, mainly due to their divergence. Here
we show that this issue can be solved by using a laser-plasma lens, in which
the field gradients are five order of magnitude larger than in conventional
optics. We demonstrate a reduction of the divergence by nearly a factor of
three, which should allow for an efficient coupling of the beam with a
conventional beam transport line.
[Show abstract][Hide abstract] ABSTRACT: X-ray radiation emitted by electrons during their acceleration in a laser-plasma accelerator was used to evidence two distincts self-injection mechanisms (longitudinal and transverse) and to identify one source of angular-momentum growth in laser-plasma accelerators.
[Show abstract][Hide abstract] ABSTRACT: The transverse properties of an electron beam are characterized by two quantities, the emittance which indicates the electron beam extent in the phase space and the angular momentum which allows for nonplanar electron trajectories. Whereas the emittance of electron beams produced in a laser-plasma accelerator has been measured in several experiments, their angular momentum has been scarcely studied. It was demonstrated that electrons in a laser-plasma accelerator carry some angular momentum, but its origin was not established. Here we identify one source of angular-momentum growth and we present experimental results showing that the angular-momentum content evolves during the acceleration.
[Show abstract][Hide abstract] ABSTRACT: While laser-plasma accelerators have demonstrated a strong potential in the
acceleration of electrons up to giga-electronvolt energies, few experimental
tools for studying the acceleration physics have been developed. In this paper,
we demonstrate a method for probing the acceleration process. A second laser
beam, propagating perpendicular to the main beam is focused in the gas jet few
nanosecond before the main beam creates the accelerating plasma wave. This
second beam is intense enough to ionize the gas and form a density depletion
which will locally inhibit the acceleration. The position of the density
depletion is scanned along the interaction length to probe the electron
injection and acceleration, and the betatron X-ray emission. To illustrate the
potential of the method, the variation of the injection position with the
plasma density is studied.
Physics of Plasmas 09/2013; 20(6). DOI:10.1063/1.4810791 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Laser-plasma accelerators can produce high-quality electron beams, up to giga electronvolts in energy, from a centimetre scale device. The properties of the electron beams and the accelerator stability are largely determined by the injection stage of electrons into the accelerator. The simplest mechanism of injection is self-injection, in which the wakefield is strong enough to trap cold plasma electrons into the laser wake. The main drawback of this method is its lack of shot-to-shot stability. Here we present experimental and numerical results that demonstrate the existence of two different self-injection mechanisms. Transverse self-injection is shown to lead to low stability and poor-quality electron beams, because of a strong dependence on the intensity profile of the laser pulse. In contrast, longitudinal injection, which is unambiguously observed for the first time, is shown to lead to much more stable acceleration and higher-quality electron beams.
[Show abstract][Hide abstract] ABSTRACT: 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.
11th International Conference on Synchrotron Radiation Instrumentation (SRI 2012), Lyon, France; 03/2013
[Show abstract][Hide abstract] ABSTRACT: Relativistic interaction of short-pulse lasers with underdense plasmas has
recently led to the emergence of a novel generation of femtosecond x-ray
sources. Based on radiation from electrons accelerated in plasma, these sources
have the common properties to be compact and to deliver collimated, incoherent
and femtosecond radiation. In this article we review, within a unified
formalism, the betatron radiation of trapped and accelerated electrons in the
so-called bubble regime, the synchrotron radiation of laser-accelerated
electrons in usual meter-scale undulators, the nonlinear Thomson scattering
from relativistic electrons oscillating in an intense laser field, and the
Thomson backscattered radiation of a laser beam by laser-accelerated electrons.
The underlying physics is presented using ideal models, the relevant parameters
are defined, and analytical expressions providing the features of the sources
are given. Numerical simulations and a summary of recent experimental results
on the different mechanisms are also presented. Each section ends with the
foreseen development of each scheme. Finally, one of the most promising
applications of laser-plasma accelerators is discussed: the realization of a
compact free-electron laser in the x-ray range of the spectrum. In the
conclusion, the relevant parameters characterizing each sources are summarized.
Considering typical laser-plasma interaction parameters obtained with currently
available lasers, examples of the source features are given. The sources are
then compared to each other in order to define their field of applications.
Review of Modern Physics 01/2013; 85(1). DOI:10.1103/RevModPhys.85.1 · 29.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Betatron x-ray emission in laser-plasma accelerators is a promising compact
source that may be an alternative to conventional x-ray sources, based on large
scale machines. In addition to its potential as a source, precise measurements
of betatron emission can reveal crucial information about relativistic
laser-plasma interaction. We show that the emission length and the position of
the x-ray emission can be obtained by placing an aperture mask close to the
source, and by measuring the beam profile of the betatron x-ray radiation far
from the aperture mask. The position of the x-ray emission gives information on
plasma wave breaking and hence on the laser non-linear propagation. Moreover,
the measurement of the longitudinal extension helps one to determine whether
the acceleration is limited by pump depletion or dephasing effects. In the case
of multiple injections, it is used to retrieve unambiguously the position in
the plasma of each injection. This technique is also used to study how, in a
capillary discharge, the variations of the delay between the discharge and the
laser pulse affect the interaction. The study reveals that, for a delay
appropriate for laser guiding, the x-ray emission only occurs in the second
half of the capillary: no electrons are injected and accelerated in the first
[Show abstract][Hide abstract] ABSTRACT: Bright and high-energy femtosecond x-ray beams were produced from Betatron oscillations and Compton scattering in laser-plasma accelerators. Their use as a diagnostic for laser-plasma accelerators and for applications was demonstrated.
[Show abstract][Hide abstract] ABSTRACT: Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser
pulse demonstrated quasi-monoenergetic electron spectra with maximum energy
over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp
density ramp generated by a high intensity laser crossing pre-pulse focused 3
ns before the main laser pulse. By adjusting this crossing pre-pulse position
inside the gas jet, among the laser shots with electron injection more than 40%
can produce quasi-monoenergetic spectra. This could become a relatively
straight forward technique to control laser wakefield electron beams
[Show abstract][Hide abstract] ABSTRACT: LUNEX5 (free electron Laser Using a New accelerator for the Exploitation of X-ray radiation of 5th generation) aims at investigating the production of short, intense, and coherent pulses in the soft X-ray region. The project consists of a Free Electron Laser (FEL) line enabling the most advanced seeding configurations: High order Harmonic in Gas (HHG) seeding and Echo Enable Harmonic Generation (EEHG) with in-vacuum (potentially cryogenic) undulators of 15 and 30 mm period. Two accelerator types feed this FEL line : a 400 MeV Conventional Linear Accelerator (CLA) using superconducting cavities compatible with a future upgrade towards high repetition rate, for the investigations of the advanced FEL schemes; and a 0.4 - 1 GeV Laser Wake Field Accelerator (LWFA), to be qualified in view of FEL application, in the single spike or seeded regime. Two pilot user experiments for time- resolved studies of isolated species and solid state matter dynamics will take benefit of LUNEX5 FEL radiation and provide feedback of the performance of the different schemes under real user conditions.
34th International Free-Electron Laser Conference, Nara, Japan; 08/2012
[Show abstract][Hide abstract] ABSTRACT: One of the major goals of research for laser-plasma accelerators is the
realization of compact sources of femtosecond X-rays. In particular,
using the modest electron energies obtained with existing laser systems,
Compton scattering a photon beam off a relativistic electron bunch has
been proposed as a source of high-energy and high-brightness photons.
However, laser-plasma based approaches to Compton scattering have not,
to date, produced X-rays above 1 keV. Here, we present a simple and
compact scheme for a Compton source based on the combination of a
laser-plasma accelerator and a plasma mirror. This approach is used to
produce a broadband spectrum of X-rays extending up to hundreds of keV
and with a 10,000-fold increase in brightness over Compton X-ray sources
based on conventional accelerators. We anticipate that this technique
will lead to compact, high-repetition-rate sources of ultrafast
(femtosecond), tunable (X- through gamma-ray) and low-divergence
(~1°) photons from source sizes on the order of a micrometre.
[Show abstract][Hide abstract] ABSTRACT: We study pump requirements to produce femtosecond X-ray laser pulses at
saturation from inner-shell transitions in the amplified spontaneous
emission regime. Since laser-based betatron radiation is considered as
the pumping source, we first study the impact of the driving laser power
on its intensity. Then we investigate the amplification behavior of the
K- α transition of nitrogen at 3.2 nm (395 eV) from radiative
transfer calculations coupled with kinetics modeling of the ion
population densities. We show that the saturation regime may be
experimentally achieved by using PW-class laser-accelerated electron
bunches. Finally, we show that this X-ray laser scheme can be extended
to heavier atoms and we calculate pump requirements to reach saturation
at 1.5 nm (849 eV) from the K- α transition of neon.
Applied Physics B 03/2012; 106(4):809-816. DOI:10.1007/s00340-012-4912-1 · 1.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A density perturbation produced in an underdense plasma was used to improve
the quality of electron bunches produced in the laser-plasma wakefield
acceleration scheme. Quasi-monoenergetic electrons were generated by controlled
injection in the longitudinal density gradients of the density perturbation. By
tuning the position of the density perturbation along the laser propagation
axis, a fine control of the electron energy from a mean value of 60 MeV to 120
MeV has been demonstrated with a relative energy-spread of 15 +/- 3.6%,
divergence of 4 +/- 0.8 mrad and charge of 6 +/- 1.8 pC.
Physics of Plasmas 01/2012; 19(6). DOI:10.1063/1.4725421 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High intensity femtosecond laser pulses can be used to generate X-ray
radiation. In the laser wakefield process, when a high intensity laser
pulse (<1018 W/cm2) is focused onto a gas jet
target, it interacts with the instantaneously created under-dense plasma
and excites a wakefield wave. In the wakefield electrons are trapped and
accelerated to high energies in short distances. The electrons trapped
in the wakefield can perform Betatron oscillations across the
propagation axis and emit X-ray photons. The Betatron X-ray beam is
broadband as the radiation emission has a synchrotron distribution. The
X-ray beam is collimated and its pulse duration is femtosecond. For high
resolution and phase contrast X-ray imaging applications, the important
feature of the X-ray Betatron beam is the μm source size. Using ALLS
100 TW class laser system we demonstrate that the Betatron X-ray beam is
both energetic and bright enough to produce single laser shot phase
contrast imaging of complex objects located in air.
Proceedings of SPIE - The International Society for Optical Engineering 01/2012; DOI:10.1117/12.2001554 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bright femtosecond X-ray beams, with energies up to a few hundreds of keV, have been produced from Betatron oscillations and Compton scattering in a laser-plasma accelerator. The potential of Betatron radiation for phase contrast imaging has been demonstrated.
Lasers and Electro-Optics (CLEO), 2012 Conference on; 01/2012
[Show abstract][Hide abstract] ABSTRACT: We show that the control and the mapping of the x-ray emission reveals unique features of the laser-plasma accelerator physics, including strong correlations between electron and x-ray beams, and density-dependence of electron injection position.
Lasers and Electro-Optics (CLEO), 2012 Conference on; 01/2012