148 research outputs found
Two-dimensional simulations of laser-plasma interaction and hot electron generation in the context of shock-ignition research
Laser-plasma interaction and hot electron generation play a crucial role in the context of inertial confinement fusion and in particular in the shock-ignition concept. Here we present a fully kinetic large-scale two-dimensional simulation studying laser-plasma interaction and hot electron generation in a relatively long and hot coronal plasma. The simulation shows saturation of the reflectivity of an intense spike pulse and absorption taking place close to a quarter critical density in particular, due to cavitation and stimulated Raman scattering. The signatures of steady two-plasmon decay are observed, but the hot electron number produced by this instability is low in comparison with the other two processes. The spectral and angular distribution of the back-scattered light is presented and the energy and angular characteristics of hot electrons due to individual absorption processes are studied
Passive memristor synaptic circuits with multiple timing dependent plasticity mechanisms
Adaptation of synaptic strength is central to memory and learning in biological systems, enabling important high-level processes such as the ability of animals to respond to a changing environment. Memristor devices are a promising new, nanoscale technology that emulates the function of synapses and can therefore be used to represent synaptic elements in analog artificial neural networks. The main mechanism to carry out unsupervised adaptation of weights in memristive synapses currently involves artificial spiking neural network designs relying on spike-timing dependent plasticity (STDP). We present and analyze a new memristive circuit that in addition to STDP learning rules also implements competitive adjustment based on relative timing of presynaptic inputs. The cooperative effect of multiple learning rules in the new circuit may ameliorate the problem of erasure of synaptic weights
Compact in-vacuum gamma-ray spectrometer for high-repetition rate PW-class laser-matter interaction
With the advent of high repetition rate laser facilities, novel diagnostic
tools compatible with these advanced specifications are in demand. This paper
presents the design of an active gamma-ray spectrometer intended for these high
repetition rate experiments, with particular emphasis on functionality within a
PW level laser-plasma interaction chamber's extreme conditions. The
spectrometer uses stacked scintillators to accommodate a broad range of
gamma-ray energies, demonstrating its adaptability for various experimental
setups. Additionally, it has been engineered to maintain compactness,
electromagnetic pulse resistance, and ISO-5 cleanliness requirements while
ensuring high sensitivity. The paper also outlines the unfolding process, to
recover the gamma-ray spectrum from the spectrometer's captured image thanks to
a calibration using a Co source
Design of plasma shutters for improved heavy ion acceleration by ultra-intense laser pulses
In this work, we investigate the application of the plasma shutters for heavy
ion acceleration driven by a high-intensity laser pulse. We use
particle-in-cell (PIC) and hydrodynamic simulations. The laser pulse,
transmitted through the opaque shutter, gains a steep-rising front and its peak
intensity is locally increased at the cost of losing part of its energy. These
effects have a direct influence on subsequent ion acceleration from the
ultrathin target behind the shutter. In our 3D simulations of silicon nitride
plasma shutter and a silver target, the maximal energy of high-Z ions increases
significantly when the shutter is included for both linearly and circularly
polarized laser pulses. Moreover, application of the plasma shutter for
linearly polarized pulse results in focusing of ions towards the laser axis in
the plane perpendicular to the laser polarization. The generated high energy
ion beam has significantly lower divergence compared to the broad ion cloud,
generated without the shutter. The effects of prepulses are also investigated
assuming a double plasma shutter. The first shutter can withstand the assumed
sub-ns prepulse (treatment of ns and ps prepulses by other techniques is
assumed) and the pulse shaping occursvia interaction with the second shutter.
On the basis of our theoretical findings, we formulated an approach towards
designing a double plasma shutter for high-intensity and high-power laser
pulses and built a prototype.Comment: 30 pages 13 figure
Evidence of resonant surface wave excitation in the relativistic regime through measurements of proton acceleration from grating targets
The interaction of laser pulses with thin grating targets, having a periodic
groove at the irradiated surface, has been experimentally investigated.
Ultrahigh contrast () pulses allowed to demonstrate an enhanced
laser-target coupling for the first time in the relativistic regime of
ultra-high intensity >10^{19} \mbox{W/cm}^{2}. A maximum increase by a factor
of 2.5 of the cut-off energy of protons produced by Target Normal Sheath
Acceleration has been observed with respect to plane targets, around the
incidence angle expected for resonant excitation of surface waves. A
significant enhancement is also observed for small angles of incidence, out of
resonance.Comment: 5 pages, 5 figures, 2nd version implements final correction
Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma
Laser–plasma interaction (LPI) at intensities 1015–1016 W cm2 is dominated by parametric instabilities which can be
responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal
electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the
shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS)
facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD)
instabilities, driven by the interaction of an infrared laser pulse at an intensity 1:2 1016 W cm2 with a 100 mm
scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high
value of plasma temperature (4 keV) expected from hydrodynamic simulations make these results interesting for a
deeper understanding of LPI in shock ignition conditions. Experimental results show that absolute TPD/SRS, driven at
a quarter of the critical density, and convective SRS, driven at lower plasma densities, are well separated in time, with
absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and
persisting all over the tail of the pulse. Side-scattering SRS, driven at low plasma densities, is also clearly observed.
Experimental results are compared to fully kinetic large-scale, two-dimensional simulations. Particle-in-cell results,
beyond reproducing the framework delineated by the experimental measurements, reveal the importance of filamentation
instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of
collisionless absorption in the LPI energy balance
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