634 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
Viscosity in the excluded volume hadron gas model
The shear viscosity in the van der Waals excluded volume
hadron-resonance gas model is considered. For the shear viscosity the result of
the non-relativistic gas of hard-core particles is extended to the mixture of
particles with different masses, but equal values of hard-core radius r. The
relativistic corrections to hadron average momenta in thermal equilibrium are
also taken into account. The ratio of the viscosity to the entropy
density s is studied. It monotonously decreases along the chemical freeze-out
line in nucleus-nucleus collisions with increasing collision energy. As a
function of hard-core radius r, a broad minimum of the ratio near fm is found at high collision energies. For the
charge-neutral system at MeV, a minimum of the ratio is reached for fm. To justify a hydrodynamic approach to
nucleus-nucleus collisions within the hadron phase the restriction from below,
fm, on the hard-core hadron radius should be fulfilled in the
excluded volume hadron-resonance gas.Comment: 12 pages, 3 figure
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
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
Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis
A sufficiently large unpolarized current can cause a spin-wave instability in
thin nanomagnets with asymmetric contacts. The dynamics beyond the instability
is understood in the perturbative regime of small spin-wave amplitudes, as well
as by numerically solving a discretized model. In the absence of an applied
magnetic field, our numerical simulations reveal a hierarchy of instabilities,
leading to chaotic magnetization dynamics for the largest current densities we
consider.Comment: 14 pages, 10 figures; revtex
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
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
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
Short Intense Laser Pulse Collapse in Near-Critical Plasma
It is observed that the interaction of an intense ultra-short laser pulse
with an overdense gas jet results in the pulse collapse and the deposition of a
significant part of energy in a small and well localized volume in the rising
part of the gas jet, where the electrons are efficiently accelerated and
heated. A collisionless plasma expansion over 150 microns at a sub-relativistic
velocity (~c/3) has been optically monitored in time and space, and attributed
to the quasistatic field ionization of the gas associated to the hot electron
current. Numerical simulations in good agreement with the observations suggest
the acceleration in the collapse region of relativistic electrons, along with
the excitation of a sizeable magnetic dipole that sustains the electron current
over several picoseconds. Perspectives of ion beam generation at high
repetition rate directly from gas jets are discussed
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