412 research outputs found
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
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
Computationally efficient methods for modelling laser wakefield acceleration in the blowout regime
Electron self-injection and acceleration until dephasing in the blowout
regime is studied for a set of initial conditions typical of recent experiments
with 100 terawatt-class lasers. Two different approaches to computationally
efficient, fully explicit, three-dimensional particle-in-cell modelling are
examined. First, the Cartesian code VORPAL using a perfect-dispersion
electromagnetic solver precisely describes the laser pulse and bubble dynamics,
taking advantage of coarser resolution in the propagation direction, with a
proportionally larger time step. Using third-order splines for macroparticles
helps suppress the sampling noise while keeping the usage of computational
resources modest. The second way to reduce the simulation load is using
reduced-geometry codes. In our case, the quasi-cylindrical code CALDER-CIRC
uses decomposition of fields and currents into a set of poloidal modes, while
the macroparticles move in the Cartesian 3D space. Cylindrical symmetry of the
interaction allows using just two modes, reducing the computational load to
roughly that of a planar Cartesian simulation while preserving the 3D nature of
the interaction. This significant economy of resources allows using fine
resolution in the direction of propagation and a small time step, making
numerical dispersion vanishingly small, together with a large number of
particles per cell, enabling good particle statistics. Quantitative agreement
of the two simulations indicates that they are free of numerical artefacts.
Both approaches thus retrieve physically correct evolution of the plasma
bubble, recovering the intrinsic connection of electron self-injection to the
nonlinear optical evolution of the driver
Persistence of magnetic field driven by relativistic electrons in a plasma
The onset and evolution of magnetic fields in laboratory and astrophysical
plasmas is determined by several mechanisms, including instabilities, dynamo
effects and ultra-high energy particle flows through gas, plasma and
interstellar-media. These processes are relevant over a wide range of
conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion
in stars. The disparate temporal and spatial scales where each operates can be
reconciled by scaling parameters that enable to recreate astrophysical
conditions in the laboratory. Here we unveil a new mechanism by which the flow
of ultra-energetic particles can strongly magnetize the boundary between the
plasma and the non-ionized gas to magnetic fields up to 10-100 Tesla (micro
Tesla in astrophysical conditions). The physics is observed from the first
time-resolved large scale magnetic field measurements obtained in a laser
wakefield accelerator. Particle-in-cell simulations capturing the global plasma
and field dynamics over the full plasma length confirm the experimental
measurements. These results open new paths for the exploration and modelling of
ultra high energy particle driven magnetic field generation in the laboratory
A bremsstrahlung gamma-ray source based on stable ionization injection of electrons into a laser wakefield accelerator
Laser wakefield acceleration permits the generation of ultra-short,
high-brightness relativistic electron beams on a millimeter scale. While those
features are of interest for many applications, the source remains constraint
by the poor stability of the electron injection process. Here we present
results on injection and acceleration of electrons in pure nitrogen and argon.
We observe stable, continuous ionization-induced injection of electrons into
the wakefield for laser powers exceeding a threshold of 7 TW. The beam charge
scales approximately linear with the laser energy and is limited by beam
loading. For 40 TW laser pulses we measure a maximum charge of almost 1 nC per
shot, originating mostly from electrons of less than 10 MeV energy. The
relatively low energy, the high charge and its stability make this source
well-suited for applications such as non-destructive testing. Hence, we
demonstrate the production of energetic radiation via bremsstrahlung conversion
at 1 Hz repetition rate. In accordance with Geant4 Monte-Carlo simulations, we
measure a gamma-ray source size of less than 100 microns for a 0.5 mm tantalum
converter placed at 2 mm from the accelerator exit. Furthermore we present
radiographs of image quality indicators
Cellular retinol-binding protein-1 expression in endometrial stromal cells: physiopathological and diagnostic implications
Cellular retinol-binding protein-1 (CRBP-1) contributes to the maintenance of the differentiated state of the endometrium through retinol bioavailability regulation. The aim was to analyse CRBP-1 expression in endometrial stromal cells at eutopic and ectopic sites in different physiopathological conditions
SO(4) Invariant States in Quantum Cosmology
The phenomenon of linearisation instability is identified in models of
quantum cosmology that are perturbations of mini-superspace models. In
particular, constraints that are second order in the perturbations must be
imposed on wave functions calculated in such models. It is shown explicitly
that in the case of a model which is a perturbation of the mini-superspace
which has spatial sections these constraints imply that any wave
functions calculated in this model must be SO(4) invariant. (This replaces the
previous corrupted version.)Comment: 15 page
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