856 research outputs found
Plasma wake inhibition at the collision of two laser pulses in an underdense plasma
An electron injector concept for laser-plasma accelerator was developed in
ref [1] and [2] ; it relies on the use of counter-propagating ultrashort laser
pulses. In [2], the scheme is as follows: the pump laser pulse generates a
large amplitude laser wakefield (plasma wave). The counter-propagating
injection pulse interferes with the pump laser pulse to generate a beatwave
pattern. The ponderomotive force of the beatwave is able to inject plasma
electrons into the wakefield. We have studied this injection scheme using 1D
Particle in Cell (PIC) simulations. The simulations reveal phenomena and
important physical processes that were not taken into account in previous
models. In particular, at the collision of the laser pulses, most plasma
electrons are trapped in the beatwave pattern and cannot contribute to the
collective oscillation supporting the plasma wave. At this point, the fluid
approximation fails and the plasma wake is strongly inhibited. Consequently,
the injected charge is reduced by one order of magnitude compared to the
predictions from previous models.Comment: 4 pages, 4 figure
Energy boost in laser wakefield accelerators using sharp density transitions
The energy gain in laser wakefield accelerators is limited by dephasing
between the driving laser pulse and the highly relativistic electrons in its
wake. Since this phase depends on both the driver and the cavity length, the
effects of dephasing can be mitigated with appropriate tailoring of the plasma
density along propagation. Preceding studies have discussed the prospects of
continuous phase-locking in the linear wakefield regime. However, most
experiments are performed in the highly non-linear regime and rely on
self-guiding of the laser pulse. Due to the complexity of the driver evolution
in this regime it is much more difficult to achieve phase locking. As an
alternative we study the scenario of rapid rephasing in sharp density
transitions, as was recently demonstrated experimentally. Starting from a
phenomenological model we deduce expressions for the electron energy gain in
such density profiles. The results are in accordance with particle-in-cell
simulations and we present gain estimations for single and multiple stages of
rephasing
Turbulence without pressure
We develop exact field theoretic methods to treat turbulence when the effect
of pressure is negligible. We find explicit forms of certain probability
distributions, demonstrate that the breakdown of Galilean invariance is
responsible for intermittency and establish the operator product expansion. We
also indicate how the effects of pressure can be turned on perturbatively.Comment: 12 page
Concept of a laser-plasma based electron source for sub-10 fs electron diffraction
We propose a new concept of an electron source for ultrafast electron
diffraction with sub-10~fs temporal resolution. Electrons are generated in a
laser-plasma accelerator, able to deliver femtosecond electron bunches at 5 MeV
energy with kHz repetition rate. The possibility of producing this electron
source is demonstrated using Particle-In-Cell simulations. We then use particle
tracking simulations to show that this electron beam can be transported and
manipulated in a realistic beamline, in order to reach parameters suitable for
electron diffraction. The beamline consists of realistic static magnetic optics
and introduces no temporal jitter. We demonstrate numerically that electron
bunches with 5~fs duration and containing 1.5~fC per bunch can be produced,
with a transverse coherence length exceeding 2~nm, as required for electron
diffraction
Quasimonoenergetic electron beams produced by colliding cross-polarized laser pulses in underdense plasmas
The interaction of two laser pulses in an underdense plasma has proven to be
able to inject electrons in plasma waves, thus providing a stable and tunable
source of electrons. Whereas previous works focused on the "beatwave" injection
scheme in which two lasers with the same polarization collide in a plasma, this
present letter studies the effect of polarization and more specifically the
interaction of two colliding cross-polarized laser pulses. It is shown both
theoretically and experimentally that electrons can also be pre-accelerated and
injected by the stochastic heating occurring at the collision of two
cross-polarized lasers and thus, a new regime of optical injection is
demonstrated. It is found that injection with cross-polarized lasers occurs at
higher laser intensities.Comment: 4 pages, 4 figure
Anticorrelation between Ion Acceleration and Nonlinear Coherent Structures from Laser-Underdense Plasma Interaction
In laser-plasma experiments, we observed that ion acceleration from the
Coulomb explosion of the plasma channel bored by the laser, is prevented when
multiple plasma instabilities such as filamentation and hosing, and nonlinear
coherent structures (vortices/post-solitons) appear in the wake of an
ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp
allows us to control the onset of these insabilities. We deduced that the laser
pulse is depleted into these structures in our conditions, when a plasma at
about 10% of the critical density exhibits a gradient on the order of 250
{\mu}m (gaussian fit), thus hindering the acceleration. A promising
experimental setup with a long pulse is demonstrated enabling the excitation of
an isolated coherent structure for polarimetric measurements and, in further
perspectives, parametric studies of ion plasma acceleration efficiency.Comment: 4 pages, 5 figure
Unique Fock quantization of scalar cosmological perturbations
We investigate the ambiguities in the Fock quantization of the scalar
perturbations of a Friedmann-Lema\^{i}tre-Robertson-Walker model with a massive
scalar field as matter content. We consider the case of compact spatial
sections (thus avoiding infrared divergences), with the topology of a
three-sphere. After expanding the perturbations in series of eigenfunctions of
the Laplace-Beltrami operator, the Hamiltonian of the system is written up to
quadratic order in them. We fix the gauge of the local degrees of freedom in
two different ways, reaching in both cases the same qualitative results. A
canonical transformation, which includes the scaling of the matter field
perturbations by the scale factor of the geometry, is performed in order to
arrive at a convenient formulation of the system. We then study the
quantization of these perturbations in the classical background determined by
the homogeneous variables. Based on previous work, we introduce a Fock
representation for the perturbations in which: (a) the complex structure is
invariant under the isometries of the spatial sections and (b) the field
dynamics is implemented as a unitary operator. These two properties select not
only a unique unitary equivalence class of representations, but also a
preferred field description, picking up a canonical pair of field variables
among all those that can be obtained by means of a time-dependent scaling of
the matter field (completed into a linear canonical transformation). Finally,
we present an equivalent quantization constructed in terms of gauge-invariant
quantities. We prove that this quantization can be attained by a mode-by-mode
time-dependent linear canonical transformation which admits a unitary
implementation, so that it is also uniquely determined.Comment: 19 pages, minor impovementes included, typos correcte
Early out-of-equilibrium beam-plasma evolution
We solve analytically the out-of-equilibrium initial stage that follows the
injection of a radially finite electron beam into a plasma at rest and test it
against particle-in-cell simulations. For initial large beam edge gradients and
not too large beam radius, compared to the electron skin depth, the electron
beam is shown to evolve into a ring structure. For low enough transverse
temperatures, the filamentation instability eventually proceeds and saturates
when transverse isotropy is reached. The analysis accounts for the variety of
very recent experimental beam transverse observations.Comment: to appear in Phys. Rev. Letter
Laser-plasma interactions with a Fourier-Bessel Particle-in-Cell method
A new spectral particle-in-cell (PIC) method for plasma modeling is presented
and discussed. In the proposed scheme, the Fourier-Bessel transform is used to
translate the Maxwell equations to the quasi-cylindrical spectral domain. In
this domain, the equations are solved analytically in time, and the spatial
derivatives are approximated with high accuracy. In contrast to the
finite-difference time domain (FDTD) methods that are commonly used in PIC, the
developed method does not produce numerical dispersion, and does not involve
grid staggering for the electric and magnetic fields. These features are
especially valuable in modeling the wakefield acceleration of particles in
plasmas. The proposed algorithm is implemented in the code PLARES-PIC, and the
test simulations of laser plasma interactions are compared to the ones done
with the quasi-cylindrical FDTD PIC code CALDER-CIRC.Comment: submitted to Phys. Plasma
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