235 research outputs found
Plasma wave undulator for laser-accelerated electrons
Laser-plasma accelerators have become compact sources of ultrashort electron
bunches at energies up to the gigaelectronvolt range thanks to the remarkable
progress made over the past decade. A direct application of these electron
bunches is the production of short pulse x-ray radiation sources. In this
letter, we study a fully optically driven x-ray source based on the combination
of a laser-plasma accelerator and a plasma wave undulator. The longitudinal
electric field of a laser-generated plasma wave is used to wiggle electrons
transversally. The period of this plasma undulator being equal to the plasma
wavelength, tunable photon energies in the 10 keV range can be achieved with
electron energies in the 100-200 MeV range. Considering a 10s TW class
femtosecond laser system, undulators with a strength parameter K~0.5 and with
about ten periods can be combined with a laser-plasma accelerator, resulting in
several 10^-2 emitted x-ray photons per electron.Comment: 6 pages, 4 figure
Comment on "Scalings for radiation from plasma bubbles" [Phys. Plasmas 17, 056708 (2010)]
Thomas has recently derived scaling laws for X-ray radiation from electrons
accelerated in plasma bubbles, as well as a threshold for the self-injection of
background electrons into the bubble [A. G. R. Thomas, Phys. Plasmas 17, 056708
(2010)]. To obtain this threshold, the equations of motion for a test electron
are studied within the frame of the bubble model, where the bubble is described
by prescribed electromagnetic fields and has a perfectly spherical shape. The
author affirms that any elliptical trajectory of the form x'^2/{\gamma}_p^2 +
y'^2 = R^2 is solution of the equations of motion (in the bubble frame), within
the approximation p'_y^2/p'_x^2 \ll 1. In addition, he highlights that his
result is different from the work of Kostyukov et al. [Phys. Rev. Lett. 103,
175003 (2009)], and explains the error committed by
Kostyukov-Nerush-Pukhov-Seredov (KNPS). In this comment, we show that
numerically integrated trajectories, based on the same equations than the
analytical work of Thomas, lead to a completely different result for the
self-injection threshold, the result published by KNPS [Phys. Rev. Lett. 103,
175003 (2009)]. We explain why the analytical analysis of Thomas fails and we
provide a discussion based on numerical simulations which show exactly where
the difference arises. We also show that the arguments of Thomas concerning the
error of KNPS do not hold, and that their analysis is mathematically correct.
Finally, we emphasize that if the KNPS threshold is found not to be verified in
PIC (Particle In Cell) simulations or experiments, it is due to a deficiency of
the model itself, and not to an error in the mathematical derivation.Comment: 5 pages, 5 figure
Probing electron acceleration and X-ray emission in laser-plasma accelerator
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
Femtosecond x rays from laser-plasma accelerators
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.Comment: 58 pages, 41 figure
Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators
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.Comment: 7 pages, 8 figure
Positron Acceleration in Plasma Wakefields
Plasma acceleration has emerged as a promising technology for future particle
accelerators, particularly linear colliders. Significant progress has been made
in recent decades toward high-efficiency and high-quality acceleration of
electrons in plasmas. However, this progress does not generalize to
acceleration of positrons, as plasmas are inherently charge asymmetric. Here,
we present a comprehensive review of historical and current efforts to
accelerate positrons using plasma wakefields. Proposed schemes that aim to
increase the energy efficiency and beam quality are summarised and
quantitatively compared. A dimensionless metric that scales with the
luminosity-per-beam power is introduced, indicating that positron-acceleration
schemes are currently below the ultimate requirement for colliders. The primary
issue is electron motion; the high mobility of plasma electrons compared to
plasma ions, which leads to non-uniform accelerating and focusing fields that
degrade the beam quality of the positron bunch, particularly for high
efficiency acceleration. Finally, we discuss possible mitigation strategies and
directions for future research.Comment: 24 pages (30 pages with references), 22 figure
Observation of longitudinal and transverse self-injections in laser-plasma accelerators
Laser-plasma accelerators can produce high quality electron beams, up to
giga-electronvolts in energy, from a centimeter 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.Comment: 7 pages, 7 figure
Angular momentum evolution in laser-plasma accelerators
The transverse properties of an electron beam are characterized by two
quantities, the emittance which indicates the electron beam extend in the phase
space and the angular momentum which allows for non-planar electron
trajectories. Whereas the emittance of electron beams produced in laser- plasma
accelerator has been measured in several experiments, their angular momentum
has been scarcely studied. It was demonstrated that electrons in 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
Betatron emission as a diagnostic for injection and acceleration mechanisms in laser-plasma accelerators
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
half.Comment: 8 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1104.245
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