141 research outputs found
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
Modeling granular materials as compressible nonlinear fluids: Heat transfer boundary value problems
We discuss three boundary value problems in the flow and heat transfer analysis in flowing
granular materials: (i) the flow down an inclined plane with radiation effects at the free surface;
(ii) the natural convection flow between two heated vertical walls;
(iii) the shearing motion between two horizontal flat plates with heat conduction.
It is assumed that the material behaves like a continuum, similar to a compressible nonlinear fluid
where the effects of density gradients are incorporated in the stress tensor.
For a fully developed flow the equations are simplified to a system of three nonlinear ordinary
differential equations. The equations are made dimensionless and a parametric study is
performed where the effects of various dimensionless numbers representing the effects of heat
conduction, viscous dissipation, radiation, and so forth are presented
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
Effect of experimental laser imperfections on laser wakefield acceleration and betatron source
International audienceLaser pulses in current ultra-short TW systems are far from being ideal Gaussian beams. The influence of the presence of non-Gaussian features of the laser pulse is investigated here from experiments and 3D Particle-in-Cell simulations. Both the experimental intensity distribution and wavefront are used as input in the simulations. It is shown that a quantitative agreement between experimental data and simulations requires to use realistic pulse features. Moreover, some trends found in the experiments, such as the growing of the X-ray signal with the plasma length, can only be retrieved in simulations with realistic pulses. The performances on the electron acceleration and the synchrotron X-ray emission are strongly degraded by these non-Gaussian features, even keeping constant the total laser energy. A drop on the X-ray photon number by one order of magnitude was found. This clearly put forward the limitation of using a Gaussian beam in the simulations
Properties of Light Flavour Baryons in Hypercentral quark model
The light flavour baryons are studied within the quark model using the hyper
central description of the three-body system. The confinement potential is
assumed as hypercentral coulomb plus power potential () with power
index . The masses and magnetic moments of light flavour baryons are
computed for different power index, starting from 0.5 to 1.5. The
predicted masses and magnetic moments are found to attain a saturated value
with respect to variation in beyond the power index 1.0. Further
we computed transition magnetic moments and radiative decay width of light
flavour baryons. The results are in good agreement with known experimental as
well as other theoretical models.Comment: Accepted in Pramana J. of Physic
Ultra-bright gamma-ray emission and dense positron production from two laser-driven colliding foils
Matter can be transferred into energy and the opposite transformation is also possible by use of high-power lasers. A laser pulse in plasma can convert its energy into γ-rays and then e −e + pairs via the multi-photon Breit-Wheeler process. Production of dense positrons at GeV energies is very challenging since extremely high laser intensity ∼ 1024 Wcm−2 is required. Here we propose an all-optical scheme for ultra-bright γ-ray emission and dense positron production with lasers at intensity of 1022−23 Wcm−2 . By irradiating two colliding elliptically-polarized lasers onto two diamondlike carbon foils, electrons in the focal region of one foil are rapidly accelerated by the laser radiation pressure and interact with the other intense laser pulse which penetrates through the second foil due to relativistically induced foil transparency. This symmetric configuration enables efficient Compton back-scattering and results in ultra-bright γ-photon emission with brightness of ∼ 1025 photons/s/mm2 /mrad2 /0.1%BW at 15 MeV and intensity of 5×1023 Wcm−2 . Our first three-dimensional simulation with quantum-electrodynamics incorporated shows that a GeV positron beam with density of 2.5×1022 cm−3 and flux of 1.6×1010/shot is achieved. Collective effects of the pair plasma may be also triggered, offering a window on investigating laboratory astrophysics at PW laser facilities
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