296 research outputs found
Astrophysical jets: observations, numerical simulations, and laboratory experiments
This paper provides summaries of ten talks on astrophysical jets given at the HEDP/HEDLA-08 International Conference in St. Louis. The talks are topically divided into the areas of observation, numerical modeling, and laboratory experiment. One essential feature of jets, namely, their filamentary (i.e., collimated) nature, can be reproduced in both numerical models and laboratory experiments. Another essential feature of jets, their scalability, is evident from the large number of astrophysical situations where jets occur. This scalability is the reason why laboratory experiments simulating jets are possible and why the same theoretical models can be used for both observed astrophysical jets and laboratory simulations
High-energy photoemission final states beyond the free-electron approximation
Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc. Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment
Study of shock waves generation, hot electron production and role of parametric instabilities in an intensity regime relevant for the shock ignition
We present experimental results at intensities relevant to Shock Ignition
obtained at the sub-ns Prague Asterix Laser System in 2012 . We studied shock waves
produced by laser-matter interaction in presence of a pre-plasma. We used a first beam at
1ω (1315 nm) at 7 × 10 13 W/cm 2 to create a pre-plasma on the front side of the target and
a second at 3ω (438 nm) at ∼ 10 16 W/cm 2 to create the shock wave. Multilayer targets
composed of 25 (or 40 μm) of plastic (doped with Cl), 5 μm of Cu (for Kα diagnostics)
and 20 μm of Al for shock measurement were used. We used X-ray spectroscopy of Cl
to evaluate the plasma temperature, Kα imaging and spectroscopy to evaluate spatial and
spectral properties of the fast electrons and a streak camera for shock breakout measurements.
Parametric instabilities (Stimulated Raman Scattering, Stimulated Brillouin Scattering and
Two Plasmon Decay) were studied by collecting the back scattered light and analysing its
spectrum. Back scattered energy was measured with calorimeters. To evaluate the maximum
pressure reached in our experiment we performed hydro simulations with CHIC and DUED
codes. The maximum shock pressure generated in our experiment at the front side of the
target during laser-interaction is 90 Mbar. The conversion efficiency into hot electrons was
estimated to be of the order of ∼ 0.1% and their mean energy in the order ∼50 keV.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributio
Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric -GeTe(111)
Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, -GeTe(111) is a non-centrosymmetric ferroelectric (FE) semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond (fs) light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of -GeTe(111) using time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage (SPV), namely an increase corresponding to a 13 % enhancement of the lattice distortion. This opens the route for the control of the FE polarization in -GeTe(111) and FE semiconducting materials in quantum heterostructures
Collisionless Shock Acceleration of protons in a plasma slab produced in a gas jet by the collision of two laser-driven hydrodynamic shockwaves
We recently proposed a new technique of plasma tailoring by laser-driven
hydrodynamic shockwaves generated on both sides of a gas jet [J.-R. Marqu\`es
et al., Phys. Plasmas 28, 023103 (2021)]. In the continuation of this numerical
work, we studied experimentally the influence of the tailoring on proton
acceleration driven by a high-intensity picosecond-laser, in three cases:
without tailoring, by tailoring only the entrance side of the ps-laser, or both
sides of the gas jet. Without tailoring the acceleration is transverse to the
laser axis, with a low-energy exponential spectrum, produced by Coulomb
explosion. When the front side of the gas jet is tailored, a forward
acceleration appears, that is significantly enhanced when both the front and
back sides of the plasma are tailored. This forward acceleration produces
higher energy protons, with a peaked spectrum, and is in good agreement with
the mechanism of Collisionless Shock Acceleration (CSA). The spatio-temporal
evolution of the plasma profile was characterized by optical shadowgraphy of a
probe beam. The refraction and absorption of this beam was simulated by
post-processing 3D hydrodynamic simulations of the plasma tailoring. Comparison
with the experimental results allowed to estimate the thickness and
near-critical density of the plasma slab produced by tailoring both sides of
the gas jet. These parameters are in good agreement with those required for
CSA
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